Interaction of enveloped viruses with planar bilayer membranes: observations on Sendai, influenza, vesicular stomatitis, and Semliki Forest viruses

Lipid-dependence of target membrane stability during influenza viral fusion

Although influenza kills about a half million people each year, even after excluding pandemics, there is only one set of antiviral drugs: neuraminidase inhibitors. By using a new approach utilizing giant unilamellar vesicles and infectious X-31 influenza virus, and testing for the newly identified pore intermediate of membrane fusion, we observed ∼30-87% poration, depending upon lipid composition. Testing the hypothesis that spontaneous curvature (SC) of the lipid monolayer controls membrane poration, our Poisson model and Boltzmann energetic considerations suggest a transition from a leaky to a non-leaky fusion pathway depending on the SC of the target membrane. When the target membrane SC is below approximately -0.20 nm^-1 fusion between influenza virus and target membrane is predominantly non-leaky while above that fusion is predominantly leaky, suggesting that influenza hemagglutinin (HA)-catalyzed topological conversion of target membranes during fusion is associated with a loss of membrane integrity.

Herpes Simplex Virus 1 Envelope Cholesterol Facilitates Membrane Fusion

Methyl beta-cyclodextrin (MβCD) treatment of herpes simplex virus 1 (HSV-1) reduced envelope cholesterol levels and inhibited viral entry and infectivity in several cell types, regardless of the dependence of entry on endocytosis or low pH. Viral protein composition was similar in MβCD-treated and untreated virions, and ultrastructural analysis by electron microscopy revealed that cholesterol removal did not grossly affect virion structure or integrity. Removal of envelope cholesterol greatly reduced virion fusion activity as measured by fusion-from-without, suggesting that virion cholesterol is critical for the step of membrane fusion. MβCD-treatment of HSV-1 did not reduce viral attachment to the cells nor endocytic uptake of HSV-1 from the cell surface. The pre-fusion form of gB present in the HSV-1 envelope undergoes conformational changes in response to mildly acidic pH. These gB changes occurred independently of envelope cholesterol. Removal of cholesterol compromised virion stability as measured by recovery of infectivity following cycles of freeze-thaw. Taken together, the data suggest that HSV-1 envelope cholesterol is important for viral entry and infectivity due to a critical role in membrane fusion.

Membrane uncoating of intact enveloped viruses

Experiments in the 1960s showed that Sendai virus, a paramyxovirus, fused its membrane with the host plasma membrane. After membrane fusion, the virus spontaneously "uncoated" with diffusion of the viral membrane proteins into the host plasma membrane and a merging of the host and viral membranes. This led to deposit of the viral ribonucleoprotein (RNP) and interior proteins in the cell cytoplasm. Later work showed that the common procedure then used to grow Sendai virus produced damaged, pleomorphic virions. Virions, which were grown under conditions that were not damaging, made a connecting structure between virus and cell at the region where the fusion occurred. The virus did not release its membrane proteins into the host membrane. The viral RNP was seen in the connecting structure in some cases. Uncoating of intact Sendai virus proceeds differently from uncoating described by the current standard model developed long ago with damaged virus. A model of intact paramyxovirus uncoating is presented and compared to what is known about the uncoating of other viruses.

Probing molecular level interaction of oseltamivir with H5N1-NA and model membranes by molecular docking, multinuclear NMR and DSC methods

Structure-based drug design has led to the introduction of three drugs--oseltamivir (GS-4104), zanamivir (GG-167) and peramivir (RWJ-270201) which target the enzyme neuraminidase, for treatment of influenza infections. Using comparative docking studies we propose that more potent molecules against neuraminidase can be obtained by appending extra positively charged substituents at the C5 position of the oseltamivir skeleton. This provides an additional interaction with the enzyme and may overcome the problem of resistance encountered with these drugs. To get an insight into the transport and absorption of oseltamivir--the ethyl ester prodrug (GS-4104) as well as its mechanism of action, we have carried out 1H, 13C, 31P NMR, DSC and TEM studies on GS-4104 with model membranes prepared from DMPC/DPPC/POPC. These studies reveal that interactions between GS-4104 and the membrane are both electrostatic (involving H-bonding) and hydrophobic (involving the hydrophobic chain and cyclohexene ring of GS-4104) in nature. The prodrug is seen to increase the fluidity as well as stabilize the bilayer phase of the membrane. This property may be responsible for preventing viral entry into the cells by preventing fusion of the virus outer coat with the cell membrane.

Fusion of alphaviruses with liposomes is a non-leaky process

It has been reported that low-pH-induced fusion of influenza virus with liposomes results in rapid and extensive release of both low- and high-molecular-weight substances from the liposomes [Günther-Ausborn et al., J. Biol. Chem. 270 (1995) 29279-29285; Shangguan et al., Biochemistry 35 (1996) 4956-4965]. Here, we demonstrate retention of encapsulated water-soluble compounds during fusion of Semliki Forest virus (SFV) or Sindbis virus with liposomes at low pH. Under conditions allowing complete fusion of the liposomes, a limited fluorescence dequenching of liposome-encapsulated calcein was observed, particularly for SFV. Also, radioactively labeled inulin or sucrose were largely retained. Freezing and thawing of the viruses in the absence of sucrose resulted in an enhanced leakiness of fusion. These results support the notion that the alphavirus fusion event per se is non-leaky and may well involve a discrete hemifusion intermediate.

Pores formed by influenza hemagglutinin

Low pH-induced fusion mediated by the hemagglutinin (HA) of influenza virus involves a conformational change in the protein that leads to the insertion of a "fusion peptide" of the protein into the target membrane. It has been suggested that this insertion, aided by the formation of a complex of multiple HA trimers, would lead to perturbation of the bilayer structure of the membrane, initiating fusion. Here we present data showing that the interaction of the bromelain released ectodomain of the protein (BHA) with liposomal membranes at low pH leads to pore formation, at least at low temperatures. Strongly temperature-dependent low pH-induced inactivation of BHA resulted in a complete lack of activity of BHA above 10 degrees C. Even at 0 degrees C, only about 5% of the BHA participated in pore formation. Viral HA was less rapidly inactivated and still induced pores at 37 degrees C. BHA-induced pore formation showed a sigmoidal time course. Once BHA had formed a pore in one liposome, it did not form a pore in a further liposome. Quantitative analysis of pore formation indicated that one single BHA trimer sufficed to produce a pore. These data indicate that fusion peptide insertion perturbs the membrane and that the formation of a complex of trimers is not a prerequisite for the perturbation.

Characterization of cholesterol-free insect cells infectible by baculoviruses: effects of cholesterol on VSV fusion and infectivity and on cytotoxicity induced by influenza M2 protein

The patented cell line from the cabbage looper Trichoplusia ni (High Five from Invitrogen) was found to grow readily under cholesterol-free (CF) culture conditions. Cellular cholesterol became undetectable by CF passage 4, while growth rate and overall cell morphology remained unaffected for at least 59 CF passages. The Golgi apparatus in CF cells was significantly smaller than in control cells, and the CF cells also concentrated a ceramide-based fluorescent Golgi marker to a greater extent, but endoplasmic reticulum morphology appeared unaffected. Two proteins were expressed in High Five cells from recombinant baculoviruses under CF and control conditions: the vesicular stomatitis virus (VSV) fusion glycoprotein G and the influenza virus ion channel M2. Both proteins were expressed in comparable amounts in CF and control cells. Both were properly assembled and transported to the plasma membrane in CF cells, indicating the presence of functional Golgi. Wild-type G protein expression resulted in extensive syncytia formation in both CF and control cells, showing that cholesterol is not required for VSV fusion. However, a mutant G protein lacking six transmembrane domain residues was inactive in both CF and control cells. Influenza M2 protein was functional in control cells, as indicated by its amantadine-inhibitable cytotoxicity, but cytotoxicity was absent in CF cells expressing this protein, indicating a cholesterol-dependence for the cytotoxic action of this protein. CF and control cells were both infectible with VSV. However, infected cell centers were modestly decreased (ca. 3.5-fold) in CF cells. CF cells offer a convenient and novel approach to the study of specific cholesterol functions.

The fragmentation of incoming Semliki Forest virus nucleocapsids in mosquito (Aedes albopictus) cells might be coupled to virion uncoating

The fate of Semliki Forest virus (SFV) nucleocapsid, especially the capsid protein (C-protein), was investigated during the early stages of a productive infection in mosquito Aedes albopictus cells. Infection of the cells resulted in a time dependent accumulation of a C-protein derived fragment. This fragmentation of incoming viral nucleocapsid was prevented by NH4Cl, an agent generally used to elevate the pH in acidic intracellular compartments, suggesting that a low intravesicular pH is required for this process. Density gradient analysis of the postnuclear cell lysate demonstrated that the fragmentation was associated with a cellular compartment showing a density of 1.14 +/- 0.02 g/ml. This cellular compartment was devoid from a lysosomal marker enzyme and represented the timely preceding cellular fraction through which SFV passed before encountering a lysosomal fraction. Furthermore, the intracellular distribution of the viral, 3H-uridine-labeled RNA suggested that the same fraction might represent a key cellular compartment in which the separation of the viral RNA from the viral structural proteins is primed. In conclusion, these data lead to the suggestion that the fragmentation of incoming SFV nucleocapsids in Aedes albopictus cells might be the part of the mechanism leading to the release of viral RNA into the cytosol during early stages of productive infection.

Requirement for vacuolar proton-ATPase activity during entry of influenza virus into cells

The role that endosomal acidification plays during influenza virus entry into MDCK cells has been analyzed by using the macrolide antibiotics bafilomycin A1 and concanamycin A as selective inhibitors of vacuolar proton-ATPase (v-[H+]ATPase), the enzyme responsible for the acidification of endosomes. Bafilomycin A1 and concanamycin A, present at the low concentrations of 5 x 10(-7) and 5 x 10(-9) M, respectively, prevented the entry of influenza virus into cells when added during the first minutes of infection. Attachment of virion particles to the cell surface was not the target for the action of bafilomycin A1. N,N'-Dicyclohexylcarbodiimide, a nonspecific inhibitor of proton-ATPases, also blocked virus entry, whereas elaiophylin, an inhibitor of the plasma-proton ATPase, had no effect. The inhibitory actions of bafilomycin A1 and concanamycin A were tested in culture medium at different pHs. Both antibiotics powerfully prevented influenza virus infection when the virus was added under low-pH conditions. This inhibition was reduced if the virus was bound to cells at 4 degrees C prior to the addition of warm low-pH medium. Moreover, incubation of cells at acidic pH potently blocked influenza virus infection, even in the absence of antibiotics. These results indicate that a pH gradient, rather than low pH, is necessary for efficient entry of influenza virus into cells.

Entry and uncoating of enveloped viruses

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Zanvil Alexander Cohn 1926-1993

Zanvil Alexander Cohn, an editor of this Journal since 1973, died suddenly on June 28, 1993. Cohn is best known as the father of the current era of macrophage biology. Many of his scientific accomplishments are recounted here, beginning with seminal studies on the granules of phagocytes that were performed with his close colleague and former editor of this Journal, James Hirsch. Cohn and Hirsch identified the granules as lysosomes that discharged their contents of digestive enzymes into vacuoles containing phagocytosed microbes. These findings were part of the formative era of cell biology and initiated the modern study of endocytosis and cell-mediated resistance to infection. Cohn further explored the endocytic apparatus in pioneering studies of the mouse peritoneal macrophage in culture. He described vesicular inputs from the cell surface and Golgi apparatus and documented the thoroughness of substrate digestion within lysosomal vacuoles that would only permit the egress of monosaccharides and amino acids. These discoveries created a vigorous environment for graduate students, postdoctoral fellows, and junior and visiting faculty. Some of the major findings that emerged from Cohn's collaborations included the radioiodination of the plasma membrane for studies of composition and turnover; membrane recycling during endocytosis; the origin of the mononuclear phagocyte system in situ; the discovery of the dendritic cell system of antigen-presenting cells; the macrophage as a secretory cell, including the release of proteases and large amounts of prostaglandins and leukotrienes; several defined parameters of macrophage activation, especially the ability of T cell-derived lymphokines to enhance killing of tumor cells and intracellular protozoa; the granule discharge mechanism whereby cytotoxic lymphocytes release the pore-forming protein perforin; the signaling of macrophages via myristoylated substrates of protein kinase C; and a tissue culture model in which monocytes emigrate across tight endothelial junctions. In 1983, Cohn turned to a long-standing goal of exploring host resistance directly in humans. He studied leprosy, focusing on the disease site, the parasitized macrophages of the skin. He injected recombinant lymphokines into the skin and found that these molecules elicited several cell-mediated responses. Seeing this potential to enhance host defense in patients, Cohn was extending his clinical studies to AIDS and tuberculosis. Zanvil Cohn was a consummate physician-scientist who nurtured the relationship between cell biology and infectious disease.(ABSTRACT TRUNCATED AT 400 WORDS)

Lipid composition and fluidity of the human immunodeficiency virus envelope and host cell plasma membranes

Previous studies have indicated that human immunodeficiency virus (HIV) is enclosed with a lipid envelope similar in composition to cell plasma membranes and to other viruses. Further, the fluidity, as measured by spin resonance spectroscopy, is low and the viral envelope is among the most highly ordered membranes analyzed. However, the relationship between viral envelope lipids and those of the host cell is not known. Here we demonstrate that the phospholipids within the envelopes of HIV-1RF and HIV-2-L are similar to each other but significantly different from their respective host cell surface membranes. Further, we demonstrate that the cholesterol-to-phospholipid molar ratio of the viral envelope is approximately 2.5 times that of the host cell surface membranes. Consistent with the elevated cholesterol-to-phospholipid molar ratio, the viral envelopes of HIV-1RF and HIV-2-L were shown to be 7.5% and 10.5% more ordered than the plasma membranes of their respective host cells. These data demonstrate that HIV-1 and HIV-2-L select specific lipid domains within the surface membrane of their host cells through which to emerge during viral maturation.

Changes in membrane permeability during Semliki Forest virus induced cell fusion

The infection of Aedes albopictus cells by Semliki Forest virus (SFV) is a non lytic event. Exposure of infected cells to mildly acidic pH (less than 6.2) leads to syncytium formation. This polykaryon formation is accompanied by an influx of protons into the cells (Kempf et al. Biosci. Rep. 7, 761-769, 1987). We have further investigated this permeability change using various fluorescent or radiolabeled compounds. A significant, pH dependent increase of the membrane permeability to low molecular weight compounds (M(r) less than 1000) was observed when infected cells were exposed to a pH less than 6.2. The pH dependence of the permeability change was very similar to the pH dependence of cell-cell fusion. The permeability change was sensitive to divalent cations, protons and anionic antiviral drugs such as trypan blue. The nature of this virus induced, pH dependent permeability change is discussed.

Semliki Forest virus envelope proteins function as proton channels

It has been shown that isolated nucleocapsids of Semliki Forest virus (SFV) contract upon low pH exposure (Soederlund et al., 1972). This contraction of the nucleocapsids has been used as an indicator to demonstrate that the spike proteins of SFV can translocate protons into the interior of the virus particle upon low pH (5.8) exposure. Spikeless virus particles obtained after bromelain digestion, which were used as a control, did not translocate protons. This implies that the ectodomain of the spike plays a crucial role for the proton translocation.

The role of N-acetylneuraminic (sialic) acid in the pH dependence of influenza virion fusion with planar phospholipid membranes

It is known that fusion of influenza virus to host cell membranes is strongly promoted by acidic pH. We have determined conditions required to obtain pH-dependent fusion of influenza virus to planar bilayer membranes. The rate of viral fusion was determined from the flash rate of R18-labeled virions delivered to the surface of the planar membrane by pressure-ejection from a pipette. For a bilayer formed only of phospholipids and cholesterol, the fusion rate was independent of pH and unaffected by the phospholipid composition. When the gangliosides GD1a + GT1b were included in the planar membrane, however, the fusion rate varied steeply with pH. The rate at pH 7.4 in the presence of the gangliosides was about an order of magnitude less than in their absence. At pH less than approximately 5.5, the rate was about an order of magnitude greater in the presence of gangliosides than in their absence. The fusion rate with planar membranes containing globoside, a ceramide-backboned glycolipid, was also independent of pH, indicating that the pH dependence required sialic acid on the carbohydrate moiety of the glycolipid. The gangliosides GM1a and GM3, both of which possess sialic acid, produced the same pH-dependent fusion rate as seen with GD1a + GT1b, indicating that the presence, but not the location, of terminal sialic acids is critical. Incubating virus with soluble sialyllactose blocked fusion to both ganglioside-free and ganglioside-containing planar membranes. These results show that the pH dependence of influenza virion fusion arises from the interaction of the sialic acid receptor with the influenza hemagglutinin. A model for sialic acid-hemagglutinin interactions accounting for pH-dependent fusion is presented.

pH-dependent solubility shift of rubella virus capsid protein

The mechanism of capsid uncoating in rubella virus and other togaviridae is not well understood. This study presents data which suggest that rubella virus capsid undergoes a structural change from having hydrophilic to hydrophobic properties, between pH 5 and 5.5. Such a conformational change would allow capsid uncoating to occur within the lysosome, allowing RNA penetration to occur upon fusion of the viral envelope with the limiting membrane of the lysosome.

Solubilization of matrix protein M1/M from virions occurs at different pH for orthomyxo- and paramyxoviruses

Enveloped viruses, of which the orthomyxo- and paramyxoviruses are members, are known to be uncoated by nonionic detergents in a salt concentration-dependent manner. In this study we have shown that detergent uncoating of myxoviruses depends not only on salt concentration but also on pH. Treatment of orthomyxoviruses with Nonidet-P40 or Triton N-101 at low salt concentrations results in solubilization of surface virion glycopolypeptides in alkaline and neutral pH (9.0-6.5), but in acidic pH (6.0-5.0) the viral matrix protein M1 is also removed, and the viral ribonucleoprotein complex is released. Conversely, the paramyxovirus matrix protein M is more completely solubilized in alkaline pH (pH 9.0) than in neutral and acidic pH 7.4-5.0. The described pH-dependent differences are discussed in terms of orthomyxo- and paramyxovirus uncoating in target cells.

Patch clamp studies of single cell-fusion events mediated by a viral fusion protein

To enter cells, viruses must fuse their envelope with a host cell membrane. Fusion is mediated by specific, membrane-spanning fusion proteins, of which the influenza virus haemagglutinins (HA) are the best characterized. Several HAs have been sequenced, and the crystal structure of the major part of one HA is known. The conditions for fusion and some of the rearrangements in the HA that accompany fusion are well understood, but it remains unclear how HA causes bilayers to fuse. We have observed, in real time, unitary cell-fusion events caused by HA. Fibroblasts expressing HA were induced to fuse with red blood cells by a rapid drop in pH. Fusion was monitored by fluorescence microscopy, and by measuring the membrane conductance and capacitance of the fibroblast. The earliest event observed was the sudden opening of an aqueous pore connecting the cytoplasms of the fusing cells. Initially, the pore conductance often fluctuated between zero and approximately 600 pS, as if the pore were opening and closing repeatedly. Later, it increased over tens of seconds, as if the pore dilated. We suggest that, as in exocytosis, HA-mediated membrane fusion begins with the formation of a narrow pore. Based on the conductance, we estimate the initial diameter of the pore to be no more than twice that of a gap junction channel.

Digital fluorescence imaging of fusion of influenza virus with erythrocytes

Fusion of influenza virus with human erythrocytes at pH 5.2 was followed by fluorescence microscopy using a cooled slow-scan CCD camera. The high sensitivity of the CCD permits repetitive digital imaging of the same cells with minimal photobleaching. The experimental conditions were such that only a small number of virus particles were adsorbed per cell. Quantitative analysis of the data indicated that for most cells only a single fusion event took place. This was, however, sufficient to cause haemolysis within 30 min at 20-22 degrees C for about 60% of cells. There was a highly variable time lag between fusion and haemolysis. The lateral diffusion coefficient of virus particles on the cell surface when bound at pH 7.4 was less than 2 x 10(-13) cm2.s-1. The technique should be of value for more detailed studies of the dynamics of viral and other membrane fusion events.

Entry mechanisms of enveloped viruses. Implications for fusion of intracellular membranes

Enveloped viruses infect cells by a mechanism involving membrane fusion. This process is mediated and triggered by specific viral membrane glycoproteins. Evidence is accumulating that fusion of intracellular membranes, as occurs during endocytosis and transport between intracellular organelles, also requires the presence of specific proteins. The relevance of elucidating the mechanisms of virus fusion for a better understanding of fusion of intracellular membranes is discussed.

Modification of membrane permeability by animal viruses

Animal viruses modify membrane permeability during lytic infection. There is a co-entry of macromolecules and virion particules during virus penetration and a drastic change in transport and membrane permeability at the late stages of the lytic cycle. Both events are of importance to understand different molecular aspects of viral infection, as virus entry into the cell and the interference of virus infection with cellular metabolism. Other methods of cell permeabilization of potential relevance to understand the mechanism of viral damage of the membrane are also discussed.

Virus entry into animal cells

In addition to its many other functions, the plasma membrane of eukaryotic cells serves as a barrier against invading parasites and viruses. It is not permeable to ions and to low molecular weight solutes, let alone to proteins and polynucleotides. Yet it is clear that viruses are capable of transferring their genome and accessory proteins into the cytosol or into the nucleus, and thus infect the cell. While the detailed mechanisms remain unclear for most animal viruses, a general theme is apparent like other stages in the replication cycle; their entry depends on the activities of the host cell. In order to take up nutrients, to communicate with other cells, to control the intracellular ion balance, and to secrete substances, cells have a variety of mechanisms for bypassing and modifying the barrier properties imposed by their plasma membrane. It is these mechanisms, and the molecules involved in them, that viruses exploit.

Phosphatidylserine vesicle lysis by Sendai virus at low pH is not due to virus-vesicle fusion

As a model of the fusion of Sendai virus with red cells, the interaction of the virus with phosphatidylserine (PS) vesicles at pH 5 was quantitated by the release of a trapped marker from target vesicles and by mixing of lipids of the virus and the vesicles. Release of the marker was measured as dequenching of calcein trapped at a self-quenched concentration and lipid mixing was measured as a decrease in energy transfer between fluorescent phospholipid analogs in the target membrane. At comparable virus:vesicle ratios both calcein release and lipid mixing were maximal at pH 5 and significantly reduced after trypsin, but not chymotrypsin, treatment. In contrast, these two effects differed in their PS dependence, time course, and temperature dependence, indicating that calcein release is not a consequence of the fusion of a permeable virus membrane with an impermeable target membrane. Vesicles composed of 25 to 100% PS released similar amounts of calcein, whereas fusion increased linearly as a function of PS content of the target vesicles. The half-time was 15 s for calcein release but 1.5 min for fusion. The temperature coefficient of fusion was at least three times greater than that of calcein release. These results indicate that calcein release at pH 5 may signify an interaction of the virus with PS target membranes which precedes but does not necessarily culminate in fusion, given too low a temperature or an inappropriate target membrane composition.

Fusion of influenza virus in an intracellular acidic compartment measured by fluorescence dequenching

The fusion of influenza virus with cultured cells has been investigated. The virus was labelled with the fluorescent probe octadecyl rhodamine B and fusion was monitored as fluorescence dequenching due to dilution of the probe from the viral into a cellular target membrane. Fusion with the plasma membrane does not occur, unless the extracellular pH is temporarily lowered. At neutral pH fusion occurs only after a lag phase of 10-15 min, the time required for virus internalization, and the reaction is inhibited by NH4Cl, indicating that it takes place in an intracellular acidic compartment, most likely the endosome. This suggests that influenza virus infects cells via the endocytic pathway.

Comparison of the interaction of the anti-viral chemotherapeutic agents amantadine and tromantadine with model phospholipid membranes

Amantadine and tromantadine are agents used against influenza and herpes infections, respectively. Tromantadine raises the bilayer to hexagonal phase transition temperature of synthetic phosphatidylethanolamines and is less disruptive to phospholipid packing. Tromantadine acts similar to cyclosporin A, previously demonstrated to inhibit viral-induced cell-cell fusion. We suggest the balance between the hydrophobic and hydrophilic group sizes would allow tromantadine to prevent membrane fusion more than amantadine and thus inhibit infection by viruses such as Herpes, which fuse with the plasma membrane. Study of agents which stabilize the bilayer phase of membranes may lead to efficacious inhibitors of viral infections requiring cell fusion events.

Radiation inactivation analysis of influenza virus reveals different target sizes for fusion, leakage, and neuraminidase activities

The size of the functional units responsible for several activities carried out by the influenza virus envelope glycoproteins was determined by radiation inactivation analysis. Neuraminidase activity, which resides in the glycoprotein NA, was inactivated exponentially with an increasing radiation dose, yielding a target size of 94 +/- 5 kilodaltons (kDa), in reasonable agreement with that of the disulfide-bonded dimer (120 kDa). All the other activities studied are properties of the HA glycoprotein and were normalized to the known molecular weight of the neuraminidase dimer. Virus-induced fusion activity was measured by two phospholipid dilution assays: relief of energy transfer between N-(7-nitro-2,1,3-benzoxadiazol-4-yl)dipalmitoyl-L-alpha- phosphatidylethanolamine (N-NBD-PE) and N-(lissamine rhodamine B sulfonyl)-dioleoyl-L-alpha-phosphatidylethanolamine (N-Rh-PE) in target liposomes and relief of self-quenching of N-Rh-PE in target liposomes. Radiation inactivation of fusion activity proceeded exponentially with radiation dose, yielding normalized target sizes of 68 +/- 6 kDa by assay i and 70 +/- 4 kDa by assay ii. These values are close to the molecular weight of a single disulfide-bonded (HA1 + HA2) unit (75 kDa), the "monomer" of the HA trimer. A single monomer is thus inactivated by each radiation event, and each monomer (or some part of it) constitutes a minimal functional unit capable of mediating fusion. Virus-induced leakage of calcein from target liposomes and virus-induced leakage of hemoglobin from erythrocytes (hemolysis) both showed more complex inactivation behavior: a pronounced shoulder was present in both inactivation curves, followed by a steep drop in activity at higher radiation levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Modification of membrane permeability during Semliki Forest virus infection

Modification of membrane permeability has been analyzed in Semliki Forest virus (SFV)-infected cells by means of translation inhibitors not permeable to normal cells. A higher inhibition of protein synthesis in the infected cells is only observed with those antibiotics that do not easily pass the cell membrane, but not with others, permeable to cells, such as anisomycin, cycloheximide, trichodermin, etc. It does not, therefore, seem that the suggestion of M. A. Gray, K. J. Micklem, and C. A. Pasternak [Eur. J. Biochem. 135, 299-302, (1983)] that protein synthesis in virus-infected cells is more susceptible to translation inhibitors in general is correct. Both low- and high-molecular weight compounds enter the cell very early during SFV infection. This permeabilization is blocked by compounds known to increase the pH of coated vesicles, such as NH4Cl and chloroquine. Inhibition of energy production by means of N3Na and 2'-deoxyglucose also blocks this process. The optimal external pH for this early permeabilization is around 7-8. Acidic pH inhibits the entry of these impermeant antibiotics promoted by SFV. Analysis of 86Rb+ content in SFV-infected HeLa cells also indicates that a drastic decline in this cation takes place, in agreement with previous findings, but disagreeing with the previous results. A parallel between the decrease in this cation and the blockade of protein synthesis is apparent, throughout the course of infection. In addition to the early permeabilization that takes place during virus entry, increased entry of hygromycin B and alpha-sarcin also occurs in SFV-infected cells from 2 to 3 hr postinfection, but not when late viral replication is blocked by means of interferon treatment.

Kinetics of pH-dependent fusion between influenza virus and liposomes

The pH-dependent fusion between influenza virus and liposomes (large unilamellar vesicles) has been investigated as a model for the fusion step in the infectious entry of the virus into cells. Fusion was monitored continuously, with a fluorescence assay based on resonance energy transfer (RET) [Struck, D. K., Hoekstra, D., & Pagano, R. E. (1981) Biochemistry 20, 4093-4099], which allows an accurate quantitation of the fusion process. Evidence is presented indicating that the dilution of the RET probes from the liposomal bilayer into the viral membrane is not due to transfer of individual lipid molecules. The initial rate and final extent of the fusion reaction increase dramatically with decreasing pH, fusion being virtually complete within 1 min at pH 4.5-5.0. From experiments in which the ratio of virus to liposomes was varied, it is concluded that virus-liposome fusion products continue to fuse with liposomes, but not with virus. Fusion is most efficient with liposomes consisting of negatively charged phospholipids, while phosphatidylcholine and sphingomyelin are inhibitory. The reaction is completely blocked by an antiserum against the virus and inhibited by pretreatment of the virus with trypsin. The effect of proteolytic pretreatment at pH 7.4 is enhanced after preincubation of the virus at pH 5.0, consistent with the occurrence of a low pH induced, irreversible, conformational change in the viral fusion protein, the hemagglutinin (HA), exposing trypsin cleavage sites. When, after initiation of the fusion reaction at pH 5.0, the pH is readjusted to neutral, the process is arrested instantaneously, indicating that the low pH induced conformational change in the HA protein, in itself, is not sufficient to trigger fusion activity.

Reconstitution and fusogenic properties of Sendai virus envelopes

Sendai virus membranes were reconstituted by detergent dialysis, using the non-ionic detergents Triton X-100 and octyl glucoside. Membrane reassembly was determined by measuring the surface-density-dependent efficiency of resonance energy transfer between two fluorescent phospholipid analogues, which were co-reconstituted with the viral envelopes. The functional incorporation of the viral proteins was established by monitoring the ability of the reconstitution products to fuse with erythrocyte membranes, utilizing assays based on either resonance energy transfer or on relief of fluorescence selfquenching. The persistent adherence of residual Triton X-100 with the reconstituted membrane was revealed by an artificial detergent-effect on the resonance energy transfer efficiency and the occurrence of hemolysis of human erythrocytes under conditions where fusion does not occur. Properly reconstituted Sendai virus envelopes were obtained with octyl glucoside. The fusion activity of the viral envelopes was dependent on the initial concentration of octyl glucoside used to disrupt the virus and the rate of detergent removal. Rapid removal of detergent by dialysis against large volumes of dialysis buffer (ratio 1:850) or by gel filtration produced reconstituted membranes capable of inducing hemagglutination but significant fusion activity was not detected. By decreasing the volume ratio of dialysate versus dialysis buffer to 1:250 or 1:25, fusogenic viral envelopes were obtained. The initial fusion kinetics of the reconstituted viral membrane and the parent virus were different in that both the onset and the initial rate of fusion of the reconstituted membranes were faster, whereas the extents to which both particles eventually fused with the target membrane were similar. The differences in the initial fusion kinetics lead us to suggest that the details of the fusion mechanism between Sendai virus and the target membrane involve factors other than the mere presence of glycoproteins F and HN in the viral bilayer. Finally, the results also indicate that determination of the viral fusion activity in a direct manner, rather than by an indirect assay, such as hemolysis, is imperative for a proper evaluation of the functional properties retained upon viral reconstitution.

Protein-mediated intermembrane contact facilitates fusion of lipid vesicles with planar bilayers

Fusion of phospholipid vesicles with planar bilayer membranes occurs provided there is an intermembrane contact, which can be mediated by phospholipid-binding proteins, even in the absence of calcium. The firm attachment phase is then followed by the osmotically-driven fusion. These results show that hydrophobic proteins (not necessarily Ca2+-binding proteins) may enhance fusion by promoting contact of membranes. Such proteins may operate synergistically with Ca2+ to reduce the threshold concentration of Ca2+ needed for fusion of biological membranes. Protein-mediated intermembrane contact resulting in fusion may play a crucial role in the regulation and catalysis of biological fusion events.