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Membrane Fusion

R. Blumenthal, D. S. Dimitrov

10.1002/cphy.cp140114

Source: Supplement 31: Handbook of Physiology, Cell Physiology

Originally published: 1997

Published online: January 2011

Full Article on Wiley Online Library



Abstract

The sections in this article are:

1 Observation of Fusion Requires Physical Techniques for Monitoring Mixing of Membranes and the Compartments they Enclose
1.1 Morphological Changes Following Fusion Are Observed by Light Microscopy but Membrane Fusion May Occur without such Changes
1.2 Fluorescence Microscopy and Spectrofluorometry Allow Quantitation of Membrane Fusion Events in Living Cells
1.3 Electron Microscopy Provides Direct Observation of Structural Rearrangements Due to Fusion
1.4 Patch‐Clamp Techniques Allow the Monitoring of Very Fast Openings of Fusion Pores
2 What Do We Learn from “Nonbiological” Fusion Processes?
2.1 Ca2+ Induces Aggregation Destabilization, and Fusion of Liposomes Containing Phospholipids with Negatively Charged Head‐groups
2.2 Fusion of Lipid Membranes by Amphipathic and Nonpolar Molecules Correlates with Their Lytic and Aggregational Activity
2.3 Dehydration, Aggregation, and Destabilization of Membranes by Polyethelene Glycol Are Essential for Fusion of Lipid Membranes
2.4 Destabilization by High‐Voltage Electric Pulses Leads to Fusion of Adjoining Membranes
2.5 Molecular Rearrangements in the Lipid Bilayers during the Very Act of Fusion May Involve Intermediate Structures
3 Specialized Proteins Mediate Fusion in Life Processes
4 Viral Envelope Proteins Contain Hydrophobic “Fusion Peptide” Sequences
4.1 To Enter a Cell a Virus Must Find the Receptor That Invites It In
4.2 Some Viruses Require More Than One Type of Envelope Protein for Entry
4.3 Influenza Hemagglutinin Was the Only Fusion Protein with Known Three‐Dimensional Structure
4.4 The Process of HA‐mediated Membrane Fusion Can Be Dissected into a Number of Elementary Steps
4.5 Human Immunodeficiency Virus Type 1 (HIV‐1), the Primary Etiological Agent of the Acquired Immunodeficiency Syndrome (AIDS), Enters Cells by Membrane Fusion at Neutral pH
4.6 The Receptor CD4 Plays Both a Passive and an Active Role in Allowing Entry of the Virus into the Cell
4.7 Stable Envelope Glycoprotein‐Receptor Complex Formation Is Rate‐limiting in the Overall Fusion Process
4.8 Multiple Copies of the HIV‐1 Envelope Glycoprotein May Be Required for Fusion Pore Formation
5 Sperm Membrane Proteins Involved in Sperm‐Egg Fusion May Resemble Viral Fusion Proteins
6 Toward A Resolution of Fusion Proteins in Exocytosis
7 Multiple Proteins May Be Required for Intracellular Fusion
8 Toward A Physicochemical Analysis of Fusion Kinetics
8.1 Delays in Fusion Are Proportional to the Fusion Barriers and Decrease with an Increase in the Strength of the Fusogen
8.2 Rates of Fusion Can Provide Information for the Time Course of Membrane Merging and Fusion Pore Expansion
8.3 Fusion Yields and Delays Are Related but May Reflect Different Properties of the Fusing Membranes
9 Does Understanding Membrane Fusion Need New Breakthroughs in Methodology?
10 Note Added in Proof
Figure 1. Figure 1.

Formation of syncytia mediated by the human immunodeficiency virus (HIV‐1) envelope glycoprotein. CD4‐negative T lymphocytes (12E1), expressing the HIV‐1 envelope glycoprotein encoded by a recombinant vaccinia virus (vPE16), were mixed with CD4‐positive T cells (Molt3). The pictures were taken under DIC 16 h after mixing.
Figure 2. Figure 2.

Redistribution of fluorescent dyes from labeled human red blood cells (RBC) to unlabeled GP4F cells following fusion as detected by fluorescence microscopy. Panel 1. (a) shows a RBC double‐labeled with a water‐soluble fluorescent dye, NBD, and a membrane‐soluble dye, R18, and attached to a GP4F cell in phase contrast incubated at pH 7.4 for 90 sec. (b‐d) show pictures taken under fluorescence of NBD, R18, and both, respectively. Note that the fluorophores are confined to the RBC cytoplasm and membrane, respectively; there is no transfer to either label to the fibroblast. (e‐h) are photos of GP4F cells decorated with double‐labeled RBC after 90 s at pH 5.0 and 37°C. Note redistribution of the two labels to the originally unlabeled GP4F cells. The opposing arrows show that the membrane between the fusing cells is still visible with the membrane label (g and h), but not in (f) where the cytoplasmic dye has redistributed. The red rhodamine fluorescence appears yellow in h, because of the overlying green of the NBD. Panel 2 shows that hemoglobin does not move from RBC to GP4F after fusion detected by NBD redistribution. (a) is taken under phase contrast and shows that the hemoglobin, which appears pink, is still contained inside the RBC; (b) is taken under fluorescence and shows the redistribution of the NBD.

From 290
Figure 3. Figure 3.

Fluorescence dequenching after fusion of influenza with erythrocyte ghosts as measured by a spectrofluorometer. The rapid kinetics of fluorescence changes (in arbitrary units [a.u.]) upon fusion was triggered by mixing (using a stop‐flow technique) of equal volumes of an R18‐influenza virus‐ghost suspension and a PBS‐citrate solution to reach the indicated pH. Nine data sets were averaged for each pH. The temperature was 37°C.

From 60
Figure 4. Figure 4.

Human immunodeficiency virus (HIV‐1) fusion with plasma membranes. The right panels show ultrathin sections of H9 cells incubated with purified HIV‐1 and embedded in Epon. The left panels represent schematically stages in the fusion process. (a) Adsorption of HIV‐1 to the cell membrane after 2 min at 4°C. (b,c) Fusion after 1–3 min at 37°C (d) Empty viral envelope after fusion with the cell membrane and release of the viral ribonucleoprotein complex. Bar=150 nm. Kindly provided by Dr. C. Grewe.

With permission from 134
Figure 5. Figure 5.

Freeze‐fracture replicas of membrane fusion during exocytosis in rat peritoneal mast cells. Mast cells were stimulated with 8 μg/ml of the histamine releaser 48/80, then rapidly frozen 15 s later. A. In the unstimulated cell the plasma membrane (pm) is separated from underlying granule membrane (gm) by a layer of cytoplasm. Magnification × 270,000. B. Prior to membrane fusion and pore formation, the plasma membrane dimples inward toward the granule. Initial pore formation is thought to occur within a highly localized area of contact between the two membranes probably no greater than 50 nm in diameter. × 220,000. C. A long narrow pore (10 nm inner diameter by 40 nm in length) connects the plasma membrane with the granule membrane (gm) bringing the extracellular space (ecs) into continuity with the granule interior. At this early stage of growth, pores are quite variably in morphology. × 200,000. D,E. Even as the pore widens to 30 nm (D) and 200 nm (E) the remainder of the granule membrane is well separated from the plasma membrane. × 190,000 (D) and × 100,000 (E). F. Finally, the pore grows to produce the typical omega figure characteristic of exocytosis in its final stages. Note the etched matrix of the granule emerging into the extracellular space. × 56,000.

Micrographs were reproduced by permission from 52 (A and D): 50 (B and E): 75 (C); and 49 (F), and kindly provided by Dr. D. E. Chandler
Figure 6. Figure 6.

Capacitance increase following fusion of secretory vesicles during exocytosis of beige mice mast cells. The capacitance numbers 6.90, 7.30, 8.05, and 10.95 (in pF) are next to the oscilloscope graphs and left to the eight‐digit timer with hr, min, sec and 0.01 sec. The upper oscilloscope trace shows capacitance; the lower, conductance. The oscilloscope parameters are 50 ms/trace, persistence of 50 ms. The microscopy images were taken with a video camera and stored on tape for later analysis. Four sequential frames are shown. In (a) no activity occurs and it provides a reference for the granule size. The arrow indicates the granule. In (b) the capacitance increased from 6.90 to 7.30. The arrow indicates the initial opening of a fusion pore. The capacitance further increased in (c) and (d) indicating widening of the fusion pore(s). Swelling of the granule (d) was observed after fusion 368. Conductance, which is indicated with numbers above the capacitance, transiently changes during the experiment, but no average change corresponds to the fusion event.

The photo was kindly provided by Dr. J. Zimmerberg
Figure 7. Figure 7.

Intermembrane forces between egg phosphatidylcholine lipid bilayers as function of the interbilayer separation. The bilayer separation is also expressed as molar ratio of water to lipid. The intermembrane forces are expressed in four different ways: (i) osmotic stress π (ii) chemical potential μw; (iii) equivalent relative humidity, and (iv) temperature increase that would correspond to the same changes in equivalent relative humidity and osmotic stress. Note the tremendous change in the intermembrane repulsion with a decrease in the intermembrane separation: more than 105 times for an about ten‐fold decrease in the separation. Note also the almost linear dependence of the logarithm of the repulsive force on the separation.

Kindly provided by Dr. V. A. Parsegian, with permission
Figure 8. Figure 8.

Correlation between the threshold transmembrane voltage V of electrofusion (triangles), of electroporation (circles), and of cell destruction (squares) as function of the pulse duration τ. The square of the threshold voltage is inversely proportional to the pulse duration in agreement with the fluctuation wave mechanism of electroporation.

From 365
Figure 9. Figure 9.

Hypothetical intermediates in fusion of lipid bilayers. The monolayers of the membranes are drawn as slabs, and all the structures are cylindrically symmetric about the vertical axis. A. The stalk intermediate 57. The structure is a catenoidal monolayer sandwiched between two flat monolayers. r is the minor radius of the catenoid. B. Evolution of the stalk toward an activated stalk intermediate. The flat monolayers start to dimple, and the radius of the catenoid in the plane of the membranes expands. C. Activated stalk. The dimples of the trans monolayers meet, and the two axially symmetric voids transform into a ring of trigonally symmetric void of radius r*. The high curvature stress, and the stress due to stabilization of the voids, is concentrated in a small monolayer area of the two dimples, driving rupture of these two monolayers to form a fusion pore (D)

From 306
Figure 10. Figure 10.

Crystal structure of soluble hemagglutinin (HA). a. Schematic diagram of the 1968 hemagglutinin trimer showing carbohydrate attachment sites (CHO), antigenic sties (Ab site), the host‐receptor binding site and the arrangement relative to the membrane. b. The eight‐stranded β‐sheet structure and looped‐out region in the globular domain. c. HA2 residues 36–130, including two α‐helices (cylinders), form part of the fibrous region. Three of the long helices, one from each monomer, pack together as a triple‐stranded coiledcoil that stabilizes the timer. d. The membrane end of the molecule contains a five‐stranded β‐sheet. The central strand is the N‐terminal of HA1 and the adjacent strands come from the C‐terminal chain of HA2. Broken lines suggest the path of the hydrophobic anchoring peptide cleaved by bromelain. The possible attachment of the N‐terminus to the membrane by the signal sequence is also indicated. The site of the first oligosaccharide chain (residue 8) is shown as a triangle. Note the large separation (more than 8.2 nm) of the end of the fusion peptide from the globular domain that contains the receptor binding sites.

Kindly provided by Dr. D. C. Wiley with permission from reference 363
Figure 11. Figure 11.

Electron micrographs (100 kV) of unstained, frozen, hydrated influenza virus. A and B. Virus from the Japan strain. C and D. Virus from the X:31 strain. Panels A and C contain virus at pH 7.4 and 37°C; panels B and D contain virus incubated for 15 min at pH 4.9 and 37°C and subsequently neutralized. Note the morphological changes in the envelope spikes of the X:31 strain at the low pH and the lack of visible changes in the spikes of the Japan strain.

From 269
Figure 12. Figure 12.

Cartoon of steps in HA‐mediated fusion. The HA trimers are activated by the hydrogen ions. The activated HA trimers undergo a complex cascade of phenomena to form a committed state after which the fusion proceeds even if the pH is reversed back to neutral. Then fusion junctions and pores form that further expand to a wide opening.
Figure 13. Figure 13.

A structural model of HA‐mediated membrane fusion. The left panels show a view from the top; the right panels—a side view cross section. A and B represent the initial assembly of HA trimers around a fusion site. C and D show the initial interaction of fusion peptides with target membranes. E and F show formation of a fusion pore. Note the bending of the membrane that is needed to get the fusion peptides (in red in the right side panels) at close proximity to the target membrane 136.

Kindly provided by Dr. H. R. Guy
Figure 14. Figure 14.

Crystal structure of the first two domains of the human immunodeficiency virus (HIV‐1) receptor, CD4. The upper left panel is a backbone representation of CD4 1,2,3,4,5,6,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,31,32,33,34,35,36,37,38,39,40,41,43,44,45,46,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,81,82,83,84,85,86,88,90,91,92,93,94,95,96,97,98,99,100,102,103,104,105,106,107,108,109,110,111,112,113,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191. Domain 1 is in red, domain 2 in blue; β strands are indicated by letters, separately in each domain. Strand A of domain 2 is continuous with strand G of domain 1. Note that domains 1 and 2 are related by a rotation of approximately 160° and a translation along the axis of the molecule. Disulfide bonds are shown as solid lines; only the trace is visible of the disulfide bond between strands B and F in domain 1. The right upper panel is a solid representation of CD4 1,2,3,4,5,6,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,31,32,33,34,35,36,37,38,39,40,41,43,44,45,46,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,81,82,83,84,85,86,88,90,91,92,93,94,95,96,97,98,99,100,102,103,104,105,106,107,108,109,110,111,112,113,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191. The C′ ridge of domain 1, implicated in the binding of HIV‐1 gp120, is highlighted. The lower two panels show representations of domains 1 and 2 oriented to show the similarity of their folded structure. First and last residues in each strand are indicated by single‐letter code and sequence numbers.

Kindly provided by Dr. S. C. Harrison with permission from reference 342
Figure 15. Figure 15.

A model of possible interactions between HIV‐1 coreceptors, viral gp120‐gp41, and CD4 resulting in fusion of the viral and host cell membranes. The coreceptors are depicted to interact with the V3 loop of gp120, which determines the viral tropism. They may also interact, albeit weakly, with CD4. Those interactions lead to exposure of the fusion peptide, which induces fusion of the viral and cell membranes. While two HIV‐1 coreceptor‐CD4‐gp120‐gp41 complexes are shown, their actual number in the fusion complex is unknown.

Modified from 87
Figure 16. Figure 16.

Human immunodeficiency virus (HIV‐1) envelope glycoprotein‐mediated cell fusion cartoon. The initial binding of gp120 to CD4 leads to close membrane contact and conformational changes that result in formation of fusion pores and their wide opening.
Figure 17. Figure 17.

Kinetics of morphological changes following fusion as monitored by videomicroscopy under DIC. Cells expressing human immunodeficiency virus (HIV‐1) envelop glycoprotein encoded by a vaccinia recombinant were mixed with CD4‐expressing cells at time 0:00 (min:sec). The cells stayed in contact for 52 min. The contact area expanded (compare 15:00 with 52:00 to 53:00) and within about a minute 57,32,58,11 the visible boundary between the cells disappeared. The cells then underwent morphological changes leading to rounding of the fusion product (54:30, 55:00) and formation of small syncytium after fusion with other cells (80:00).
Figure 18. Figure 18.

A sketch of kinetic pathways of membrane fusion. 0. Two membranes approach each other under the action of a driving force due to intermembrane attraction or external fields (e.g., electric). 1. The membranes approach each other by gradually changing their shapes to almost flat membranes that surround a liquid layer of almost uniform thickness. 2. Fast localized membrane approach due to unstable shape undulations. 1a. A single bilayer membrane formed by “semi‐fusion” of two membranes that can expand to reach an equilibrium state. 1b. Nonuniform “nonwavy” liquid layer between approach membranes. 1–2. A liquid layer between membranes that has an uniform thickness. 2a. Localized “semifusion” of membranes and formation of “lenses.” 2b. Destabilization of the membranes and formation of pores. 1c,2c. Fusion of membranes and formation of intracellular vesicles of other structures. 3a,b. Post‐membrane fusion phenomena leading to final fusion products.

Modified from 85
Figure 19. Figure 19.

Delays and kinetics of fluorescent dye redistribution from a labeled to an originally unlabeled erythrocyte ghost after fusion induced by a high‐voltage electric pulse. The space locations of the points of measurement are shown in the inset. The distances, from the contact with the labeled membrane are (in μm) A,0 (the contact); B, 1.7; C, 3.3 (the center of the erythrocyte ghost); D, 5; and E, 6.6 (the far end of the membrane). The fluorescence intensity is normalized to that at the center of the labeled membrane (point 0 in the inset). The fluorescence intensity at the membrane contact (point A in the inset) is higher than zero even before the pulse because of the close proximity of the labeled membrane. The increase in fluorescence begins after a delay at a point in time that coincides with a characteristic appearance of the fluorescence as “horns” (see Fig. 13). The dye diffuses until reaching the far ends of the membranes (after time te) and an almost uniform distribution (after time tu).

From 97 with permission


Figure 1.

Formation of syncytia mediated by the human immunodeficiency virus (HIV‐1) envelope glycoprotein. CD4‐negative T lymphocytes (12E1), expressing the HIV‐1 envelope glycoprotein encoded by a recombinant vaccinia virus (vPE16), were mixed with CD4‐positive T cells (Molt3). The pictures were taken under DIC 16 h after mixing.



Figure 2.

Redistribution of fluorescent dyes from labeled human red blood cells (RBC) to unlabeled GP4F cells following fusion as detected by fluorescence microscopy. Panel 1. (a) shows a RBC double‐labeled with a water‐soluble fluorescent dye, NBD, and a membrane‐soluble dye, R18, and attached to a GP4F cell in phase contrast incubated at pH 7.4 for 90 sec. (b‐d) show pictures taken under fluorescence of NBD, R18, and both, respectively. Note that the fluorophores are confined to the RBC cytoplasm and membrane, respectively; there is no transfer to either label to the fibroblast. (e‐h) are photos of GP4F cells decorated with double‐labeled RBC after 90 s at pH 5.0 and 37°C. Note redistribution of the two labels to the originally unlabeled GP4F cells. The opposing arrows show that the membrane between the fusing cells is still visible with the membrane label (g and h), but not in (f) where the cytoplasmic dye has redistributed. The red rhodamine fluorescence appears yellow in h, because of the overlying green of the NBD. Panel 2 shows that hemoglobin does not move from RBC to GP4F after fusion detected by NBD redistribution. (a) is taken under phase contrast and shows that the hemoglobin, which appears pink, is still contained inside the RBC; (b) is taken under fluorescence and shows the redistribution of the NBD.

From 290


Figure 3.

Fluorescence dequenching after fusion of influenza with erythrocyte ghosts as measured by a spectrofluorometer. The rapid kinetics of fluorescence changes (in arbitrary units [a.u.]) upon fusion was triggered by mixing (using a stop‐flow technique) of equal volumes of an R18‐influenza virus‐ghost suspension and a PBS‐citrate solution to reach the indicated pH. Nine data sets were averaged for each pH. The temperature was 37°C.

From 60


Figure 4.

Human immunodeficiency virus (HIV‐1) fusion with plasma membranes. The right panels show ultrathin sections of H9 cells incubated with purified HIV‐1 and embedded in Epon. The left panels represent schematically stages in the fusion process. (a) Adsorption of HIV‐1 to the cell membrane after 2 min at 4°C. (b,c) Fusion after 1–3 min at 37°C (d) Empty viral envelope after fusion with the cell membrane and release of the viral ribonucleoprotein complex. Bar=150 nm. Kindly provided by Dr. C. Grewe.

With permission from 134


Figure 5.

Freeze‐fracture replicas of membrane fusion during exocytosis in rat peritoneal mast cells. Mast cells were stimulated with 8 μg/ml of the histamine releaser 48/80, then rapidly frozen 15 s later. A. In the unstimulated cell the plasma membrane (pm) is separated from underlying granule membrane (gm) by a layer of cytoplasm. Magnification × 270,000. B. Prior to membrane fusion and pore formation, the plasma membrane dimples inward toward the granule. Initial pore formation is thought to occur within a highly localized area of contact between the two membranes probably no greater than 50 nm in diameter. × 220,000. C. A long narrow pore (10 nm inner diameter by 40 nm in length) connects the plasma membrane with the granule membrane (gm) bringing the extracellular space (ecs) into continuity with the granule interior. At this early stage of growth, pores are quite variably in morphology. × 200,000. D,E. Even as the pore widens to 30 nm (D) and 200 nm (E) the remainder of the granule membrane is well separated from the plasma membrane. × 190,000 (D) and × 100,000 (E). F. Finally, the pore grows to produce the typical omega figure characteristic of exocytosis in its final stages. Note the etched matrix of the granule emerging into the extracellular space. × 56,000.

Micrographs were reproduced by permission from 52 (A and D): 50 (B and E): 75 (C); and 49 (F), and kindly provided by Dr. D. E. Chandler


Figure 6.

Capacitance increase following fusion of secretory vesicles during exocytosis of beige mice mast cells. The capacitance numbers 6.90, 7.30, 8.05, and 10.95 (in pF) are next to the oscilloscope graphs and left to the eight‐digit timer with hr, min, sec and 0.01 sec. The upper oscilloscope trace shows capacitance; the lower, conductance. The oscilloscope parameters are 50 ms/trace, persistence of 50 ms. The microscopy images were taken with a video camera and stored on tape for later analysis. Four sequential frames are shown. In (a) no activity occurs and it provides a reference for the granule size. The arrow indicates the granule. In (b) the capacitance increased from 6.90 to 7.30. The arrow indicates the initial opening of a fusion pore. The capacitance further increased in (c) and (d) indicating widening of the fusion pore(s). Swelling of the granule (d) was observed after fusion 368. Conductance, which is indicated with numbers above the capacitance, transiently changes during the experiment, but no average change corresponds to the fusion event.

The photo was kindly provided by Dr. J. Zimmerberg


Figure 7.

Intermembrane forces between egg phosphatidylcholine lipid bilayers as function of the interbilayer separation. The bilayer separation is also expressed as molar ratio of water to lipid. The intermembrane forces are expressed in four different ways: (i) osmotic stress π (ii) chemical potential μw; (iii) equivalent relative humidity, and (iv) temperature increase that would correspond to the same changes in equivalent relative humidity and osmotic stress. Note the tremendous change in the intermembrane repulsion with a decrease in the intermembrane separation: more than 105 times for an about ten‐fold decrease in the separation. Note also the almost linear dependence of the logarithm of the repulsive force on the separation.

Kindly provided by Dr. V. A. Parsegian, with permission


Figure 8.

Correlation between the threshold transmembrane voltage V of electrofusion (triangles), of electroporation (circles), and of cell destruction (squares) as function of the pulse duration τ. The square of the threshold voltage is inversely proportional to the pulse duration in agreement with the fluctuation wave mechanism of electroporation.

From 365


Figure 9.

Hypothetical intermediates in fusion of lipid bilayers. The monolayers of the membranes are drawn as slabs, and all the structures are cylindrically symmetric about the vertical axis. A. The stalk intermediate 57. The structure is a catenoidal monolayer sandwiched between two flat monolayers. r is the minor radius of the catenoid. B. Evolution of the stalk toward an activated stalk intermediate. The flat monolayers start to dimple, and the radius of the catenoid in the plane of the membranes expands. C. Activated stalk. The dimples of the trans monolayers meet, and the two axially symmetric voids transform into a ring of trigonally symmetric void of radius r*. The high curvature stress, and the stress due to stabilization of the voids, is concentrated in a small monolayer area of the two dimples, driving rupture of these two monolayers to form a fusion pore (D)

From 306


Figure 10.

Crystal structure of soluble hemagglutinin (HA). a. Schematic diagram of the 1968 hemagglutinin trimer showing carbohydrate attachment sites (CHO), antigenic sties (Ab site), the host‐receptor binding site and the arrangement relative to the membrane. b. The eight‐stranded β‐sheet structure and looped‐out region in the globular domain. c. HA2 residues 36–130, including two α‐helices (cylinders), form part of the fibrous region. Three of the long helices, one from each monomer, pack together as a triple‐stranded coiledcoil that stabilizes the timer. d. The membrane end of the molecule contains a five‐stranded β‐sheet. The central strand is the N‐terminal of HA1 and the adjacent strands come from the C‐terminal chain of HA2. Broken lines suggest the path of the hydrophobic anchoring peptide cleaved by bromelain. The possible attachment of the N‐terminus to the membrane by the signal sequence is also indicated. The site of the first oligosaccharide chain (residue 8) is shown as a triangle. Note the large separation (more than 8.2 nm) of the end of the fusion peptide from the globular domain that contains the receptor binding sites.

Kindly provided by Dr. D. C. Wiley with permission from reference 363


Figure 11.

Electron micrographs (100 kV) of unstained, frozen, hydrated influenza virus. A and B. Virus from the Japan strain. C and D. Virus from the X:31 strain. Panels A and C contain virus at pH 7.4 and 37°C; panels B and D contain virus incubated for 15 min at pH 4.9 and 37°C and subsequently neutralized. Note the morphological changes in the envelope spikes of the X:31 strain at the low pH and the lack of visible changes in the spikes of the Japan strain.

From 269


Figure 12.

Cartoon of steps in HA‐mediated fusion. The HA trimers are activated by the hydrogen ions. The activated HA trimers undergo a complex cascade of phenomena to form a committed state after which the fusion proceeds even if the pH is reversed back to neutral. Then fusion junctions and pores form that further expand to a wide opening.



Figure 13.

A structural model of HA‐mediated membrane fusion. The left panels show a view from the top; the right panels—a side view cross section. A and B represent the initial assembly of HA trimers around a fusion site. C and D show the initial interaction of fusion peptides with target membranes. E and F show formation of a fusion pore. Note the bending of the membrane that is needed to get the fusion peptides (in red in the right side panels) at close proximity to the target membrane 136.

Kindly provided by Dr. H. R. Guy


Figure 14.

Crystal structure of the first two domains of the human immunodeficiency virus (HIV‐1) receptor, CD4. The upper left panel is a backbone representation of CD4 1,2,3,4,5,6,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,31,32,33,34,35,36,37,38,39,40,41,43,44,45,46,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,81,82,83,84,85,86,88,90,91,92,93,94,95,96,97,98,99,100,102,103,104,105,106,107,108,109,110,111,112,113,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191. Domain 1 is in red, domain 2 in blue; β strands are indicated by letters, separately in each domain. Strand A of domain 2 is continuous with strand G of domain 1. Note that domains 1 and 2 are related by a rotation of approximately 160° and a translation along the axis of the molecule. Disulfide bonds are shown as solid lines; only the trace is visible of the disulfide bond between strands B and F in domain 1. The right upper panel is a solid representation of CD4 1,2,3,4,5,6,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,31,32,33,34,35,36,37,38,39,40,41,43,44,45,46,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,81,82,83,84,85,86,88,90,91,92,93,94,95,96,97,98,99,100,102,103,104,105,106,107,108,109,110,111,112,113,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191. The C′ ridge of domain 1, implicated in the binding of HIV‐1 gp120, is highlighted. The lower two panels show representations of domains 1 and 2 oriented to show the similarity of their folded structure. First and last residues in each strand are indicated by single‐letter code and sequence numbers.

Kindly provided by Dr. S. C. Harrison with permission from reference 342


Figure 15.

A model of possible interactions between HIV‐1 coreceptors, viral gp120‐gp41, and CD4 resulting in fusion of the viral and host cell membranes. The coreceptors are depicted to interact with the V3 loop of gp120, which determines the viral tropism. They may also interact, albeit weakly, with CD4. Those interactions lead to exposure of the fusion peptide, which induces fusion of the viral and cell membranes. While two HIV‐1 coreceptor‐CD4‐gp120‐gp41 complexes are shown, their actual number in the fusion complex is unknown.

Modified from 87


Figure 16.

Human immunodeficiency virus (HIV‐1) envelope glycoprotein‐mediated cell fusion cartoon. The initial binding of gp120 to CD4 leads to close membrane contact and conformational changes that result in formation of fusion pores and their wide opening.



Figure 17.

Kinetics of morphological changes following fusion as monitored by videomicroscopy under DIC. Cells expressing human immunodeficiency virus (HIV‐1) envelop glycoprotein encoded by a vaccinia recombinant were mixed with CD4‐expressing cells at time 0:00 (min:sec). The cells stayed in contact for 52 min. The contact area expanded (compare 15:00 with 52:00 to 53:00) and within about a minute 57,32,58,11 the visible boundary between the cells disappeared. The cells then underwent morphological changes leading to rounding of the fusion product (54:30, 55:00) and formation of small syncytium after fusion with other cells (80:00).



Figure 18.

A sketch of kinetic pathways of membrane fusion. 0. Two membranes approach each other under the action of a driving force due to intermembrane attraction or external fields (e.g., electric). 1. The membranes approach each other by gradually changing their shapes to almost flat membranes that surround a liquid layer of almost uniform thickness. 2. Fast localized membrane approach due to unstable shape undulations. 1a. A single bilayer membrane formed by “semi‐fusion” of two membranes that can expand to reach an equilibrium state. 1b. Nonuniform “nonwavy” liquid layer between approach membranes. 1–2. A liquid layer between membranes that has an uniform thickness. 2a. Localized “semifusion” of membranes and formation of “lenses.” 2b. Destabilization of the membranes and formation of pores. 1c,2c. Fusion of membranes and formation of intracellular vesicles of other structures. 3a,b. Post‐membrane fusion phenomena leading to final fusion products.

Modified from 85


Figure 19.

Delays and kinetics of fluorescent dye redistribution from a labeled to an originally unlabeled erythrocyte ghost after fusion induced by a high‐voltage electric pulse. The space locations of the points of measurement are shown in the inset. The distances, from the contact with the labeled membrane are (in μm) A,0 (the contact); B, 1.7; C, 3.3 (the center of the erythrocyte ghost); D, 5; and E, 6.6 (the far end of the membrane). The fluorescence intensity is normalized to that at the center of the labeled membrane (point 0 in the inset). The fluorescence intensity at the membrane contact (point A in the inset) is higher than zero even before the pulse because of the close proximity of the labeled membrane. The increase in fluorescence begins after a delay at a point in time that coincides with a characteristic appearance of the fluorescence as “horns” (see Fig. 13). The dye diffuses until reaching the far ends of the membranes (after time te) and an almost uniform distribution (after time tu).

From 97 with permission
References
 1. Ahkong, Q. F., F. C. Cramp, D. Fisher, J. I. Howell, and J. A. Lucy. Studies on chemically induced fusion. J. Cell Sci. 10: 769–787, 1972.
 2. Ahkong, Q. F., D. Fisher, W. Tampion, and J. A. Lucy. The fusion of erythrocytes by fatty acids, retinol, and alpha‐tocopherol. Biochem. J. 136: 147–155, 1973.
 3. Ahkong, Q. F., D. Fisher, W. Tampion, and J. A. Lucy. Mechanisms of cell fusion. Nature 253: 194–195, 1975.
 4. Ahkong, Q. F., J. I. Howell, J. A. Lucy, F. Safwat, M. R. Davey, and E. C. Cocking. Fusion of hen erythrocytes with yeast protoplasts induced by polyethylene glycol. Nature 255: 266–267, 1975.
 5. Alder, J., B. Lu, F. Valtorta, P. Greengard, and M‐M. Poo. Calcium‐dependent transmitter secretion reconstituted in Xenopus oocytes: requirement for synaptophysin. Science 257: 657–661, 1992.
 6. Alkhatib, G., C. Richardson, and S. H. Shen. Intracellular processing, glycosylation, and cell‐surface expression of the measles virus fusion protein (F) encoded by a recombinant adenovirus. Virology. 175: 262–270, 1990.
 7. Alkhatib, G., C. Combadiere, C. C. Broder, Y. Feng, P. E. Kennedy, P. M. Murphy, and E. A. Berger. CC CKR5: a RANTES, MIP‐1alpha, MIP‐1beta receptor as a fusion cofactor for macrophage‐tropic HIV‐1. Science 272: 1955–1958, 1996.
 8. Allan, J. S., J. Strauss, and D. W. Buck. Enhancement of SIV infection with soluble receptor molecules. Science 247: 1084–1088, 1990.
 9. Almers, W. Exocytosis. Annu. Rev. Physiol. 52: 607–624, 1990.
 10. Almers, W., L. J. Breckenridge, A. Iwata, A. K. Lee, A. E. Spruce, and F. W. Tse. Millisecond studies of single membrane fusion events. Ann. NY Acad. Sci. 318: 327, 1991.
 11. Almers, W., and F. W. Tse. Transmitter release from synapses: does a preassembled fusion pore initiate exocytosis. Neuron 4: 813–818, 1900.
 12. Arnold, K., A. Hermann, L. Pratsch, and K. Gawrisch. The dielectric properties of aqueous solutions of polyethylene glycol and their influence on membrane structure. Biochim. Biophys. Acta 815: 515–518, 1985.
 13. Arthos, J., K. C. Deen, M. A. Chaikin, J. A. Fornwald, G. Sathe, Q. J. Sattentau, P. R. Clapham, R. Weiss, J. S. McDougal, C. Pietropaolo, R. Axel, A. Truneh, P. J. Maddon, and R. W. Sweet. Identification of the residues in human CD4 critical for the binding of HIV. Cell 57: 469–481, 1989.
 14. Ashorn, P. A., E. A. Berger, and B. Moss. Human immunodeficiency virus envelope glycoprotein/CD4‐mediated fusion of non‐primate cells with human cells. J. Virol. 64: 2149–2156, 1990.
 15. Ashwell, G., and J. Harford. Carbohydrate‐specific receptors of the liver. Annu. Rev. Biochem. 51: 531–554, 1982.
 16. Bagai, S., A. Puri, R. Blumenthal, and D. P. Sarkar. Hemagglutinin‐neuraminidase enhances F protein‐mediated membrane fusion of reconstituted Sendai virus envelopes with cells. J. Virol. 67: 3312–3318, 1994.
 17. Bangham, A. D., M. M. Standish, and J. C. Watkins. Diffusion of univalent ions across lamellae of swollen phospholipids. J. Mol. Biol. 13: 238–252, 1968.
 18. Baran, A. A., I. M. Solomentseva, V. V. Mank, and O. D. Kurilenko. Role of the solvation factor in stabilizing disperse systems containing water‐soluble polymers. Dokl. Akad. Nauk SSSR, 1972, p. 363–366.
 19. Barre‐Sinoussi, F., J. C. Chermann, F. Rey, M. T. Nugeyre, S. Chamaret, J. Gruest, C. Dauguet, C. Axler‐Blin, F. Vezinet‐Brun, C. Rouzioux, W. Rozenbaum, and L. Montagnier. Isolation of a T‐lymphotropic retrovirus from a patient at risk for Acquired Immune Deficiency Syndrome (AIDS). Science 220: 868–871, 1983.
 20. Barski, G., S. Sorieul, and F. Cornefert. “Hybrid” type cells in combined cultures of two different mammalian cell strains. J. Natl. Cancer Inst. 26: 1269–1291, 1961.
 21. Beckers, C. J., M. R. Block, B. S. Glick, J. E. Rothman, and W. E. Balch. Vesicular transport between the endoplasmic reticulum and the Golgi stack requires the NEM‐sensitive fusion protein. Nature 339: 397–398, 1989.
 22. Bentz, J. Viral Fusion Mechanisms. Boca Raton, Florida: CRC Press, 1993.
 23. Bergelson, J. M., M. P. Shepley, B. M. Chan, M. E. Hemler, and R. W. Finberg. Identification of the integrin VLA‐2 as a receptor for echovirus 1 [see comments]. Science 255: 1718–1720, 1992.
 24. Blobel, C. P., T. G. Wolfsberg, C. W. Turck, D. G. Myles, P. Primakoff, and J. M. White. A potential fusion peptide and an integrin ligand domain in a protein active in sperm‐egg fusion. Nature 356: 248–252, 1992.
 25. Blumenthal, R. Membrane fusion. Curr. Top. Membr. Transp. 29: 203–254, 1987.
 26. Blumenthal, R., A. Bali‐Puri, A. Walter, D. Covell, and O. Eidelman. pH‐Dependent fusion of vesicular stomatitis virus with Vero cells: measurement by dequenching of octadecylrhodamine fluorescence. J. Biol. Chem. 262: 13614–13619, 1987.
 27. Blumenthal, R., and A. Loyter. Reconstituted viral envelopes—“Trojan Horses” for drug delivery and gene therapy. TibTech 9: 41–45, 1991.
 28. Blumenthal, R., A. Puri, and D. S. Dimitrov. Initial steps of enveloped virus entry into animal cells. In: Biotechnology of Cell Regulation, edited by R. Verna and Y. Nishizuka, New York: Raven Press 1991, p. 115–133.
 29. Blumenthal, R., C. Schoch, A. Puri, and M. J. Claque. A dissection of steps leading to viral envelope protein‐mediated membrane fusion. Ann. NY Acad. Sci. 635: 285–296, 1991.
 30. Blumenthal, R., D. P. Sarkar, S. Durell, D. E. Howard, and S. J. Morris. Dilation of the influenza hemagglutinin fusion pore revealed by the kinetics of individual cell‐cell fusion events. J. Cell. Biol. 135: 63–72, 1996.
 31. Boni, L. T., J. S. Hah, S. W. Hui, P. Mukherjee, J. T. Ho, and C. Y. Jung. Aggregation and fusion of unilamellar vesicles by polyethylene glycol. Biochim. Biophys. Acta. 775: 409–418, 1984.
 32. Boni, L. T., T. P. Stewart, J. L. Alderfer, and S. W. Hui. Lipid‐polyethylene glycol interactions: I. Induction of fusion between liposomes. J. Membrane Biol. 62: 65–70, 1981.
 33. Boni, L. T., T. P. Stewart, and S. W. Hui. Lipid‐polyethylene glycol interactions: II. Formation of defects in bilayers. J. Membrane Biol. 62: 71–77, 1981.
 34. Boni, L. T., T. P. Stewart, and S. W. Hui. Alterations in phospholipid polymorphism by polyethylene glycol. J. Membrane Biol. 80: 91–104, 1984.
 35. Bonnett, H. T., and T. Eriksson. Transfer of algal chloroplasts into protoplasts of higher plants. Planta 120: 71–79, 1974.
 36. Bosch, M. L., P. L. Earl, K. Fargnoli, S. Picciafuoco, F. Giombini, F. Wong‐Staal, and G. Franchini. Identification of the fusion peptide of primate immunodeficiency viruses. Science 244: 694–697, 1989.
 37. Brasseur, R. Differentiation of lipid‐associating helices by use of three‐dimensional molecular hydrophobicity potential calculations. J. Biol. Chem. 266: 16120–16127, 1991.
 38. Brasseur, R., M. Vandenbranden, B. Cornet, A. Burny, and J. M. Ruysschaert. Orientation into the lipid bilayer of an asymmetric amphipathic helical peptide located at the N‐terminus of viral fusion proteins. Biochim. Biophys. Acta 1029: 267–273, 1990.
 39. Breckenridge, L. J., and W. Almers. Currents through the fusion pore that forms during exocytosis of a secretory vesicle. Nature 328: 814–817, 1987.
 40. Broder, C. C., and E. A. Berger. CD4 molecules with a diversity of mutations encompassing the CDR3 region efficiently support the human immunodeficiency virus type 1 envelope glycoprotein‐mediated cell fusion. J. Virol. 67: 913–926, 1993.
 41. Broder, C. C., D. S. Dimitrov, R. Blumenthal, and E. A. Berger. The block to HIV‐1 envelope glycoprotein‐mediated membrane fusion in animal cells expressing human CD4 can be overcome by human cell components. Virology 193: 483–491, 1993.
 42. Bullough, P. A., F. M. Hughson, J. J. Skehel, and D. C. Wiley. Structure of influenza haemagglutinin at the pH of membrane fusion. Nature 371: 37–43, 1994.
 43. Burgoyne, R. D. Secretory vesicle‐associated proteins and their role in exocytosis. Annu. Rev. Physiol. 52: 647–659, 1990.
 44. Burkly, L. C., D. Olson, R. Shapiro, G. Winkler, J. J. Rosa, D. W. Thomas, C. Williams, and P. Chisholm. Inhibition of HIV entry by a CD4 domain 2 specific monoclonal antibody: evidence for structural alterations upon CD4/HIV gp120 binding. J. Immunol. 149: 1779–1787, 1992.
 45. Burns, A. L., K. Magendzo, A. K. Shirvan, A. Srivastava, E. Rojas, M. R. Alijani, and H. B. Pollard. Calcium channel activity of purified human synexin and structure of the human synexin gene. Proc. Natl. Acad. Sci. U. S. A. 86: 3798–3802, 1989.
 46. Capers, C. R. Multinucleation of skeletal muscle in vitro. J. Biophys. Biochem. Cytol. 7: 559–566, 1960.
 47. Carr, C. M., and P. S. Kim. A spring‐loaded mechanism for the conformational change of influenza hemagglutinin. Cell 73: 823–832, 1993.
 48. Celada, F., C. Cambiaggi, J. Maccari, S. Burastero, T. Gregory, E. Patzer, J. Porter, C. McDanal, and T. Matthews. Antibody raised against soluble CD4‐rgp120 complex recognizes the CD4 moiety and blocks membrane fusion without inhibiting CD4‐gp120 binding. J. Exp. Med. 172: 1143–1150, 1990.
 49. Chandler, D. E. Exocytosis involves highly localized membrane fusions. Biochem. Soc. Trans. 12: 961–963, 1984.
 50. Chandler, D. E. Membrane fusion as seen in rapidly frozen secretory cells. Ann. NY Acad. Sci. 635: 234–245, 1991.
 51. Chandler, D. E., and J. E. Heuser. Membrane fusion during secretion. Cortical granule exocytosis in sea urchin eggs as studied by quick freezing and freeze‐fracture. J. Cell Biol. 83: 91–108, 1979.
 52. Chandler, D. E., and J. E. Heuser. Arrest of membrane fusion events in mast cells by quick freezing. J. Cell Biol. 86: 666–674, 1980.
 53. Chang, D. C., B. M. Chassy, J. A. Saunders, and A. E. Sowers. Guide to Electroporation and Electrofusion. San Diego: Academic Press, 1992.
 54. Chen, Y., and R. Blumenthal. On the use of self‐quenching fluorophores in the study of membrane fusion kinetics. Biophys. Chem. 34: 283–292, 1989.
 55. Chen, Y. H., C. Ebenbichler, R. Vornhagen, T. F. Schulz, F. Steindl, G. Bock, H. Katinger, and M. P. Dierich. HIV‐1 gp41 contains 2 sites for interaction with several proteins on the helper T‐lymphoid cell line, H9. AIDS 6: 533–539, 1992.
 56. Chernomordik, L., S. S. Vogel, E. Leukina, and J. Zimmerberg. Inhibition of biological membrane fusion by amphipathic compounds. Biophys. J. 61: a499, 1992.
 57. Chernomordik, L. V., G. B. Melikyan, and Y. A. Chizmadzhev. Biomembrane fusion: a new concept derived from model studies using two interacting planar lipid bilayers. Biochim. Biophys. Acta 906: 309–352, 1987.
 58. Chernomordik, L. V., and A. E. Sowers. Evidence that the spectrin network and a nonosmotic force control the fusion product morphology in electrofused erythrocyte ghosts. Biophys. J. 60: 1026–1037, 1991.
 59. Chow, R. H., L. Von Ruden, and E. Neher. Delay in vesicle fusion revealed by electrochemical monitoring of single secretory events in adrenal chromaffin cells. Nature 356: 60–63, 1992.
 60. Clague, M. J., C. Schoch, and R. Blumenthal. Delay time for influenza hemagglutinin‐induced membrane fusion depends on the haemagglutinin surface density. J. Virol. 65: 2402–2407, 1991.
 61. Clague, M. J., C. Schoch, L. Zech, and R. Blumenthal. Gating kinetics of pH‐activated membrane fusion of vesicular stomatitis virus with cells: stopped flow measurements by dequenching of octadecylrhodamine fluorescence. Biochemistry 29: 1303–1308, 1990.
 62. Clapham, P. R. Human immunodeficiency virus infection of non‐haematopoietic cells. The role of CD4‐independent entry. Rev. Med. Virol. 1: 51–58, 1991.
 63. Clary, D. O., I. C. Griff, and J. E. Rothman. SNAPs, a family of NSF attachment proteins involved in intracellular membrane fusion in animals and yeast. Cell 61: 709–721, 1990.
 64. Coakley, T. W., H. Darmani, and A. J. Baker. Membrane contact induced between erythrocytes by polycations, lectins and dextran. In: Cell and Model Membrane Interactions, edited by S. Ohki, New York: Plenum Press, 1991, p. 25–45.
 65. Coakley, W. T., and D. Gallez. Membrane‐membrane contact: involvement of interfacial instability in the generation of discrete contacts. Biosc. Rep. 9: 675–691, 1989.
 66. Cohen, F. S., M. H. Akabas, J. Zimmerberg, and A. Finkelstein. Parameters affecting the fusion of unilamellar phospholipid vesicles with planar bilayer membranes. J. Cell Biol. 98: 1054–1062, 1984.
 67. Colombo, M. I., L. S. Mayorga, P. J. Casey, and P. D. Stahl. Evidence of a role for heterotrimeric GTP‐binding proteins in endosome fusion. Science 255: 1695–1697, 1992.
 68. Colwin, L. H., and A. L. Colwin. Formation of sperm entry holes in the vitelline membrane of Hydroides hexagonus (Annelida) and evidence of their lytic origin. J. Biophys. Biochem. Cytol. 7: 315–320, 1960.
 69. Compans, R. W., A. Helenius, and M. Oldstone. Cell Biology of Virus Entry, Replication and Pathogenesis. New York: Alan R. Liss, 1989, p. 1–449.
 70. Creutz, C. Cis‐unsaturated fatty acids induce the fusion of chromaffin granules aggregated by synexin. J. Cell Biol. 91: 247–256, 1981.
 71. Creutz, C. E. The annexins and exocytosis. Science 258: 924–931, 1992.
 72. Creutz, C. E., L. G. Dowling, J. J. Sando, C. Villar‐Palasi, J. H. Whiple, and W. J. Zaks. Characterization of the chromobindins: Soluble proteins that bind to the chromaffin granule membrane in the presence of Ca2+ J. Biol. Chem. 258: 14664–14674, 1983.
 73. Creutz, C. E., C. J. Pazoles, and H. B. Pollard. Identification and purification of an adrenal medullary protein (synexin) that causes calcium dependent aggregation of isolated chromaffin granules. J. Biol. Chem. 253: 2858–2866, 1978.
 74. Cunningham, J. M. Cellular entry by murine retroviruses. Seminars in Virology 3: 85–89, 1992.
 75. Curran, M., F. S. Cohen, D. E. Chandler, P. Munson, and J. Zimmerberg. Exocytotic fusion pores exhibit semi‐stable states. J. Membrane Biol. 133: 61–75, 1993.
 76. Curran, M. J., M. S. Brodwick, and C. Edwards. Direct visualization of exocytosis in mast cells. Biophys. J. 45: 170a, 1984.
 77. Daiz, R., L. Mayorga, P. J. Weidman, J. E. Rothman, and P. D. Stahl. Vesicle fusion following receptor‐mediated endocytosis requires a protein active in Golgi transport. Nature 339: 398–400, 1989.
 78. Dalgleish, A. G., P.C.L. Beverley, P. R. Clapham, D. H. Crawford, M. F. Greaves, and R. A. Weiss. The CD4 (T4) antigen is an essential component of the receptor for the AIDS retrovirus. Nature 312: 763–767, 1984.
 79. Del Castillo, J., and B. Katz. Quantal components of the end‐plate potential. J. Physiol. 124: 560–573, 1954.
 80. Deng, H., R. Liu, W. Ellmeier, S. Choe, D. Unutmaz, M. Burkhart, P. Di Marzio, S. Marmon, R. E. Sutton, C. M. Hill, C. B. Davis, S. C. Peiper, T. J. Schall, D. R. Littman, and N. R. Landau. Identification of a major co‐receptor for primary isolates of HIV‐1. Nature 381: 661–666, 1996.
 81. Dimitrov, D. S. A hydrodynamic theory of the bilayer membrane formation. Biophys. J. 36: 21–25, 1981.
 82. Dimitrov, D. S. Instability of thin liquid films between membranes. Coll. Pol. Sci. 260: 1137–1144, 1982.
 83. Dimitrov, D. S. Dynamic interactions between approaching surfaces of biological interest. Progr. Surface Sci. 14: 295–424, 1983.
 84. Dimitrov, D. S. Approach and instability of membranes: physicochemical mechanisms and applications to adhesion, fusion, dielectrophoresis, electroporation and liposome formation. Proc. 8th School Bioph. Membrane Transport, Mierki, Poland 1: 51–82, 1986.
 85. Dimitrov, D. S., Membrane fusion kinetics. In: Guide to Electroporation and Electrofusion, edited by D. C. Chang, B. M. Chassy, J. A. Saunders, and A. E. Sowers. Orlando: Academic Press, 1992, p. 155–166.
 86. Dimitrov, D. S., Electroporation and electrofusion of membranes. In: Handbook of Physics of Biological Systems, Structure and Dynamics of Membranes, edited by E. Sackmann and R. Lipowsky, Amsterdam: Elsevier, Vol. 1b Ch.18 852–901, 1995.
 87. Dimitrov, D. S. Fusin—a place for HIV‐1 and T4 cells to meet. Nature Med. 2: 640–641, 1996.
 88. Dimitrov, D. S., and R. Blumenthal. Kinetics of intermembrane interactions leading to fusion. In Cell and Model Membrane Interactions. edited by S. Ohki. New York: Plenum Press, 1991, p. 115–133.
 89. Dimitrov, D. S., and R. Blumenthal. Photoinactivation and kinetics of membrane fusion mdiated by the human immunodeficiency virus type I envelope glycoprotein. J. Virol. 68: 1956–1961, 1994.
 90. Dimitrov, D. S., and R. K. Jain. Membrane stability. Biochim. Biophys. Acta 779: 437–468, 1984.
 91. Dimitrov, D. S., C. C. Broder, E. A. Berger, and R. Blumenthal. Calcium ions are required for cell fusion mediated by the CD4‐HIV‐1 envelope glycoprotein interaction. J. Virol. 67: 1647–1652, 1993.
 92. Dimitrov, D. S., H. Golding, and R. Blumenthal. Initial steps in HIV‐1 envelope glycoprotein mediated cell fusion monitored by a new assay based on redistribution of fluorescence markers. AIDS Res. Human Retroviruses 7: 799–805, 1991.
 93. Dimitrov, D. S., K. Hillman, J. Manischewitz, R. Blumenthal, and H. Golding. Kinetics of interaction of sCD4 with cells expressing HIV‐1 envelope and inhibition of syncytia formation. AIDS 60: 249–256, 1992.
 94. Dimitrov, D. S., K. Hillman, J. Manischewitz, R. Blumenthal, and H. Golding. Kinetics of binding of sCD4 to cells expressing HIV‐1 envelope glycoprotein. J. Virol. 66: 132–138, 1992.
 95. Dimitrov, D. S., I. Panaiotov, P. Richmond, and L. Ter‐Minassian‐Saraga. Dynamics of insoluble monolayers. I. Dilational or elastic modulus, friction coefficient and Marangoni effect for dipalmitoyllecithin. J. Colloid Interface Sci. 65: 483–494, 1978.
 96. Dimitrov, D. S., and A. E. Sowers. Membrane electroporation—fast molecular exchange by electroosmosis. Biochim. Biophys. Acta 1022: 381–392, 1990.
 97. Dimitrov, D. S., and A. E. Sowers. A delay in membrane fusion: lag times observed by fluorescence microscopy of individual fusion events induced by an electric field pulse. Biochemistry 29: 8337–8344, 1990.
 98. Dimitrov, D. S., R. Willey, M. Martin and R. Blumenthal. Kinetics of HIV‐1 interactions with sCD4 and CD4 + cells: implications for inhibition of virus infection and initial steps of virus entry into cells. Virology 187: 398–406, 1992.
 99. Dimitrov, D. S., R. L. Willey, H. Sato, L.‐Ji. Chang, R. Blumenthal, and M. A. Martin. Quantitation of HIV‐1 infection kinetics. J. Virol. 67: 2182–2190, 1993.
 100. Doms, R. W., A. Helenius, and J. White. Membrane fusion activity of the influenza virus hemagglutinin: the low pH‐induced conformational change. J. Biol. Chem. 260: 2973–2981, 1985.
 101. Dragic, T., V. Litwin, G. P. Allaway, S. R. Martin, Y. Huang, K. A. Nagashima, C. Cayanan, P. J. Maddon, R. A. Koup, J. P. Moore, and W. A. Paxton. HIV‐1 entry into CD4+ cells is mediated by the chemokine receptor CC‐CKR‐5. Nature 381: 667–673, 1996.
 102. Dragic, T., P. Charneau, F. Clavel, and M. Alizon. Complementation of murine cells for human immunodeficiency virus envelope/CD4‐mediated fusion in human/murine heterokaryons. J. Virol. 66: 4794–4802, 1992.
 103. Dukhin, S. S., N. N. Rulyov, and D. S. Dimitrov. Coagulation and Dynamics of Thin Layers. Kiev: Naukova Dumka, 1986.
 104. Duzgunes, N., and J. Bentz. Fluorescence assays for membrane fusion. In: Spectroscopic Membrane Probes, edited by L. M. Loew, Boca Raton, Florida: CRC Press, 1988, p. 117–159.
 105. Duzgunes, N., and F. Bronner. Membrane Fusion in Fertilization, Cellular Transport, and Viral Infection: Current Topics in Membranes and Transport, vol. 32. San Diego: Academic Press, 1988.
 106. Earl, P. L., R. W. Doms, and B. Moss. Multimeric CD4 binding exhibited by human and simian immunodeficiency virus envelope protein dimers. J. Virol. 66: 5610–5614, 1992.
 107. Ebata, S. N., M. J. Cote, C. Y. Kang, and K. Dimock. The fusion and hemagglutinin‐neuraminidase glycoproteins of human parainfluenza virus 3 are both required for fusion. Virology. 183: 437–441, 1991.
 108. Edwards, W., J. H. Phillips, and S. J. Morris. Structural changes in chromaffin granules induced by divalent cations. Biochim. Biophys. Acta 356: 164–173, 1974.
 109. Ehrenstein, G., E. F. Stanley, S. L. Pocotte, M. Jia, K. H. Iwasa, and K. E. Krebs. Evidence for a model of exocytosis that involves calcium‐activated channels. Ann. NY Acad. Sci. 635: 297–306, 1991.
 110. Ellens, H., J. Bentz, D. Mason, F. Zhang, and J. M. White. Fusion of influenza hemagglutinin‐expressing fibroblasts with glycophorin‐bearing liposomes: role of hemagglutinin surface density. Biochemistry. 29: 9697–9707, 1990.
 111. Epel, D., and V. D. Vacquier. Membrane fusion events during invertebrate fertilization. Cell Surface Rev. 5: 1–63, 1978.
 112. Evered, D., and J. Whelan. Cell Fusion. London: Pitman, 1984.
 113. Fauci, A. S. The human immunodeficiency virus: infectivity and mechanisms of pathogenesis. Science 239: 617–622, 1988.
 114. Feng, Y., C. C. Broder, P. E. Kennedy, and E. A. Berger. HIV‐1 entry cofactor: functional cDNA cloning of a seven‐transmembrane, G protein‐coupled receptor. Science 272: 872–877, 1996.
 115. Fernandez, J. M., E. Neher, and B. D. Gomperts. Capacitance measurements reveal stepwise fusion events in degranulating mast cells. Nature 312: 453–455, 1984.
 116. Fernandez‐Larsson, R., K. K. Srivastava, S. Lu, and H. L. Robinson. Replication of patient isolates of human immunodeficiency virus type 1 in T cells: a spectrum of rates and efficiencies of entry. Proc. Natl. Acad. Sci. U.S.A. 89: 2223–2226, 1992.
 117. Formanowski, F., S. A. Wharton, L. J. Calder, C. Hofbauer, and H. Meier‐Ewert. Fusion characteristics of influenza C viruses. J. Gen. Virol. 71: 1181–1188, 1990.
 118. Freed, E. O., E. L. Delwart, G. L. Buchschacher, Jr. and A. T. Panganiban. A mutation in the human immunodeficiency virus type 1 transmembrane glycoprotein gp41 dominantly interferes with fusion and infectivity. Proc. Natl. Acad. Sci. U. S. A. 89: 70–74, 1992.
 119. Freed, E. O., D. J. Myers, and R. Risser. Characterization of the fusion domain of the human immunodeficiency virus type 1 envelope glycoprotein gp41. Proc. Natl. Acad. Sci. U. S. A. 87: 4650–4654, 1990.
 120. Freistadt, M. S., and V. R. Racaniello. Mutational analysis of the cellular receptor for poliovirus. J. Virol. 65: 3873–3876, 1991.
 121. Fridberger, A., J. Sundeling, V. D. Vacquier, and P. A. Peterson. Amino acid sequence of an egg lysin protein from abalone spermatozoa that solubilizes the vitelline layer. J. Biol. Chem. 260: 9092–9099, 1985.
 122. Fujioka, A., M. Ohtsuki, M. Nagano, and S. Mori. Membrane fusion events during endocytosis in mouse kidney tubule cells detected by rapid freezing followed by freeze‐substitution. J. Electron Micr. 39: 356–362, 1990.
 123. Fuller, A. O., and W. C. Lee. Herpes simplex virus type 1 entry through a cascade of virus‐cell interactions requires different roles of gD and gH in penetration. J. Virol. 66: 5002–5012, 1992.
 124. Gad, A. E., B. L. Silver, and G. D. Eytan. Polycation‐induced fusion of negatively‐charged vesicles. Biochim. Biophys. Acta 690: 124–132, 1982.
 125. Gallo, R. C., S. Z. Salahuddin, M. Popovic, G. M. Shearer, M. Kaplan, B. F. Haynes, T. J. Palker, R. Redfield, J. Oleske, B. Safai, G. White, P. Foster, and P. D. Markham. Frequent detection and isolation of cytopathic retroviruses (HTLV‐III) from patients with AIDS and at risk for AIDS. Science 224: 500–503, 1984.
 126. Gelderblom, H. R. Assembly and morphology of HIV: potential effect of structure on viral function. AIDS 5: 617–638, 1991.
 127. Gelderblom, H. R., M. Ozel, and G. Pauli. Morphogenesis and morphology of HIV. Structure‐function relations. Arch. Virol. 106: 1–13, 1989.
 128. Gething, M. J., R. W. Doms, D. York, and J. White. Studies on the mechanism of membrane fusion: site‐specific mutagenesis of the hemagglutinin of influenza virus. J. Cell Biol. 102: 11–23, 1986.
 129. Gething, M. J., and J. Sambrook. Cell‐surface expression of influenza haemagglutinin from a cloned DNA copy of the RNA gene. Nature 293: 620–625, 1981.
 130. Glabe, C. C. Interaction of the sperm adhesive protein, bindin, with phospholipid vesicles. II. Bindin induces the fusion of mixed phase vesicles that contain phosphatidylcholine and phosphatidylserine in vitro. J. Cell Biol. 100: 800–806, 1985.
 131. Godley, L., J. Pfeifer, D. Steinhauer, B. Ely, G. Shaw, R. Kaufmann, E. Suchanek, C. Pabo, J. J. Skehel, D. C. Wiley, and A. L. Et. Introduction of intersubunit disulfide bonds in the membrane‐distal region of the influenza hemagglutinin abolishes membrane fusion activity. Cell 68: 635–645, 1992.
 132. Golding, H., D. S. Dimitrov, R. Blackburn, J. Manischewitz, R. Blumenthal, and B. Golding. Fusion of human B cell lines with HIV‐1 envelope‐expressing T cells is enhanced by antigen‐specific immunoglobulin receptors: a possible mechanism for elimination of gp120‐specific B cells in vivo. J. Immunol. 150: 2506–2516, 1993.
 133. Golding, H., D. S. Dimitrov, and R. Blumenthal. LFA‐1 adhesion molecules are not involved in early stages of HIV‐1‐env‐mediated cell membrane fusion. AIDS Res. Human Retroviruses 8: 1607–1612, 1992.
 134. Grewe, C., A. Beck, and H. R. Gelderblom. HIV: early virus‐cell interaction. J. AIDS 3: 965–974, 1990.
 135. Gruenberg, J., and M. J. Clague. Regulation of intracellular membrane transport. Curr. Opin. Cell Biol. 4: 593–599, 1992.
 136. Guy, H. R., S. R. Durell, C. Schoch, and R. Blumenthal. Analyzing the fusion process of influenza hemagglutinin by mutagenesis and molecular modeling. Biophys. J. 62: 113–116, 1992.
 137. Healey, D., L. Dianda, J. P. Moore, J. S. McDougal, M. J. Moore, P. Estess, D. Buck, P. D. Kwong, P.C.L. Beverley, and Q. J. Sattentau. Novel anti‐CD4 monoclonal antibodies separate human immunodeficiency virus infection and fusion of CD4+ cells from virus binding. J. Exp. Med. 172: 1233–1242, 1990.
 138. Herrmann, A., M. J. Clague, and R. Blumenthal. The role of the target membrane structure in fusion with influenza virus: effect of modulating erythrocyte transbilayer phospholipid structure. Membr. Biochem. 10: 3–15, 1993.
 139. Heuser, J. E., T. S. Reese, M. J. Dennis, Y. Jan, L. Jan and L. Evans. Synaptic vesicle exocytosis captured by quick freezing and correlated with quantal transmitter release. J. Cell Biol. 81: 275–300, 1979.
 140. Ho, D. D., J. C. Kaplan, I. E. Rackauskas, and M. E. Gurney. Second conserved domain of gp120 is important for HIV infectivity and antibody neutralization. Science 239: 1021–1023, 1988.
 141. Hoekstra, D., T. De Boer, K. Klappe, and J. Wilschut. Fluorescence method for measuring the kinetics of fusion between biological membranes. Biochemistry 23: 5675–5681, 1984.
 142. Hoekstra, D., K. Klappe, H. Hoff, and S. Nir. Mechanism of fusion of Sendai virus: role of hydrophobic interactions and mobility constraints of viral membrane proteins: effects of PEG. J. Biol. Chem. 264: 6786–6792, 1989.
 143. Hoekstra, D., and J. W. Kok. Entry mechanisms of enveloped viruses. Implications for fusion of intracellular membranes. Biosci. Rep. 9: 273–305, 1989.
 144. Hoggan, M. D., and B. Roizman. The isolation and properties of a variant of herpes simplex virus producing multinucleate giant cells in monolayer cultures in the presence of antibody. Am. J. Hyg. 70: 208–219, 1959.
 145. Holtzer, H., J. Abbott, and J. Lash. The formation of multinucleate myotubes. Anat. Rec. 131: 567, 1958.
 146. Hong, K., N. Duzgunes, R. Ekerdt, and D. Papahadjopoulos. Modulation of membrane fusion by calcium‐binding proteins. Biophys. J. 37: 297–305, 1982.
 147. Hong, K., N. Duzgunes, and D. Papahadjopoulos. Role of synexin in membrane fusion. J. Biol. Chem. 256: 3641–3644, 1981.
 148. Hong, K., N. Duzgunes, and D. Papahadjopoulos. Synexin facilitates fusion of specific phospholipid vesicles at divalent cation concentrations found intracellulary. Proc. Natl. Acad. Sci. U. S. A. 79: 4942–4944, 1982.
 149. Hong, K., and V. D. Vacquier. Fusion of liposomes induced by a cationic protein from the acrosome granule of abalone spermatozoa. Biochemistry 25: 543–549, 1986.
 150. Horth, M., B. Lambrecht, M. C. Khim, F. Bex, C. Thiriart, J. M. Ruysschaert, A. Burny, and R. Brasseur. Theoretical and functional analysis of the SIV fusion peptide. EMBO. J. 10: 2747–2755, 1991.
 151. Horvath, C. M., and R. A. Lamb. Studies on the fusion peptide of a paramyxovirus fusion glycoprotein: roles of conserved residues in cell fusion. J. Virol. 66: 2443–2455, 1992.
 152. Hsu, M.‐C., A. Scheid, and P. Choppin. Reconstruction of membranes with individual paramyxovirus glycoproteins and phospholipid and cholate solution. Virology 95: 476–491, 1979.
 153. Hu, X. L., R. Ray and R. W. Compans. Functional interactions between the fusion protein and hemagglutinin‐neuraminidase of human parainfluenza viruses. J. Virol. 66: 1528–1534, 1992.
 154. Hui, S. W., and L. T. Boni. Membrane fusion induced by polyethylene glycol. In: Membrane Fusion, edited by J. Wilschut and D. Hoekstra, New York: Marcel Dekker, 1991, p. 231–253.
 155. Hui, S. W., T. Isac, L. T. Boni, and A. Sen. Action of polyethylene glycol on the fusion of human erythrocyte membranes. J. Membrane Biol. 84: 137–146, 1985.
 156. Hui, S. W., T. P. Stewart, and L. T. Boni. The nature of lipidic particles and their roles in polymorphic transitions. Chem. Phys. Lipids 33: 113–126, 1983.
 157. Hui, S. W., T. P. Stewart, L. T. Boni, and P. L. Yeagle. Membrane fusion through point defects in bilayers. Science 212: 921–923, 1981.
 158. Ivanov, I. B. Thin Liquid Films. New York: Marcel Dekker, 1988.
 159. Ivanov, I. B., and D. S. Dimitrov. Hydrodynamics of thin liquid films. Effects of surface viscosity on thinning and rupture of foam films. Coll. Pol. Sci. 252: 982–990, 1974.
 160. Ivanov, I. B., and D. S. Dimitrov. Thin film drainage. In: Thin Liquid Films, edited by I. B. Ivanov, New York: Marcel Dekker, 1988.
 161. Ivanov, I. B., B. P. Radoev, E. Manev, and A. Scheludko. Dynamics and stability of thin liquid films. Trans. Faraday Soc. 66: 1262–1273, 1970.
 162. Jaffe, L. A., S. Hagiwara, and R. T. Kado. The time course of cortical vesicle fusion in sea urchin eggs observed as membrane capacitance changes. Dev. Biol. 67: 243–248, 1978.
 163. Jasin, M., K. A. Page, and D. R. Littman. Glycosylphosphatidylinositol‐anchored CD4/Thy‐1 chimeric molecules serve as human immunodeficiency virus receptors in human, but not mouse, cells and are modulated by gangliosides. J. Virol. 65: 440–444, 1991.
 164. Johann, S. V., J. J. Gibbons, and B. O'Hara. GLVR1, a receptor for gibbon ape leukemia virus, is homologous to a phosphate permease of Neurospora crassa and is expressed at high levels in the brain and thymus. J. Virol. 66: 1635–1640, 1992.
 165. Kahn, R. A. Fluoride is not an activator of the smaller (20–25 kDa) GTP‐binding proteins. J. Biol. Chem. 266: 15595–15597, 1991.
 166. Kao, K. N., and M. R. Michayluk. A method for high‐frequency intergeneric fusion of plant protoplasts. Planta 115: 355–367, 1974.
 167. Kaplan, D., J. Zimmerberg, A. Puri, D. P. Sarkar, and R. Blumenthal. Single cell fusion events induced by influenza hemagglutinin: studies with rapid‐flow, quantitative fluorescence microscopy. Exp. Cell. Res. 195: 137–144, 1991.
 168. Katz, B., and R. Miledi. The effect of temperature on the synaptic delay at the neuromuscular junction. J. Physiol. 181: 656–670, 1965.
 169. Keller, P. M., S. Person, and W. Snipes. A fluorescence enhancement assay of cell fusion. J. Cell Sci. 28: 167–177, 1977.
 170. Kemble, G. W., D. L. Bodian, J. Rose, I. A. Wilson, and J. M. White. Intermonomer disulfide bonds impair the fusion activity of influenza virus hemagglutinin. J. Virol. 66: 4940–4950, 1992.
 171. Klatzmann, D., E. Champagne, S. Chamaret, J. Gruest, D. Guetard, T. Hercend, J.‐C. Gluckman, and L. Montagnier. T‐lymphocyte T4 molecule behaves as the receptor for human retrovirus LAV. Nature 312: 767–768, 1984.
 172. Knoll, G., and H. Plattner. Ultrastructural analysis of biological membrane fusion and a tentative correlation with biochemical and biophysical aspects. In: Electron Microscopy of Subsellular Dynamics, edited by H. Plattner, Boca Raton, FL: CRC Press, 1989, p. 95–117.
 173. Knutton, S. Studies of membrane fusion. V. Fusion of erythrocytes with non‐haemolytic Sendai virus. J. Cell Sci. 36: 85–96, 1979.
 174. Knutton, S. Studies of membrane fusion. IV. Fusion of HeLa cells with Sendai virus. J. Cell Sci. 36: 73–84, 1979.
 175. Knutton, S. Studies of membrane fusion. III. Fusion of erythrocytes with polyethylene glycol. J. Cell Sci. 36: 61–72, 1979.
 176. Konigsberg, I. R., N. McElvain, M. Tootle, and H. Hermann. The dissociability of deoxyribonucleic acid synthesis from the development of multinuclearity of muscle cells in culture. J. Biophys. Biochem. Cytol. 8: 333–343, 1960.
 177. Kowalski, M., J. Potz, L. Basiripour, T. Dorfman, W. C. Goh, E. Terwilliger, A. Dayton, C. Rosen, W. Haseltine, and J. Sodroski. Functional regions of the envelope glycoprotein of human immunodeficiency virus type 1. Science 237: 1351–1355, 1987.
 178. Kozlov, M. M., and V. S. Markin. Possible mechanism of membrane fusion. Biophysics 28: 255–261, 1983.
 179. Kuhn, T. S. The Structure of Scientific Revolutions. Chicago: University of Chicago Press, 1970.
 180. Laggner, P. X‐Ray studies on biological membranes using synchrotron radiation. Top. Curr. Chem. 145: 173–202, 1988.
 181. Laggner, P., and M. Kriechbaum. Phospholipid phase transitions: kinetics and structural mechanisms. Chem. Phys. Lipids 57: 121–145, 1991.
 182. Langhans, T. Uber Resenzellen mit wandstandigen Kernen in Tuberkelen und die fibrose Form des Tuberkels. Arch. Pathol. Anat. Physiol. Klin. Med. 42: 382–404, 1868.
 183. Lawson, D., M. C. Raff, B. Gomperts, C. Fewtrell, and N. B. Gilula. Molecular events during membrane fusion: a study of exocytosis in rat peritoneal mast cells. J. Cell Biol. 72: 242–259, 1977.
 184. Layne, S. P., M. J. Merges, M. Dembo, J. L. Spouge, and P. L. Nara. HIV requires multiple gp120 molecules for CD4‐mediated infection. Nature 346: 277–279, 1990.
 185. Levy, J. A., A. D. Hoffman, S. M. Kramer, J. M. Shimabukuro, and L. S. Oshiro. Isolation of lymphocytopathic retroviruses from San Francisco patients with AIDS. Science 225: 840–842, 1984.
 186. Lewis, W. H. The formation of giant cells in tissue culture and their similarity to those in tuberculous lesions. Am. Rev. Tuberc. 15: 616–628, 1927.
 187. Liao, M.‐J., and J. H. Prestegard. Fusion of phosphatidic acid phosphatidylcholine mixed lipid vesicles. Biochim. Biophys. Acta 550: 157–173, 1979.
 188. Lifson, J. D., M. B. Feinberg, G. R. Reyes, L. Rabins, B. Banapour, S. Chakrabarti, B. Moss, F. Wong‐Staal, K. S. Steimer, and E. G. Engleman. Induction of CD4‐dependent cell fusion by the HTLV‐III/LAV envelope glycoprotein. Nature 323: 725–728, 1986.
 189. Lifson, J. D., G. R. Reyes, M. S. McGrath, S. B. Stein, and E. G. Engleman. AIDS retrovirus induced cytopathology: giant cell formation and involvement of CD4 antigen. Science 232: 1123–1127, 1986.
 190. Lindau, M. Time‐resolved capacitance measurements: monitoring exocytosis in single cells. Q. Rev. Biophys. 24: 75–101, 1991.
 191. Lindau, M., and E. Neher. Patch‐clamp techniques for time‐resolved capacitance measurements in single cells. Pflugers Arch. 411: 137–146, 1988.
 192. Lineberger, D. W., D. J. Graham, J. E. Tomassini, and R. J. Colonno. Antibodies that block rhinovirus attachment map to domain 1 of the major group receptor. J. Virol. 64: 2582–2587, 1990.
 193. Lowy, R. J., D. P. Sarkar, and R. Blumenthal. Influenza virus‐cell fusion: Observations of protein and lipid movements using low light‐level fluorescent video microscopy. J. Cell Biochem. 16: 131–131, 1992.
 194. Lowy, R. J., D. P. Sarkar, Y. Chen, and R. Blumenthal. Observation of single influenza virus‐cell fusion and measurement by fluorescence video microscopy. Proc. Natl. Acad. Sci. U.S.A. 87: 1850–1854, 1990.
 195. Loyter, A., V. Citovsky, and R. Blumenthal. The use of fluorescence dequenching methods to follow viral fusion events. Methods Biochem. Anal. 33: 128–164, 1988.
 196. Loyter, A., M. Tomasi, A. G. Gitman, L. Etinger, and O. Nussbaum. The use of specific antibodies to mediate fusion between Sendai virus envelopes and living cells. Ciba Found. Symp. 103: 163–180, 1984.
 197. Loyter, A., and D. J. Volsky. Reconstituted Sendai virus envelopes as carriers for the introduction of biological material into animal cells. In: Cell Surface Reviews, vol 8, Membrane Reconstitution, edited by G. Poste and G. L. Nicolson, Amsterdam: Elsevier/North‐Holland, 1982, p. 215–266.
 198. Lucy, J. A. Mechanism of chemically induced fusion. Cell Surface Rev. 5: 267–304, 1978.
 199. Lucy, J. A. Do hydrophobic sequences cleaved from cellular polypeptides induce membrane fusion reaction in vivo? FEBS Lett. 166: 223–231, 1984.
 200. Maddon, P. J., A. G. Dalgleish, J. S. McDougal, P. R. Clapham, R. A. Weiss, and R. Axel. The T4 gene encodes the AIDS virus receptor and is expressed in the immune system and the brain. Cell 47: 333–348, 1986.
 201. Malhotra, V., L. Orci, B. S. Glick, M. R. Block, and J. E. Rothman. Role of an N‐ethylmaleimide‐sensitive transport component in promoting fusion of transport vesicles with cisternae of the Golgi stack. Cell 54: 221–227, 1988.
 202. Markwell, M. A., A. Portner, and A. L. Schwartz. An alternative route of infection for viruses: entry by means of the asialoglycoprotein receptor of a Sendai virus mutant lacking its attachment protein. Proc. Natl. Acad. Sci. U.S.A. 82: 978–982, 1985.
 203. Marsh, M., and A. Helenius. Virus entry into animal cells. Adv. Virus Res. 36: 107–151, 1989.
 204. Maruyama, Y. Ca2+‐induced excess capacitance fluctuation studies by phase‐sensitive detection method in exocrine pancreatic acinar cells. Pflugers Arch. 407: 561–569, 1986.
 205. Mayorga, L. S., R. Diaz, and P. D. Stahl. Regulatory role for GTP‐binding proteins in endocytosis. Science 244: 1475–1477, 1989.
 206. McClure, M. O., M. Marsh, and R. Weiss. Human immunodeficiency virus infection of CD4‐bearing cells occurs by a pH‐independent mechanism. EMBO J. 7: 513–518, 1988.
 207. McDougal, J. S., M. S. Kennedy, J. N. Sligh, S. P. Cort, A. Mawle, and J.K.A. Nicholson. Binding of HTLV‐III/LAV to T4+ cells by a complex of the 110K viral protein and the T4 molecule. Science 231: 382–385, 1986.
 208. McKeating, J. A., A. McKnight, and J. P. Moore. Differential loss of envelope glycoprotein gp120 from virions of human immunodeficiency virus type 1 isolates: effects on infectivity and neutralization. J. Virol. 65: 852–860, 1991.
 209. Meers, P., K. Hong, and D. Papahadjopoulos. Free fatty acid enhancement of cation‐induced fusion of liposomes: sinergism with synexin and other promoters of vesicle aggregation. Biochemistry 27: 6784–6794, 1988.
 210. Meers, P., K. L. Hong, and D. Papahadjopoulos. Role of specific lipids and annexins in calcium‐dependent membrane fusion. Ann. NY Acad. Sci. 635: 259–272, 1991.
 211. Melancon, P., B. S. Glick, V. Malhotra, P. J. Weidman, T. Serafini, M. L. Gleason, L. Orci, and J. E. Rothman. Involvement of GTP‐binding “G” proteins in transport through the Golgi stack. Cell 51: 1053–1062, 1987.
 212. Miller, N., and L. M. Hutt‐Fletcher. A monoclonal antibody to glycoprotein gp85 inhibits fusion but not attachment of Epstein‐Barr virus. J. Virol. 62: 2366–2372, 1988.
 213. Moore, J. P., B. A. Jameson, R. A. Weiss, and Q. J. Sattentau. The HIV‐cell fusion reaction. In: Viral Fusion Mechanisms, edited by J. Bentz, Boca Raton, FL: CRC Press, p. 233–289, 1993.
 214. Morgan, A., and R. D. Burgoyne. Relationship between arachidonic acid release and Ca2+‐dependent exocytosis in digitonin‐permeabilized bovine adrenal chromaffin cells. Biochem. J. 271: 571–574, 1990.
 215. Morgan, A., and R. D. Burgoyne. Exo1 and Exo2 proteins stimulate calcium‐dependent exocytosis in permeabilized adrenal chromaffin cells. Nature 355: 833–836, 1992.
 216. Morris, S. J., D. Bradley, G. C. Gibson, P. D. Smith, and R. Blumenthal. Use of membrane‐associated fluorescence probes to monitor fusion of vesicles: rapid kinetics of aggregation and fusion using pyrene excimer/monomer fluorescence. In: Spectroscopic Membrane Probes, edited by L. Loew, Boca Raton, FL: CRC Press, 1988, p. 161–191.
 217. Morris, S. J., C. C. Gibson, P. D. Smith, P. C. Greif, C. W. Stirk, D. Bradley, D. M. Haynes, and R. Blumenthal. Rapid kinetics of CA2+‐induced fusion of phosphatidylserine‐phosphatidylethanol vesicles. J. Biol. Chem. 260: 4122–4127, 1985.
 218. Morris, S. J., D. P. Sarkar, J. M. White, and R. Blumenthal. Kinetics of pH‐dependent fusion between 3T3 fibroblasts expressing influenza hemagglutinin and red blood cells. J. Biol. Chem. 264: 3972–3978, 1989.
 219. Morrison, T., C. McQuain, and L. McGinnes. Complementation between avirulent Newcastle disease virus and a fusion protein gene expressed from a retrovirus vector: requirements for membrane fusion. J. Virol. 65: 813–822, 1991.
 220. Moscona, A., and R. W. Peluso. Fusion properties of cells infected with human parainfluenza virus type 3: receptor requirements for viral spread and virus‐mediated membrane fusion. J. Virol. 66: 6280–6287, 1992.
 221. Munson, J. B., and G. W. Sypert. Properties of single fibre excitatory post‐synaptic potentials in triceps surae motoneourons. J. Physiol. 296: 329–342, 1979.
 222. Murata, M., S. Takahashi, Y. Shirai, S. Kagiwada, R. Hishida, and S. Ohnishi. Specificity of amphiphilic anionic peptides for fusion of phospholipid vesicles. Biophys. J. 64: 724–34, 1993.
 223. Neher, E., and A. Marty. Discrete changes of cell membrane capacitance observed under conditions of enhanced secretion in bovine adrenal chromaffin cells. Proc. Natl. Acad. Sci. U.S.A. 79: 6712–6716, 1982.
 224. Nemerow, G. R., and N. R. Cooper. CR2 (CD21) mediated infection of B lymphocytes by Epstein‐Barr virus. Semin. Virol. 3: 117–124, 1992.
 225. Neumann, E., and K. Rosenheck. Permeability changes induced by electric pulses in vesicular membranes. J. Membrane Biol. 10: 279–290, 1972.
 226. Neumann, E., G. Gerisch, and K. Opatz. Cell fusion induced by high electric impulses applied to Dictyostelium. Naturwissenschaften 67: 414–415, 1980.
 227. Neumann, E., M. Schaefer‐Ridder, Y. Wang and P. H. Hofschneider. Gene transfer into mouse lyoma cells by electroporation in high electric fields. EMBO J. 1: 841–845, 1982.
 228. Neumann, E., A. E. Sowers, and C. A. Jordan. Electroporation and Electrofusion in Cell Biology. New York: Plenum Press, 1989.
 229. Nusse, O., and M. Lindau. The dynamics of exocytosis in human neutrophils. J. Cell Biol. 107: 2117–2126, 1989.
 230. Oberhauser, A. F., J. R. Monck, and J. M. Fernandez. Events leading to the opening and closing of the exocytotic fusion pore have markedly different temperature dependencies. Kinetic analysis of single fusion events in patch‐clamped mouse mast cells. Biophys. J. 61: 800–809, 1992.
 231. Ohki, S. Effects of divalent cations, temperature, osmotic pressure gradient, and vesicle curvature on phosphatidylserine vesicle fusion. J. Membrane Biol. 77: 265–275, 1984.
 232. Ohki, S. Cell and Model Membrane Interactions. New York: Plenum Press, 1991.
 233. Ohki, S., D. Doyle, T. D. Flanagan, S. W. Hui, and E. Mayhew. Molecular Mechanisms of Membrane Fusion. New York: Plenum Press, 1988.
 234. Ohnishi, S. Fusion of viral envelopes with cellular membranes. Curr. Top. Membr. Transp. 32: 257–296, 1988.
 235. Okada, Y. The fusion of Ehrlich's tumor cells caused by HVJ virus in vitro. Biken's J. 1: 103–110, 1958.
 236. Okada, Y. Analysis of giant polynuclear cell formation caused by HVJ virus from Ehrlich's ascites tumor cells I. Microscopic observation of giant polynuclear cell formation. Exp. Cell Res. 26: 98–107, 1962.
 237. Okada, Y. Sendai virus‐mediated cell fusion. Curr. Top. Membr. Transp. 32: 297–336, 1988.
 238. Orci, L., and A. Perrelet. Ultrastructural aspects of exocytotic membrane fusion. Cell Surface Rev. 5: 629–656, 1978.
 239. Orloff, G. M., S. L. Orloff, M. S. Kennedy, P. J. Maddon, and J. S. McDougal. Penetration of CD4 T cells by HIV‐1. The CD4 receptor does not internalize with HIV, and CD4‐related signal transduction events are not required for entry. J. Immunol. 146: 2578–2587, 1991.
 240. Ornberg, R. L., and T. S. Reese. Beginning of exocytosis captured by rapid‐freezing of Limulus amebocytes. J. Cell Biol. 90: 40–54, 1981.
 241. Pak, C. C., M. Krumbiegel, and R. Blumenthal. Intermediates in influenza virus PR/8 haemagglutinin‐induced membrane fusion. J. Gen. Virol. 75: 395–399, 1994.
 242. Palade, G. Intracellular aspects of the process of protein synthesis. Science 189: 347–358, 1975.
 243. Palade, G. E., and R. R. Bruns. Structural modulations of plasmalemmal vesicles. J. Cell Biol. 37: 633–649, 1968.
 244. Panaiotov, I., D. S. Dimitrov, and M. Ivanova. Influence of bulk‐to‐surface diffusion interchange on dynamic surface tension excess at uniformly compressed interface. J. Colloid Interface Sci. 69: 318–325, 1979.
 245. Panaiotov, I., D. S. Dimitrov, and L. Ter‐Minassian‐Saraga. Dynamics of insoluble monolayers. II. Viscoelastic behavior and Marangoni effect for mixed protein phospholipid films. J. Colloid Interface Sci. 72: 49–53, 1979.
 246. Pang, D. T., J.K.T. Wang, F. Valtorta, F. Benfenati, and P. Greengard. Protein tyrosine phosphorylation in synaptic vesicles. Proc. Natl. Acad. Sci. U.S.A. 85: 762–766, 1988.
 247. Papahadjopoulos, D. Calcium‐induced phase changes and fusion in natural and model membranes. Cell Surface Rev. 5: 765–790, 1978.
 248. Papahadjopoulos, D., S. Nir, and N. Duzgunes. Molecular mechanisms of calcium‐induced membrane fusion. J. Bioenerg. Biomembranes 22: 157–179, 1990.
 249. Papahadjopoulos, D., G. Poste, B. E. Schaeffer, and W. J. Vail. Membrane fusion and molecular segregation in phospholipid vesicles. Biochim. Biophys. Acta 352: 10–28, 1974.
 250. Papahadjopoulos, D., W. J. Vail, K. Jacobson, and G. Poste. Cochleate lipid cylinders: formation by fusion of unilamellar lipid vesicles. Biochim. Biophys. Acta 394: 483–491, 1975.
 251. Papahadjopoulos, D., W. J. Vail, C. Newton, S. Nir, K. Jacobson, G. Poste, and R. Lazo. Studies on membrane fusion. III. The role of calcium‐induced phase changes. Biochim. Biophys. Acta 465: 579–598, 1977.
 252. Papahadjopoulos, D., W. J. Vail, W. A. Pangborn, and G. Poste. Studies on membrane fusion. II. Induction of fusion in pure phospholipid membranes by calcium ions and other divalent metals. Biochim. Biophys. Acta 448: 245–264, 1976.
 253. Parsegian, V. A., and R. P. Rand. Membrane interaction and deformation. Surface forces in biological systems. Ann. NY Acad. Sci. 416: 1–12, 1983.
 254. Parsegian, V. A., and R. P. Rand. Forces governing lipid interaction and rearrangement. In: Membrane Fusion, edited by J. Wilschut and D. Hoekstra, New York: Marcel Dekker, 1991, p. 65–85.
 255. Paterson, R. G., S. W. Hiebert, and R. A. Lamb. Expression at the cell surface of biologically active fusion and hemagglutinin/neuraminidase proteins of the paramyxovirus simian virus 5 from cloned cDNA. Proc. Natl. Acad. Sci. U.S.A. 82: 7520–7524, 1985.
 256. Pauza, C. D., and T. M. Price. Human immunodeficiency virus infection of T cells and monocytes proceeds via receptor‐mediated endocytosis. J. Cell Biol. 107: 959–968, 1988.
 257. Pinto Da Silva, P., and M. L. Nogueira. Membrane fusion during secretion. A hypothesis based on electron microscopy observation of Phytophthora palmivora zoospores during encystment. J. Cell Biol. 73: 161–181, 1976.
 258. Plattner, H. Regulation of membrane fusion during exocytosis. Int. Review of Cytology 119: 197–286, 1989.
 259. Pollard, H. B., A. L. Burns, and E. Rojas. A molecular basis for synexin driven calcium‐dependent membrane fusion. J. Exp. Biol. 139: 267–286, 1988.
 260. Pollard, H. B., E. Rojas, and A. L. Burns. Synexin and chromaffin granule membrane fusion. A new “hydrophobic bridge” hypothesis for driving and directing of the fusion process. Ann. NY Acad. Sci. 493: 524–541, 1987.
 261. Pollard, H. B., E. Rojas, R. W. Pastor, E. M. Rojas, H. R. Guy, and A. L. Burns. Synexin: molecular mechanism of calcium‐dependent membrane fusion and voltage‐dependent calcium‐channel activity. Evidence in support of the “hydrophobic bridge hypothesis” for exocytotic membrane fusion. Ann. NY Acad. Sci. 635: 328–351, 1991.
 262. Pollard, H. B., O. Zinder, O. G. Hoffman, and O. Nikodejevic. Regulation of the transmembrane potential of isolated chromaffin granules by ATP, ATP analogs and external pH. J. Biol. Chem. 251: 4544–4550, 1976.
 263. Pontecorvo, G. Production of indefinitely multiplying mammalian somatic cell hybrids by polyethylene glycol (PEG) treatment. Somatic. Cell Genet. 1: 397–400, 1975.
 264. Poole, A. R., J. I. Howell, and J. A. Lucy. Lysolecithin in cell fusion. Nature 227: 810–814, 1970.
 265. Popovic, M., M. G. Sarngadharan, E. Read, and R. C. Gallo. Detection, isolation and continuous production of cytopathic retroviruses (HTLV‐III) from patients with AIDS and pre‐AIDS. Science 224: 497–500, 1984.
 266. Poste, G., and L. Nicolson. Membrane Fusion. Amsterdam: Elsevier North‐Holland Biomedical Press, 1978.
 267. Poulin, L., L. A. Evans, S. B. Tang, A. Barboza, H. Legg, D. R. Littman, and J. A. Levy. Several CD4 domains can play a role in human immunodeficiency virus infection in cells. J. Virol. 65: 4893–4901, 1991.
 268. Primakoff, P., H. Hyatt, and J. Tredick‐Kline. Identification of a sperm surface protein with a potential role in sperm‐egg membrane fusion. J. Cell Biol. 104: 141–149, 1987.
 269. Puri, A., F. Booy, R. W. Doms, J. M. White, and R. Blumenthal. Conformational changes and fusion activity of influenza hemagglutinin of the H2 and H3 subtypes: effects of acid pretreatment. J. Virol. 64: 3824–3832, 1990.
 270. Puri, A., M. J. Clague, C. Schoch and R. Blumenthal. Kinetics of fusion of enveloped viruses with cells. Methods. Enzymol. 220: 277–287, 1993.
 271. Puri, A., S. J. Morris, P. Jones, M. Ryan, and R. Blumenthal. Heat‐resistant factors in human erythrocyte membranes mediate CD4‐dependent fusion with cells expressing HIV‐1 envelope glycoprotein. Virology 219: 262–267, 1996.
 272. Qureshi, N. M., D. H. Coy, R. F. Garry, and L. A. Henderson. Characterization of a putative cellular receptor for HIV‐1 transmembrane glycoprotein using synthetic peptides. AIDS 4: 553–558, 1990.
 273. Radoev, B. P., D. S. Dimitrov, and I. B. Ivanov. Hydrodynamics of thin liquid films. Effects of surfactants on the rate of thinning. Colloid Polymer Sci. 252: 50–55, 1974.
 274. Rafalski, M., A. Ortiz, A. Rockwell, L. C. Van Ginkel, J. D. Lear, W. F. Degrado, and J. Wilschut. Membrane fusion activity of the influenza virus hemagglutinin: interaction of HA2 N‐terminal peptides with phospholipid vesicles. Biochemistry. 30: 10211–10220, 1991.
 275. Rand, R. P., and V. A. Parsegian. Hydration forces between phospholipid bilayers. Biochem. Biophys. Acta 988: 351–376, 1989.
 276. Rey, F. A., F. X. Heinz, C. Mandl, C. Kunz, and S. C. Harrison. The envelope glycoprotein from tick‐borne encephalitis virus at 2 Å resolution. Nature 375: 291–298, 1995.
 277. Ringertz, N. R., and R. E. Savage. Cell Hybrids. New York: Academic Press, 1976.
 278. Roizman, B. Polykaryocytosis, Cold Spring Harbor Symp. Quant. Biol. 27: 327–342, 1962.
 279. Roos, D. S., and P. W. Choppin. Biochemical studies on cell fusion. J. Cell Biol. 101: 1578–1598, 1985.
 280. Roos, D. S., R. Davidson, and P. Choppin. Control of membrane fusion in polyethylene glycol‐resistant cell mutants: application to fusion technology. In: Cell Fusion, edited by A. E. Sowers, New York: Plenum Press, 1987, p. 123–144.
 281. Rothman, J. E., and L. Orci. Molecular dissection of the secretory pathway. Nature 355: 409–415, 1992.
 282. Rubin, R. J., and Y. Chen. Diffusion and redistribution of lipid‐like molecules between membranes in virus‐cell and cell‐cell fusion systems. Biophys. J. 58: 1157–1167, 1990.
 283. Ruigrok, R. W., S. R. Martin, S. A. Wharton, J. J. Skehel, and P. M. Bayley. Conformational changes in the hemagglutinin of influenza virus which accompany heat‐induced fusion of virus with liposomes. Virology 155: 484–497, 1986.
 284. Ryu, S.‐E., P. D. Kwong, A. Truneh, T. G. Porter, J. Arthos, M. Rosenberg, X. Dai, N.‐H. Xuong, R. Axel, R. W. Sweet, and W. A. Hendrickson. Crystal structure of an HIV‐binding recombinant fragment of human CD4. Nature 348: 419–426, 1990.
 285. Sackmann, E. Molecular and global structure and dynamics of membranes and lipid bilayers. Can. J. Phys. 68: 999–1012, 1990.
 286. Sakai, Y., and H. Shibuta. Syncytium formation by recombinant vaccinia viruses carrying bovine parainfluenza 3 virus envelope protein genes. J. Virol. 63: 3661–3668, 1989.
 287. Sale, A.J.H., and W. A. Hamilton. Effects of high electric fields on microorganisms. I. Killing of bacteria and yeasts. Biochim. Biophys. Acta 148: 781–788, 1967.
 288. Sale, A.J.H., and W. A. Hamilton. Effects of high electric fields on microorganisms. III. Lysis of erythrocytes and protoplasts. Biochim. Biophys. Acta 163: 37–43, 1968.
 289. Saling, P. M., G. Irons, and R. Waibel. Mouse sperm antigens that participate in fertilization. I. Inhibition of sperm fusion with the egg plasma membrane using monoclonal antibodies. Biol. Reprod. 33: 515–526, 1985.
 290. Sarkar, D. P., S. J. Morris, O. Eidelman, J. Zimmerberg, and R. Blumenthal. Initial stages of influenza hemagglutinin‐induced cell fusion monitored simultaneously by two fluorescent events: cytoplasmic continuity and lipid mixing. J. Cell Biol. 109: 113–122, 1989.
 291. Satir, B. Ultrastructural aspects of membrane fusion. J. Supramol. Structr. 2: 529–537, 1974.
 292. Sato, H., J. Orstein, D. Dimitrov, and M. Martin. Cell‐to‐cell spread of HIV‐1 occurs within minutes and may not involve the participation of virus particles. Virology 186: 712–724, 1992.
 293. Sattentau, Q. J. CD4 activation of HIV fusion. Int. J. Cell Cloning 10: 323–332, 1992.
 294. Sattentau, Q. J., and J. P. Moore. Conformational changes induced in the human immunodeficiency virus envelope glycoproteins by soluble CD4 binding. J. Exp. Med. 174: 407–415, 1991.
 295. Sattentau, Q. J., and R. A. Weiss. The CD4 antigen: physiological ligand and HIV receptor. Cell 60: 697–700, 1988.
 296. Sauter, N. K. Hemagglutinins from two influenza virus variants bind to sialic acid derivatives with millimolar dissociation constants: a 500‐MHz proton nuclear magnetic resonance study. Biochemistry. 28: 8388–8396, 1989.
 297. Scheid, A., and P. W. Choppin. Two disulfide‐linked polypeptide chains constitute the active F protein of paramyxoviruses. Virology, 80: 54–66, 1977.
 298. Scheludko, A. Colloid Chemistry. Amsterdam: North Holland, 1967.
 299. Schierenberg, E. Altered cell‐division rates after laser‐induced cell fusion in nematode embryos. Dev. Biol. 101: 240–245, 1984.
 300. Schoch, C., R. Blumenthal, and M. J. Clague. Identification and characterization of a long lived state for influenza virus‐erythrocyte complexes committed to fusion a neutral pH. FEBS Lett. 311: 221–225, 1992.
 301. Scholtissek, C. Stability of infectious influenza A viruses at low pH and at elevated temperature. Vaccine 3: 215–218, 1985.
 302. Sechoy, O., M. Vidal, J. R. Philippot, and A. Bienvenue. Interactions of human lymphoblasts with targeted vesicles containing Sendai virus envelope proteins. Exp. Cell Res. 185: 122–131, 1989.
 303. Segawa, A., S. Terakawa, S. Yamashina, and C. R. Hopkins. Exocytosis in living salivary glands: direct visualization by video‐enhanced microscopy and confocal laser microscopy. Eur. J. Cell Biol. 54: 322–330, 1991.
 304. Senda, M., J. Takeda, S. Abe, and T. Nakamura. Induction of cell fusion of plant protoplasts by electrical stimulation. Plant Cell Physiol. 20: 1441–1443, 1979.
 305. Siegel, D. P. Inverted micellar structures in bilayer membranes: formation rates and half‐lives. Biophys. J. 45: 399–420, 1984.
 306. Siegel, D. P., Modeling protein‐induced fusion mechanisms: insights from the relative stability of lipidic structures. In: Viral Fusion Mechanisms, edited by J. Bentz, Boca Raton, FL: CRC Press, 1993, p. 475–512.
 307. Siegel, D. P., J. Banschbach, and P. L. Yeagle. Stabilization of H2 phases by low levels of diglycerides and alkanes: an NMR, calorimetric, and X‐ray diffraction study. Biochemistry 28: 5010–5019, 1989.
 308. Sinangil, F., A. Loyter, and D. J. Volsky. Quantitative measurement of fusion between human immunodeficiency virus and cultured cells using membrane fluorescence dequenching. FEBS Lett. 239: 88–92, 1988.
 309. Skehel, J. J., P. M. Bayley, E. B. Brown, S. R. Martin, M. D. Waterfield, J. M. White, I. A. Wilson, and D. C. Wiley. Changes in the conformation of influenza virus hemagglutinin at the pH optimum of virus‐mediated membrane fusion. Proc. Natl. Acad. Sci. U.S.A. 79: 968–972, 1982.
 310. Sodroski, J., W. C. Goh, C. Rosen, K. Campbell, and W. A. Haseltine. Role of the HTLV‐III/LAV envelope in syncytium formation and cytopathicity. Nature 322: 470–474, 1986.
 311. Sorieul, S., and B. Ephrussi. Karyological demonstration of hybridization of mammalian cells in vitro. Nature 190: 653–654, 1961.
 312. Sowers, A. E. Characterization of electric field‐induced fusion in erythrocyte ghost membranes. J. Cell Biol. 99: 1989–1996, 1984.
 313. Sowers, A. E. A long‐lived fusogenic state is induced in erythrocyte ghosts by electric pulses. J. Cell Biol. 102: 1358–1362, 1986.
 314. Sowers, A. E. Cell Fusion. New York: Plenum Press, 1987.
 315. Spear, P. G., Membrane fusion induced by herpes simplex virus. In: Viral Fusion Mechanisms, edited by J. Bentz, Boca Raton, FL: CRC Press, 1993, p. 201–232.
 316. Spruce, A. E., L. J. Breckenridge, A. K. Lee and W. Almers. Properties of the fusion pore that forms during exocytosis of a mast cell secretory vesicle. Neuron 4: 643–654, 1990.
 317. Spruce, A. E., A. Iwata, and W. Almers. The first milliseconds of the pore formed by a fusogenic viral envelope protein during membrane fusion. Proc. Natl. Acad. Sci. U. S. A. 88: 3623–3627, 1991.
 318. Spruce, A. E., A. Iwata, J. M. White, and W. Almers. Patch clamp studies of single cell‐fusion events mediated by a viral fusion protein. Nature 342: 555–558, 1989.
 319. Steffy, K. R., G. Kraus, D. J. Looney, and F. Wong‐Staal. Role of the fusogenic peptide sequence in syncytium induction and infectivity of human immunodeficiency virus type 2. J. Virol. 66: 4532–4535, 1992.
 320. Stegmann, T., J. M. Delfino, F. M. Richards, and A. Helenius. The HA2 subunit of influenza hemagglutinin inserts into the target membrane prior to fusion. J. Biol. Chem. 266: 18404–18410, 1991.
 321. Stegmann, T., J. M. White, and A. Helenius. Intermediates in influenza‐induced membrane fusion. EMBO J. 9: 4231–4241, 1990.
 322. Stein, B. S., S. D. Gowda, J. D. Lifson, R. C. Penhallow, K. G. Bensch, and E. G. Engleman. pH‐independent HIV entry into CD4‐positive T cells via virus envelope fusion to the plasma membrane. Cell 49: 659–668, 1987.
 323. Steinhauer, D. A., N. K. Sauter, J. J. Skehel, and D. C. Wiley. Receptor binding and cell entry by influenza viruses. Semin. Virol. 3: 91–100, 1992.
 324. Stenger, D. A., and S. W. Hui. Kinetics of ultrastructural changes during electrically‐induced fusion of human erythrocytes. J. Membrane Biol. 93: 43–53, 1986.
 325. Stenger, D. A., and S. W. Hui. Human erythrocyte electrofusion kinetics monitored by aqueous contents mixing. Biophys. J. 53: 833–838, 1988.
 326. Stenger, D. A., and S. W. Hui. Electrofusion kinetics: studies using electron microscopy and fluorescence contents mixing. In: Electroporation and Electrofusion in Cell Biology, edited by E. Neumann, A. E. Sowers, and C. A. Jordan, New York: Plenum Press, 1989, p. 167–180.
 327. Stow, J. L., J. B. De Almeida, N. Narula, E. J. Holtzman, L. Ercolani, and D. A. Ausiello. A heterotrimeric G protein, G alpha i‐3, on Golgi membranes regulates the secretion of a heparan sulfate proteoglycan in LLC‐PK1 epithelial cells. J. Cell Biol. 114: 1113–1124, 1991.
 328. Sugar, I. P., W. Forster, and E. Neumann. Model of cell electrofusion: membrane electroporation, pore coalescence and percolation. Biophys. Chem. 26: 321–335, 1987.
 329. Tate, M. W., E. Shyamsunder, and S. M. Gruner. Kinetics of the lamellar‐inverse hexagonal phase transition determined by time‐resolved X‐ray diffraction. Biochemistry 31: 1081–1092, 1992.
 330. Teissie, I., and M. P. Rols. Fusion of mammalian cells in culture is obtained by creating the contact between cells after their electropermeabilization. Biochem. Biophys. Res. Commun. 140: 258–266, 1986.
 331. Thali, M., C. Furman, E. Helseth, H. Repke, and J. Sodroski. Lack of correlation between soluble CD4‐induced shedding of the human immunodeficiency virus type 1 exterior envelope glycoprotein and subsequent membrane fusion events. J. Virol. 66: 5516–5524, 1992.
 332. Thomas, L., K. Harting, D. Langosch, H. Rehm, E. Bamberg, W. W. Franke, and H. Betz. Identification of synaptophysin—a hexameric channel protein of the synaptic vesicle membrane. Science 2073: 1050–1053, 1988.
 333. Tilcock, C.P.S., and D. Fisher. The interaction of phospholipid membranes with polyethylene glycol vesicle aggregation and lipid exchange. Biochim. Biophys. Acta 688: 645–652, 1982.
 334. Tse, F. W., A. Iwata, and W. Almers. Membrane flux through the pore formed by a fusogenic viral envelope protein during cell fusion. J. Cell. Biol. 121: 543–552, 1993.
 335. Vacquier, V. D., and G. W. Moy. Isolation of bindin: the protein responsible for adhesion of sperm to sea urchin eggs. Proc. Natl. Acad. Sci. U. S. A. 74: 2456–2460, 1977.
 336. Van Meer, G., J. Davoust, and K. Simons. Parameters affecting low‐pH‐mediated fusion of liposomes with the plasma membrane of cells infected with influenza virus. Biochemistry. 24: 3593–3602, 1985.
 337. Verkleij, A. J., and P.H.J. Th. Ververgaert. Freeze‐fracture morphology of biological membrane. Biochim. Biophys. Acta 515: 303–327, 1978.
 338. Verwey, E. J. W., and T. Th. G. Overbeek. Theory of Stability of Lyophobic Colloids. Amsterdam: Elsevier, 1948.
 339. Walter, A., D. Margolis, R. Mohan, and R. Blumenthal. Apocytochrome c induces pH‐dependent vesicle fusion. Membr. Biochem. 6: 217–237, 1986.
 340. Walter, A., C. J. Steer, and R. Blumenthal. Polylysine induces pH‐dependent fusion of acidic phospholipid vesicles: a model for polycation‐induced fusion. Biochim. Biophys. Acta 861: 319–330, 1986.
 341. Wang, H., M. P. Kavanaugh, R. A. North, and D. Kabat. Cell‐surface receptor for ecotropic murine retroviruses is a basic amino‐acid transporter [see comments]. Nature 352: 729–731, 1991.
 342. Wang, J., Y. Yan, T.P.J. Garrett, J. Liu, D. W. Rodgers, R. L. Garlick, G. E. Tarr, Y. Husain, E. L. Reinherz, and S. C. Harrison. Atomic structure of a fragment of human CD4 containing two immunoglobulin‐like domains. Nature 348: 411–418, 1990.
 343. Wang, K. S., R. J. Kuhn, E. G. Strauss, S. Ou, and J. H. Strauss. High‐affinity laminin receptor is a receptor for Sindbis virus in mammalian cells. J. Virol. 66: 4992–5001, 1992.
 344. Weaver, J. C., and K. T. Powell. Theory of electroporation. In: Electroporation and Electrofusion in Cell Biology, edited by E. Neumann, A. E. Sowers, and C. A. Jordan, New York: Plenum Press, 1989, p. 111–126.
 345. Weber, H., W. Forster, H.‐E. Jacob, and H. Berg. Microbiological implications of electric field effects. Z. Allg. Microbiol. 21: 555–562, 1981.
 346. Weidman, P. J., P. Melancon, M. R. Block, and J. E. Rothman. Binding of an N‐ethylmaleimide‐sensitive fusion protein to Golgi membranes requires both a soluble protein(s) and an integral membrane receptor. J. Cell Biol. 108: 1589–1596, 1989.
 347. Weis, W., J. H. Brown, S. Cusack, J. C. Paulson, J. J. Skehel, and D. C. Wiley. Structure of the influenza virus haemagglutinin complexed with its receptor, sialic acid. Nature 333: 426–431, 1988.
 348. Weiss, C. D., J. A. Levy, and J. M. White. Oligomeric organization of gp120 on infectious human immunodeficiency virus type 1 particles. J. Virol. 64: 5674–5677, 1990.
 349. Weiss, R. A. Human immunodeficiency virus receptors. Semin. Virol. 3: 79–84, 1992.
 350. Wharton, S. A., S. R. Martin, R. W. Ruigrok, J. J. Skehel, and D. C. Wiley. Membrane fusion by peptide analogues of influenza virus haemagglutinin. J. Gen. Virol. 69: 1847–1857, 1988.
 351. White, J. M. Viral and cellular fusion proteins. Annu. Rev. Physiol. 52: 675–697, 1990.
 352. White, J. M., and I. A. Wilson. Anti‐peptide antibodies detect steps in a protein conformational change: low‐pH activation of the influenza virus hemagglutinin. J. Cell Biol. 56: 365–394, 1987.
 353. White, J., M. Kielian, and A. Helenius. Membrane fusion proteins of enveloped animal viruses. Q. Rev. Biophys. 16: 151–195, 1983.
 354. Whitt, M. A., P. Zagouras, B. Crise, and J. K. Rose. A fusion‐defective mutant of the vesicular stomatitis virus glycoprotein. J. Virol. 64: 4907–4913, 1990.
 355. Wild, T. F., E. Malvoisin, and R. Buckland. Measles virus: both the haemagglutinin and fusion glycoproteins are required for fusion. J. Gen. Virol. 72: 439–442, 1991.
 356. Wiley, D. C., and J. J. Skehel. The structure and function of the hemagglutinin membrane glycoprotein of influenza virus. Annu. Rev. Biochem. 56: 365–394, 1987.
 357. Willey, R., D. H. Smith, L. A. Lasky, T. S. Theodore, P. L. Earl, B. Moss, D. J. Capon, and M. A. Martin. In vitro mutagenesis identifies a region within the envelope gene of the human immunodeficiency virus that is critical for infectivity. J. Virol. 62: 139–147, 1988.
 358. Wilschut, J., Membrane fusion in lipid vesicle systems: an overview. In: Membrane Fusion, edited by J. Wilschut and D. Hoekstra, New York: Marcel Dekker, 1991, p. 89–126.
 359. Wilschut, J., N. Duzgunes, R. Fraley, and D. Papahadjopoulos. Studies on the mechanism of membrane fusion: kinetics of calcium ion induced fusion of phosphatidylserine vesicles followed by a new assay for mixing of aqueous vesicle contents. Biochemistry 19: 6011–6021, 1980.
 360. Wilschut, J., N. Duzgunes, and D. Papahadjopoulos. Calcium/magnesium specificity in membrane fusion: kinetics of aggregation and fusion of phosphatidylserine vesicles and the role of bilayer curvature. Biochemistry 20: 3126–3133, 1981.
 361. Wilschut, J., and D. Hoekstra. Membrane Fusion. New York: Marcel Dekker, 1991.
 362. Wilson, D. W., C. A. Wilcox, G. C. Flynn, E. Chen, W.‐J. Kuang, et al. A fusion protein required for vesicle‐mediated transport in both mammalian cells and yeast. Nature 339: 355–359, 1989.
 363. Wilson, I. A., J. J. Skehel, and D. C. Wiley. Structure of the hemagglutinin membrane glycoprotein of influenza virus at 3 A resolution. Nature 289: 366–373, 1981.
 364. Zaks, W. J., and C. E. Creutz. Evaluation of the annexins as potential mediators of membrane fusion in exocytosis. J. Bioenerg. Biomembr. 22: 97–120, 1990.
 365. Zhelev, D. V., D. S. Dimitrov, and P. Doinov. Correlation between physical parameters in electrofusion and electroporation of protoplasts. Bioelectrochem. Bioenerg. 20: 155–167, 1988.
 366. Zieseniss, E., and H. Plattner. Synchronous exocytosis in Paramecium cells involves very rapid (<1 s), reversible dephosphorylation of a 65 kD‐phosphoprotein in exocytosis‐competent strains. J. Cell Biol. 101: 2028–2035, 1985.
 367. Zimmerberg, J., M. Curran, and F. S. Cohen. A lipid/protein hypothesis for exocytotic fusion pore formation. Ann. NY Acad. Sci. 635: 307–317, 1991.
 368. Zimmerberg, J., M. Curran, F. S. Cohen, and M. Brodwick. Simultaneous electrical and optical measurements show that membrane fusion precedes secretory granule swelling during exocytosis of beige mouse mast cells. Proc. Natl. Acad. Sci. USA 84: 1585–1589, 1987.
 369. Zimmerberg, J., R. Blumenthal, D. P. Sarkar, M. Curran, and S. J. Morris. Restricted movement of lipid and aqueous dyes through pores formed by influenza hemagglutinin during cell fusion. J. Cell. Biol. 127: 1885–1894, 1994.
 370. Zimmermann, U., and P. Scheurich. High frequency fusion of plant protoplasts by electric fields. Planta 151: 26–32, 1981.
 371. Zimmermann, U., W. M. Arnold, and W. Mehrle. Biophysics of electroinjection and electrofusion. J. Electrostatics 21: 309–345, 1988.
 372. Zimmermann, U., G. Pilwat, and F. Rienmann. Reversibler dielektrischer Durchbruch von Zellmembranen in electrostatischen Feldern. Z. Naturforsch. 29c: 304–305, 1974.
 373. Zimmermann, U., J. Schulz, and G. Pilwat. Transcellular ion flow in Escherichia coli B and electrical sizing of bacterias. Biophys. J. 13: 1005–1013, 1973.

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Article Title: Membrane Fusion
 

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R. Blumenthal, D. S. Dimitrov. Membrane Fusion. Compr Physiol 2011, Supplement 31: Handbook of Physiology, Cell Physiology: 563-603. First published in print 1997. doi: 10.1002/cphy.cp140114