A dissection of steps leading to viral envelope protein-mediated membrane fusion

Crystal Structure of the Pre-fusion Nipah Virus Fusion Glycoprotein Reveals a Novel Hexamer-of-Trimers Assembly

Nipah virus (NiV) is a paramyxovirus that infects host cells through the coordinated efforts of two envelope glycoproteins. The G glycoprotein attaches to cell receptors, triggering the fusion (F) glycoprotein to execute membrane fusion. Here we report the first crystal structure of the pre-fusion form of the NiV-F glycoprotein ectodomain. Interestingly this structure also revealed a hexamer-of-trimers encircling a central axis. Electron tomography of Nipah virus-like particles supported the hexameric pre-fusion model, and biochemical analyses supported the hexamer-of-trimers F assembly in solution. Importantly, structure-assisted site-directed mutagenesis of the interfaces between F trimers highlighted the functional relevance of the hexameric assembly. Shown here, in both cell-cell fusion and virus-cell fusion systems, our results suggested that this hexamer-of-trimers assembly was important during fusion pore formation. We propose that this assembly would stabilize the pre-fusion F conformation prior to cell attachment and facilitate the coordinated transition to a post-fusion conformation of all six F trimers upon triggering of a single trimer. Together, our data reveal a novel and functional pre-fusion architecture of a paramyxoviral fusion glycoprotein.

Single particle assay of coronavirus membrane fusion with proteinaceous receptor-embedded supported bilayers

Total internal reflection microscopy combined with microfluidics and supported bilayers is a powerful, single particle tracking (SPT) platform for host-pathogen membrane fusion studies. But one major inadequacy of this platform has been capturing the complexity of the cell membrane, including membrane proteins. Because of this, viruses requiring proteinaceous receptors, or other unknown cellular co-factors, have been precluded from study. Here we describe a general method to integrate proteinaceous receptors and cellular components into supported bilayers for SPT fusion studies. This method is general to any enveloped virus-host cell pair, but demonstrated here for feline coronavirus (FCoV). Supported bilayers are formed from mammalian cell membrane vesicles that express feline aminopeptidase N (the viral receptor) using a cell blebbing technique. SPT is then used to identify fusion intermediates and measure membrane fusion kinetics for FCoV. Overall, the fusion results recapitulate what is observed in vivo, that coronavirus entry requires binding to specific receptors, a low-pH environment, and that membrane fusion is receptor- and protease-dependent. But this method also provides quantitative kinetic rate parameters for intermediate steps in the coronavirus fusion pathway, which to our knowledge have not been obtained before. Moreover, the platform offers versatile, precise control over the sequence of triggers for fusion; these triggers may define the fusion pathway, tissue tropism, and pathogenicity of coronaviruses. Systematically varying these triggers in this platform provides a new route to study how viruses rapidly adapt to other hosts, and to identify factors that led to the emergence of zoonotic viruses, such as human SARS-CoV and the newly emerging human MERS-CoV.

ATR-FTIR studies in pore forming and membrane induced fusion peptides

Infrared (IR) spectroscopy has been shown to be very reliable for the characterization, identification and quantification of structural data. Particularly, the Attenuated Total Reflectance (ATR) technique which became one of the best choices to study the structure and organization of membrane proteins and membrane-bound peptides in biologically relevant membranes. An important advantage of IR spectroscopy is its ability to analyze material under a very wide range of conditions including solids, liquids and gases. This method allows elucidation of component secondary structure elements of a peptide or protein in a global manner, and by using site specific isotope labeling allows determination of specific regions. A few advantages in using ATR-FTIR spectroscopy include; a relatively simple technique, allow the determination of peptide orientation in the membrane, allow the determination of secondary structures of very small peptides, and importantly, the method is sensitive to isotopic labeling on the scale of single amino acids. Many studies were reported on the use of ATR-FTIR spectroscopy in order to study the structure and orientation of membrane bound hydrophobic peptides and proteins. The list includes native and de-novo designed peptides, as well as those derived from trans-membrane domains of various receptors (TMDs). The present review will focus on several examples that demonstrate the potential and the simplicity in using the ATR-FTIR approach to determine secondary structures of proteins and peptides when bound, inserted, and oligomerized within membranes. The list includes (i) a channel forming protein/peptide: the Ca(2+) channel phospholamban, (ii) a cell penetrating peptide, (iii) changes in the structure of a transmembrane domain located within ordered and non-ordered domains, and (iv) isotope edited FTIR to directly assign structure to the membrane associated fusion peptide in context of a Key gp41 Structural Motif. Importantly, a unique advantage of infrared spectroscopy is that it allows a simultaneous study of the structure of lipids and proteins in intact biological membranes without an introduction of foreign perturbing probes. Because of the long IR wavelength, light scattering problems are virtually non-existent. This allows the investigation of highly aggregated materials or large membrane fragments. This article is part of a Special Issue entitled: FTIR in membrane proteins and peptide studies.

Influenza virus-membrane fusion triggered by proton uncaging for single particle studies of fusion kinetics

We report a method for studying membrane fusion, focusing on influenza virus fusion to lipid bilayers, which provides high temporal resolution through the rapid and coordinated initiation of individual virus fusion events. Each fusion event proceeds through a series of steps, much like multistep chemical reaction. Fusion is initiated by a rapid decrease in pH that accompanies the "uncaging" of an effector molecule from o-nitrobenzaldehyde, a photoisomerizable compound that releases a proton to the surrounding solution within microseconds of long-wave ultraviolet irradiation. In order to quantify pH values upon UV irradiation and uncaging, we introduce a simple silica nanoparticle pH sensor, useful for reporting the pH in homogeneous nanoliter volumes under conditions where traditional organic dye-type pH probes fail. Subsequent single-virion fusion events are monitored using total internal reflection fluorescence microscopy. Statistical analysis of these stochastic events uncovers kinetic information about the fusion reaction. This approach reveals that the kinetic parameters obtained from the data are sensitive to the rate at which protons are delivered to the bound viruses. Higher resolution measurements can enhance fundamental fusion studies and aid antiviral antifusogenic drug development.

A GxxxG-like motif within HIV-1 fusion peptide is critical to its immunosuppressant activity, structure, and interaction with the transmembrane domain of the T-cell receptor

To thrive in the human body, HIV fuses to its target cell and evades the immune response via several mechanisms. The fusion cascade is initiated by the fusion peptide (FP), which is located at the N-terminal of gp41, the transmembrane protein of HIV. Recently, it has been shown that the HIV-1 FP, particularly its 5-13 amino acid region (FP(5-13)), suppresses T-cell activation and interacts with the transmembrane domain (TMD) of the T-cell receptor (TCR) complex. Specific amino acid motifs often contribute to such interactions in TMDs of membrane proteins. Using bioinformatics and experimental studies, we report on a GxxxG-like motif (AxxxG), which is conserved in the FP throughout different clades and strains of HIV-1. Biological activity studies and FTIR spectroscopy revealed that HIV FP(5-13)-derived peptides, in which the motif was altered either by randomization or by a single amino acid shift, lost their immunosuppressive activity concomitant with a loss of the β-sheet structure in a membranous environment. Furthermore, fluorescence studies revealed that the inactive mutants lost their ability to interact with their target site, namely, the TMD of TCRα, designated CP. Importantly, lipotechoic acid activated macrophages (lacking TCR) were not affected by FP, further demonstrating the specificity of the immunosuppressant activity of CP. Finally, although the AxxxG WT and the GxxxG analog both associated with the CP and immunosuppressed T-cells, the AxxxG WT but not the GxxxG analog induced lipid mixing. Overall, the data support an important role for the AxxxG motif in the function of FP and might explain the natural selection of the AxxxG motif rather than the classical GxxxG motif in FP.

Characterization of the interacting domain of the HIV-1 fusion peptide with the transmembrane domain of the T-cell receptor

HIV infection is initiated by the fusion of the viral membrane with the target T-cell membrane. The HIV envelope glycoprotein, gp41, contains a fusion peptide (FP) in the N terminus that functions together with other gp41 domains to fuse the virion with the host cell membrane. We recently reported that FP co-localizes with CD4 and T-cell receptor (TCR) molecules, co-precipitates with TCR, and inhibits antigen-specific T-cell proliferation and pro-inflammatory cytokine secretion. Molecular dynamic simulation implicated an interaction between an alpha-helical transmembrane domain (TM) of the TCRalpha chain (designated CP) and the beta-sheet 5-13 region of the 16 N-terminal amino acids of FP (FP(1-16)). To correlate between the theoretical prediction and experimental data, we synthesized a series of mutants derived from the interacting motif GALFLGFLG stretch (FP(5-13)) and investigated them structurally and functionally. The data reveal a direct correlation between the beta-sheet structure of FP(5-13) and its mutants and their ability to interact with CP and induce immunosuppressive activity; the phenylalanines play an important role. Furthermore, studies with fluorescently labeled peptides revealed that this interaction leads to penetration of the N terminus of FP and its active analogues into the hydrophobic core of the membrane. A detailed understanding of the molecular interactions mediating the immunosuppressive activity of the FP(5-13) motif should facilitate evaluating its contribution to HIV pathology and its exploitation as an immunotherapeutic tool.

Modulation of entry of enveloped viruses by cholesterol and sphingolipids (Review)

Enveloped animal viruses infect host cells by fusion of viral and target membranes. This crucial fusion event occurs either with the plasma membrane of the host cells at the physiological pH or with the endosomal membranes at low pH and is triggered by specific glycoproteins in the virus envelope. Both lipids and proteins play critical and co-operative roles in the fusion process. Interactions of viral proteins with their receptors direct which membranes fuse and viral fusion proteins then drive the process. These fusion proteins operate on lipid assemblies, whose physical and mechanical properties are equally important to the proper functioning of the process. Lipids contribute to the viral fusion process by virtue of their distinct chemical structure, composition and/or their preferred partitioning into specific microdomains in the plasma membrane called 'rafts'. An involvement of lipid rafts in viral entry and membrane fusion has been examined recently. However, the mechanism(s) by which lipids as dynamic raft components control viral envelope-glycoprotein-triggered fusion is not clear. This paper will review literature findings on the contribution of the two raft-associated lipids, cholesterol and sphingolipids in viral entry.

Lysolipids do not inhibit influenza virus fusion by interaction with hemagglutinin

The interaction of a spin-labeled lysophosphatidylcholine analog with intact and bromelain-treated influenza viruses as well as with the bromelain-solubilized hemagglutinin ectodomain has been studied. The inhibition of fusion of influenza viruses with erythrocytes by the lysophosphatidylcholine analog was similar to that observed for non-labeled lysophosphatidylcholine. Only a weak interaction of the lysophosphatidylcholine analog with the hemagglutinin ectodomain was observed even upon triggering the conformational change of the ectodomain at a low pH. A significant interaction of spin-labeled lysophosphatidylcholine with the hemagglutinin ectodomain of intact viruses was observed neither at neutral nor at low pH, whereas a strong interaction of the lipid analog with the viral lipid bilayer was evident. We suggest that the high number of lipid binding sites of the virus bilayer and their affinity compete efficiently with binding sites of the hemagglutinin ectodomain. We conclude that the inhibition of influenza virus fusion by lysolipids is not mediated by binding to the hemagglutinin ectodomain, preventing its interaction with the target membrane. The results unambiguously argue for an inhibition mechanism based on the action of lysolipid inserted into the lipid bilayer.

Role of the Glu residues of the influenza hemagglutinin fusion peptide in the pH dependence of fusion activity

To elucidate the role of the fusion peptide in influenza hemagglutinin (HA)-mediated fusion, we compared pH-dependent conformational changes and fusion mediated by wild-type and a mutant HA in which Glu residues at positions 11 and 15 of the fusion peptide are substituted for valine. The pH dependence of conformational changes and kinetics of fusion with erythrocytes was the same for both forms of HA. The time for commitment and the temperature dependence of HA-mediated fusion were also the same. However, striking differences were observed between wild-type and mutant fusion peptides in their interactions with lipid membranes at neutral and acidic pH. Since elimination of the negatively charged residues allows the exposed fusion peptide to penetrate the bilayer at pH values closer to neutral, but does not affect conformational changes and fusion activity in intact HA, we conclude that conformational changes are tightly coupled to fusion peptide insertion in the overall HA-mediated fusion cascade.

Infectious entry by amphotropic as well as ecotropic murine leukemia viruses occurs through an endocytic pathway

Infectious entry of enveloped viruses is thought to proceed by one of two mechanisms. pH-dependent viruses enter the cells by receptor-mediated endocytosis and are inhibited by transient treatment with agents that prevent acidification of vesicles in the endocytic pathway, while pH-independent viruses are not inhibited by such agents and are thought to enter the cell by direct fusion with the plasma membrane. Nearly all retroviruses, including amphotropic murine leukemia virus (MuLV) and human immunodeficiency virus type 1, are classified as pH independent. However, ecotropic MuLV is considered to be a pH-dependent virus. We have examined the infectious entry of ecotropic and amphotropic MuLVs and found that they were equally inhibited by NH4Cl and bafilomycin A. These agents inhibited both viruses only partially over the course of the experiments. Agents that block the acidification of endocytic vesicles also arrest vesicular trafficking. Thus, partial inhibition of the MuLVs could be the result of virus inactivation during arrest in this pathway. In support of this contention, we found that that the loss of infectivity of the MuLVs during treatment of target cells with the drugs closely corresponded to the loss of activity due to spontaneous inactivation at 37 degrees C in the same period of time. Furthermore, the drugs had no effect on the efficiency of infection under conditions in which the duration of infection was held to a very short period to minimize the effects of spontaneous inactivation. These results indicate that the infectious processes of both ecotropic and amphotropic MuLVs were arrested rather than aborted by transient treatment of the cells with the drugs. We also found that infectious viruses were efficiently internalized during treatment. This indicated that the arrest occurred in an intracellular compartment and that the infectious process of both the amphotropic and ecotropic MuLVs very likely involved endocytosis. An important aspect of this study pertains to the interpretation of experiments in which agents that block endocytic acidification inhibit infectivity. As we have found with the MuLVs, inhibition of infectivity may be secondary to the block of endocytic acidification. While this strongly suggests the involvement of an endocytic pathway, it does not necessarily indicate a requirement for an acidic compartment during the infectious process. Likewise, a lack of inhibition during transient treatment with the drugs would not preclude an endocytic pathway for viruses that are stable during the course of the treatment.

Delay of influenza hemagglutinin refolding into a fusion-competent conformation by receptor binding: a hypothesis

Two subunits of influenza hemagglutinin (HA), HA1 and HA2, represent one of the best-characterized membrane fusion machines. While a low pH conformation of HA2 mediates the actual fusion, HA1 establishes a specific connection between the viral and cell membranes via binding to the sialic acid-containing receptors. Here we propose that HA1 may also be involved in modulating the kinetics of HA refolding. We hypothesized that binding of the HA1 subunit to its receptor restricts the major refolding of the low pH-activated HA to a fusion-competent conformation and, in the absence of fusion, to an HA-inactivated state. Dissociation of the HA1-receptor connection was considered to be a slow kinetic step. To verify this hypothesis, we first analyzed a simple kinetic scheme accounting for the stages of dissociation of the HA1/receptor bonds, inactivation and fusion, and formulated experimentally testable predictions. Second, we verified these predictions by measuring the extent of fusion between HA-expressing cells and red blood cells. Three experimental approaches based on 1) the temporal inhibition of fusion by lysophosphatidylcholine, 2) rapid dissociation of the HA1-receptor connections by neuraminidase treatment, and 3) substitution of membrane-anchored receptors by a water-soluble sialyllactose all provided support for the proposed role of the release of HA1-receptor connections. Possible biological implications of this stage in HA refolding and membrane fusion are being discussed.

Membrane perturbation and fusion pore formation in influenza hemagglutinin-mediated membrane fusion. A new model for fusion

Low pH-induced fusion mediated by the hemagglutinin (HA) of influenza virus involves conformational changes in the protein that lead to the insertion of a "fusion peptide" domain of this protein into the target membrane and is thought to perturb the membrane, triggering fusion. By using whole virus, purified HA, or HA ectodomains, we found that shortly after insertion, pores of less than 26 A in diameter were formed in liposomal membranes. As measured by a novel assay, these pores stay open, or continue to close and open, for minutes to hours and persist after pH neutralization. With virus and purified HA, larger pores, allowing the leakage of dextrans, were seen at times well after insertion. For virus, dextran leakage was simultaneous with lipid mixing and the formation of "fusion pores," allowing the transfer of dextrans from the liposomal to the viral interior or vice versa. Pores did not form in the viral membrane in the absence of a target membrane. Based on these data, we propose a new model for fusion, in which HA initially forms a proteinaceous pore in the target, but not in the viral membrane, before a lipidic hemifusion intermediate is formed.

Role of hemagglutinin surface density in the initial stages of influenza virus fusion: lack of evidence for cooperativity

Membrane fusion mediated by influenza virus hemagglutinin (HA) is believed to proceed via the cooperative action of multiple HA trimers. To determine the minimal number of HA trimers required to trigger fusion, and to assess the importance of cooperativity between these HA trimers, we have generated virosomes containing coreconstituted HAs derived from two strains of virus with different pH dependencies for fusion, X-47 (optimal fusion at pH 5.1; threshold at pH 5.6) and A/Shangdong (optimal fusion at pH 5.6; threshold at pH 6.0), and measured fusion of these virosomes with erythrocyte ghosts by a fluorescence lipid mixing assay. Virosomes with different X-47-to-A/Shangdong HA ratios, at a constant HA-to-lipid ratio, showed comparable ghost-binding activities, and the low-pH-induced conformational change of A/Shangdong HA did not affect the fusion activity of X-47 HA. The initial rate of fusion of these virosomes at pH 5.7 increased directly proportional to the surface density of A/Shangdong HA, and a single A/Shangdong trimer per virosome appeared to suffice to induce fusion. The reciprocal of the lag time before the onset of fusion was directly proportional to the surface density of fusion-competent HA. These results support the notion that there is no cooperativity between HA trimers during influenza virus fusion.

Conformational intermediates and fusion activity of influenza virus hemagglutinin

Three strains of influenza virus (H1, H2, and H3) exhibited similar characteristics in the ability of their hemagglutinin (HA) to induce membrane fusion, but the HAs differed in their susceptibility to inactivation. The extent of inactivation depended on the pH of preincubation and was lowest for A/Japan (H2 subtype), in agreement with previous studies (A. Puri, F. Booy, R. W. Doms, J. M. White, and R. Blumenthal, J. Virol. 64:3824-3832, 1990). While significant inactivation of X31 (H3 subtype) was observed at 37 degrees C at pH values corresponding to the maximum of fusion (about pH 5.0), no inactivation was seen at preincubation pH values 0.2 to 0.4 pH units higher. Surprisingly, low-pH preincubation under those conditions enhanced the fusion rates and extents of A/Japan as well as those of X31. For A/PR 8/34 (H1 subtype), neither a shift of the pH (to >5.0) nor a decrease of the temperature to 20 degrees C was sufficient to prevent inactivation. We provide evidence that the activated HA is a conformational intermediate distinct from the native structure and from the final structure associated with the conformational change of HA, which is implicated by the high-resolution structure of the soluble trimeric fragment TBHA2 (P. A. Bullough, F. M. Hughson, J. J. Skehel, and D. C. Wiley, Nature 371:37-43, 1994).

Mechanisms of enveloped virus entry into animal cells

The ability of viruses to transfer macromolecules between cells makes them attractive starting points for the design of biological delivery vehicles. Virus-based vectors and sub-viral systems are already finding biotechnological and medical applications for gene, peptide, vaccine and drug delivery. Progress has been made in understanding the cellular and molecular mechanisms underlying virus entry, particularly in identifying virus receptors. However, receptor binding is only a first step and we now have to understand how these molecules facilitate entry, how enveloped viruses fuse with cells or non-enveloped viruses penetrate the cell membrane, and what happens following penetration. Only through these detailed analyses will the full potential of viruses as vectors and delivery vehicles be realised. Here we discuss aspects of the entry mechanisms for several well-characterised viral systems. We do not attempt to provide a fully comprehensive review of virus entry but focus primarily on enveloped viruses.

Dilation of the human immunodeficiency virus-1 envelope glycoprotein fusion pore revealed by the inhibitory action of a synthetic peptide from gp41

We have monitored fusion between cell pairs consisting of a single human immunodeficiency virus-1 (HIV-1) envelope glycoprotein-expressing cell and a CD4+ target cell, which had been labeled with both a fluorescent lipid in the membrane and a fluorescent solute in the cytosol. We developed a new three-color assay to keep track of the cell into which fluorescent lipids and/or solutes are redistributed. Lipid and solute redistribution occur as a result of opening a lipid-permissive fusion pore and a solute-permissive fusion pore (FPS), respectively. A synthetic peptide (DP178) corresponding to residues 643-678 of the HIV-1LAI gp120-gp41 sequence (Wild, C.T., D.C. Shugars, T.K. Greenwell, C.B. McDanal, and T.J. Matthews. 1994. Proc. Natl. Acad. Sci. USA. 91:12676-12680) completely inhibited FPS at 50 ng/ml, whereas at that concentration there was 20-30% fusion activity measured by the lipid redistribution. The differences detected in lipid mixing versus contents mixing are maintained up to 6 h of coculture of gp120-41-expressing cells with target cells, indicating that DP178 can "clamp" the fusion complex in the lipid mixing intermediate for very long time periods. A peptide from the NH2-terminal of gp41, DP107, inhibited HIV-1LAI gp120-gp41-mediated cell fusion at higher concentrations, but with no differences between lipid and aqueous dye redistribution at the different inhibitor concentrations. The inhibition of solute redistribution by DP178 was complete when the peptide was added to the fusion reaction mixture during the first 15 min of coculture. We have analyzed the inhibition data in terms of a fusion pore dilation model that incorporates the recently determined high resolution structure of the gp41 core.

Conformational changes in cell surface HIV-1 envelope glycoproteins are triggered by cooperation between cell surface CD4 and co-receptors

We have continuously measured CD4-induced conformational changes of cell surface-expressed human immunodeficiency virus type-1 envelope glycoprotein gp120-gp41 in situ using 4,4'-dianilino-1, 1'-binaphthyl-5,5'-disulfonic acid, a fluorescent probe that binds to hydrophobic groups. CD4-expressing human T cell lines induced significant and rapid conformational changes (<1 min delay) in gp120-gp41 from T cell-tropic strains, and little conformational changes in gp120-gp41 from macrophage-tropic strains, with equivalent levels of envelope expression. Conversely, CD4-expressing human macrophages induced significant and rapid conformational changes in gp120-gp41 from macrophage-tropic strains, and little conformational changes in gp120-gp41 from T cell-tropic strains. Thus, the conformational changes undergone by gp120-gp41, which lead to membrane fusion, are highly cooperative and require both receptor and co-receptor. We used a dye transfer assay to show that neither membrane lipid fusion or fusion pore formation can occur with host cells having different tropism from the envelope.

Membrane rearrangements in fusion mediated by viral proteins

Diverse enveloped viruses enter host cells by fusing their envelopes with cell membranes. The mechanisms of merger of lipid bilayers of two membranes mediated by influenza hemagglutinin and other viral fusion proteins apparently involve local lipidic connections that evolve into a bilayer septum in which a pore forms and expands.

Fusion peptides derived from the HIV type 1 glycoprotein 41 associate within phospholipid membranes and inhibit cell-cell Fusion. Structure-function study

The fusion domain of human immunodeficiency virus (HIV-1) envelope glycoprotein (gp120-gp41) is a conserved hydrophobic region located at the N terminus of the transmembrane glycoprotein (gp41). A V2E mutant has been shown to dominantly interfere with wild-type envelope-mediated syncytium formation and virus infectivity. To understand this phenomenon, a 33-residue peptide (wild type, WT) identical to the N-terminal segment of gp41 and its V2E mutant were synthesized, fluorescently labeled, and characterized. Both peptides inhibited HIV-1 envelope-mediated cell-cell fusion and had similar alpha-helical content in membrane mimetic environments. Studies with fluorescently labeled peptide analogues revealed that both peptides have high affinity for phospholipid membranes, are susceptible to digestion by proteinase-K in their membrane-bound state, and tend to self- and coassemble in the membranes. In SDS-polyacrylamide gel electrophoresis the WT peptide formed dimers as well as higher order oligomers, whereas the V2E mutant only formed dimers. The WT, but not the V2E mutant, induced liposome aggregation, destabilization, and fusion. Moreover, the V2E mutant inhibited vesicle fusion induced by the WT peptide, probably by forming inactive heteroaggregates. These data form the basis for an explanation of the mechanism by which the gp41 V2E mutant inhibits HIV-1 infectivity in cells when co-expressed with WT gp41.

Dilation of the influenza hemagglutinin fusion pore revealed by the kinetics of individual cell-cell fusion events

We have monitored kinetics of fusion between cell pairs consisting of a single influenza hemaglutinin (HA)-expressing cell and a single erythrocyte (RBC) that had been labeled with both a fluorescent lipid (Dil) in the membrane and a fluorescent solute (calcein) in the aqueous space. Initial fusion pore opening between the RBC and HA-expressing cell produced a change in RBC membrane potential (delta psi) that was monitored by a decrease in Dil fluorescence. This event was followed by two distinct stages of fusion pore dilation: the flux of fluorescent lipid (phi L) and the flux of a large aqueous fluorescent dye (phi s). We have analyzed the kinetics of events that occur as a result of transitions between a fusion pore (FP) and a solute permissive fusion pore (FPs). Our data are consistent with a fusion pore comprising six HA trimers.

Resonance energy transfer imaging of phospholipid vesicle interaction with a planar phospholipid membrane: undulations and attachment sites in the region of calcium-mediated membrane--membrane adhesion

Membrane fusion of a phospholipid vesicle with a planar lipid bilayer is preceded by an initial prefusion stage in which a region of the vesicle membrane adheres to the planar membrane. A resonance energy transfer (RET) imaging microscope, with measured spectral transfer functions and a pair of radiometrically calibrated video cameras, was used to determine both the area of the contact region and the distances between the membranes within this zone. Large vesicles (5-20 microns diam) were labeled with the donor fluorophore coumarin-phosphatidylethanolamine (PE), while the planar membrane was labeled with the acceptor rhodamine-PE. The donor was excited with 390 nm light, and separate images of donor and acceptor emission were formed by the microscope. Distances between the membranes at each location in the image were determined from the RET rate constant (kt) computed from the acceptor:donor emission intensity ratio. In the absence of an osmotic gradient, the vesicles stably adhered to the planar membrane, and the dyes did not migrate between membranes. The region of contact was detected as an area of planar membrane, coincident with the vesicle image, over which rhodamine fluorescence was sensitized by RET. The total area of the contact region depended biphasically on the Ca2+ concentration, but the distance between the bilayers in this zone decreased with increasing [Ca2+]. The changes in area and separation were probably related to divalent cation effects on electrostatic screening and binding to charged membranes. At each [Ca2+], the intermembrane separation varied between 1 and 6 nm within each contact region, indicating membrane undulation prior to adhesion. Intermembrane separation distances < or = 2 nm were localized to discrete sites that formed in an ordered arrangement throughout the contact region. The area of the contact region occupied by these punctate attachment sites was increased at high [Ca2+]. Membrane fusion may be initiated at these sites of closest membrane apposition.

Analysis of delay times of hemagglutinin-mediated fusion between influenza virus and cell membranes

We have studied the kinetics of low pH-induced fusion between influenza virus A/PR 8/34 and human erythrocyte membranes in suspension by using an assay based on fluorescence dequenching (FDQ) of the lipophilic dye octadecylrhodamine B chloride (R 18). As shown previously (Clague et al. 1991) the onset of FDQ is preceded by a characteristic lag time (tlag) following pH reduction. Whereas tlag represents only a subpopulation of fusing viruses with the shortest delay time we suggest here that a representative mean lag time mu lag of virus-cell fusion can be deduced from the R 18-assay. Kinetics of FDQ reflects the cumulative distribution function of lag times tau lag of single fusion events with the mean value mu lag. We show that tau lag obtained from the onset of FDQ does not always reflect the fusion behaviour of the whole population of fusing viruses. While both lag times, taulag and mu lag, exhibit a similar temperature dependence we found a significantly different dependence of both delay times on virus inactivation by low pH-pretreatment. We conclude that the mean lag time mu lag appears to be a more appropriate parameter describing the kinetics of virus-cell fusion. The analysis of delay times offers a new approach to test the validity of different kinetic models of HA-mediated fusion and to gain valuable information about HA-mediated fusion. The analysis confirms that the inactivation process proceeds via steps of the formation of the fusion pore. Although the increase of lag times can be explained by a depletion of fusion competent HA's, our data suggest that intermediate structures of HA along the inactivation pathway can still transform into a fusion site.

Control of baculovirus gp64-induced syncytium formation by membrane lipid composition

We have investigated the effects of membrane lipid composition on biological membrane fusion triggered by low pH and mediated by the baculovirus envelope glycoprotein gp64. Lysolipids, either added exogenously or produced in situ by phospholipase A2 treatment of cell membranes, reversibly inhibited syncytium formation. Lysolipids also decreased the baculovirus infection rate. In contrast, oleic and arachidonic acids and monoolein promoted cell-cell fusion. Membrane lipid composition affected pH-independent processes which followed the low-pH-induced change in fusion protein conformation. Inhibition and promotion of membrane fusion by a number of lipids could not be explained by mere binding or incorporation into membranes, but rather was correlated with the effective molecular shape of exogenous lipids. Our data are consistent with the hypothesis that membrane fusion proceeds through highly bent membrane intermediates (stalks) having a net negative curvature. Consequently, inverted cone-shaped lysolipids inhibit and cone-shaped cis-unsaturated fatty acids promote stalk formation and, ultimately, membrane fusion.

Transient domains induced by influenza haemagglutinin during membrane fusion

During low pH-induced fusion of influenza virus with erythrocytes we have observed differential dispersion of viral lipid and haemagglutinin (HA) into the erythrocyte membrane, and viral RNA into the erythrocyte using fluorescence video microscopy. The movement of both viral lipid and HA from virus to cell was restricted during the initial stages of fusion relative to free diffusion. This indicates the existence of relatively long-lived barriers to diffusion subsequent to fusion pore formation. Fluorescence anisotropy of phospholipid analogues incorporated into the viral membrane decreased when the pH was lowered to levels required for optimum fusion. This indicates that the restricted motion of viral membrane components was not due to rigidification of membrane lipids. The movement of HA from the fusion site was also assessed by photosensitized labelling by means of a fluorescent substrate (NBD-taurine) passing through the band 3 sialoglycoprotein (the erythrocyte anion transporter). We also examined the flow of lipid and aqueous markers during fusion of HA-expressing cells with labelled erythrocytes. During this cell-cell fusion, movement of lipid between fusing membranes begins before the fusion pore is wide enough to allow diffusion of aqueous molecules (M(r) > 500). The data indicate that HA is capable of creating domains in the membrane and controlling continuity of aqueous compartments which are bounded by such domains.

Restricted movement of lipid and aqueous dyes through pores formed by influenza hemagglutinin during cell fusion

The fusion of cells by influenza hemagglutinin (HA) is the best characterized example of protein-mediated membrane fusion. In simultaneous measurements of pairs of assays for fusion, we determined the order of detectable events during fusion. Fusion pore formation in HA-triggered cell-cell fusion was first detected by changes in cell membrane capacitance, next by a flux of fluorescent lipid, and finally by flux of aqueous fluorescent dye. Fusion pore conductance increased by small steps. A retardation of lipid and aqueous dyes occurred during fusion pore fluctuations. The flux of aqueous dye depended on the size of the molecule. The lack of movement of aqueous dyes while total fusion pore conductance increased suggests that initial HA-triggered fusion events are characterized by the opening of multiple small pores: the formation of a "sieve".

Kinetics of the low pH-induced conformational changes and fusogenic activity of influenza hemagglutinin

The decrease of the intrinsic tryptophan fluorescence intensity of purified influenza (X31 strain) hemagglutinin (HA) was used to monitor the low pH-induced conformational change of this protein. The kinetics of the fluorescence decrease depended strongly on the pH. At pH optimal for fusion, the change in tryptophan fluorescence was fast and could be fitted to a monoexponential function. We measured a rate constant of 5.78 s-1 (t1/2 = 120 ms) at pH 4.9 using rapid stopped-flow mixing. Under suboptimal conditions (higher pH), the rate constant was decreased by an order of magnitude. In addition, a slow component appeared and the fluorescence decrease followed a sum of two exponentials. The kinetics of conformational changes were compared with those of the fusion of influenza virus with red blood cell membranes as assessed by the R18-dequenching assay. At optimal pH the HA conformational change was not rate-limiting for the fusion process. However, at sub-optimal pH, the slow transition to the fusogenic conformational of HA resulted in slower kinetics and decreased extent of fusion.

A trans-dominant mutation in human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein gp41 inhibits membrane fusion when expressed in target cells

A recombinant vaccinia virus was used to express a mutation in the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein gp120-gp41. In this mutant protein, the second amino acid in the N-terminal region of gp41 has been converted from a hydrophobic valine residue to the polar glutamate. When recombinant vaccinia viruses encoding wild-type HIV-1 envelope glycoprotein infect a lymphocyte cell line lacking CD4, the cells express the HIV-1 envelope glycoprotein gp120-gp41 and are able to fuse with a CD4(4) T lymphocyte cell line. Cells expressing the mutant envelope glycoprotein are unable to fuse with CD4(4) T lymphocytes. When both viruses infect CD4- cells simultaneously, there is an inhibition of fusion to CD4+ cells with an increasing fraction of the virus encoding the mutated envelope glycoprotein. Interestingly, when the opposing, or CD4+ target cells are infected with the mutation-expressing virus, while CD4- cells are infected with wild-type envelope-expressing virus, a similar inhibition of fusion is observed. This suggests that the mutated envelope glycoprotein does not need to reside in the same membrane as the wild-type protein it inhibits.

pH-dependent binding of the fluorophore bis-ANS to influenza virus reflects the conformational change of hemagglutinin

Binding of the fluorophore 1,1'-bis(4-anilino)naphthalene-5,5'-disulfonic acid (bis-ANS) to influenza virus A/PR 8/34 is strongly enhanced at low pH. Binding is accompanied by a significant increase in fluorescence intensity. The binding and the fluorescence increase are associated with the low-pH induced conformational change of the viral spike protein, hemagglutinin, exposing hydrophobic binding sites. The data indicate that in addition to the hydrophobic N-terminus of HA2 other hydrophobic sequences of the HA ectodomain become accessible to bis-ANS at low pH. It is shown that the time course of the fluorescence increase of bis-ANS at low pH is determined by the conformational change of HA. The application of this assay for continuously monitoring the kinetics of the structural alteration in HA is discussed and its relevance for elucidating the temporal relationship between the conformational change of HA and virus-membrane fusion is outlined.

Enhancement of viral fusion by nonadsorbing polymers

Nonadsorbing polymers such as dextran and poly(ethylene glycol) enhance binding as well as extents of fusion of influenza virus with erythrocytes. Kinetics and extent of viral membrane fusion were measured using an assay based on lipid mixing of a fluorescent dye. The effects of nonadsorbing polymers were in the concentration range from 0 to 10 wt%, far below the concentration required to overcome hydration repulsion forces. The enhancing effects were dependent on the molecular weight of nonadsorbing polymer, and only occurred at molecular weight > 1500; this links the phenomena we observe to the so-called "excluded volume effect" of nonadsorbing polymers. The time delay between triggering and the onset of influenza virus fusion was significantly reduced in the presence of nonadsorbing polymers. High molecular weight poly(ethylene glycol) also induced fusion of vesicular stomatitis virus with intact erythrocytes, which do not serve as target of vesicular stomatitis virus fusion in the absence of the polymer. The forces between membranes which determine rate-limiting processes in viral fusion and how they are affected by nonadsorbing polymers are discussed.

Hemagglutinin-neuraminidase enhances F protein-mediated membrane fusion of reconstituted Sendai virus envelopes with cells

Reconstituted Sendai virus envelopes containing both the fusion (F) protein and the hemagglutinin-neuraminidase (HN) (F,HN-virosomes) or only the F protein (F-virosomes) were prepared by solubilization of the intact virus with Triton X-100 followed by its removal by using SM2 Bio-Beads. Viral envelopes containing HN whose disulfide bonds were irreversibly reduced (HNred) were also prepared by treating the envelopes with dithiothreitol followed by dialysis (F,HNred-virosomes). Both F-virosomes and F,HNred-virosomes induced hemolysis of erythrocytes in the presence of wheat germ agglutinin, but the rates and extents were markedly lower than those for hemolysis induced by F,HN-virosomes. Using an assay based on the relief of self-quenching of a lipid probe incorporated in the Sendai virus envelopes, we demonstrate the fusion of both F,HN-virosomes and F-virosomes with cultured HepG2 cells containing the asialoglycoprotein receptor, which binds to a terminal galactose moiety of F. By desialylating the HepG2 cells, the entry mediated by HN-terminal sialic acid receptor interactions was bypassed. We show that both F-virosomes and F,HN-virosomes fuse with desialylated HepG2 cells, although the rate was two- to threefold higher if HN was included in the viral envelope. We also observed enhancement of fusion rates when both F and HN envelope proteins were attached to their specific receptors.

Divalent cation-induced lipid mixing between phosphatidylserine liposomes studied by stopped-flow fluorescence measurements: effects of temperature, comparison of barium and calcium, and perturbation by DPX

To understand the mechanism of membrane fusion, it is important to study the processes that mix the lipids of two apposed membranes. We measured the rates of divalent cation-induced aggregation and lipid mixing of bovine brain phosphatidylserine (BBPS) LUV, using light scattering and a resonance energy transfer assay. The lipid and divalent cation solutions were combined by stopped-flow mixing, which permitted measuring the half-times of aggregation and lipid mixing between pairs of liposomes. The collisional quencher DPX [p-xylene-bis(pyridinium bromide)], used in a liposome contents-mixing assay, lowered the main transition temperature (Tm) of BBPS by about 10 degrees C and decreased the temperature threshold for lipid mixing. Since DPX was inside the liposomes for the latter measurements, this implies that perturbations to the inner monolayer affect the reactivity of the liposome. When palmitoyl-oleoyl-PS (POPS) was substituted for BBPS, little or no lipid mixing occurred. Ca(2+)- and Ba(2+)-induced BBPS aggregation and lipid mixing were compared as a function of temperature and divalent cation concentration. Aggregation rates were nearly insensitive to temperature and correlated with the percent of PS bound to either Ba2+ or Ca2+. Above Tm, lipid-mixing rates increased with the Ba2+ and Ca2+ concentrations and temperature, even above the Tm of the Ba2+/PS complex. Arrhenius plots were linear for both ions. The temperature dependence was greater for Ca(2+)- than Ba(2+)-induced reactions, and the slopes were independent of divalent cation concentration. When equivalent fractions of PS were bound with divalent cation at, and above, 20 degrees C, the lipid-mixing rate was greater with Ca2+ than with Ba2+. The faster rate may reflect greater activation entropies and/or greater attempt frequencies at one or more steps in the Ca(2+)-induced process. We conclude that stopped-flow mixing permits better characterization of initial interaction between liposomes, that small changes in the acyl chain region of the PS bilayer or the inner monolayer can have large effects on lipid-mixing rates, and that the differences between Ba(2+)- and Ca(2+)-induced interactions may be related to qualitative differences in the destabilization step.

Role of target membrane structure in fusion with influenza virus: effect of modulating erythrocyte transbilayer phospholipid distribution

To study the role of the target membrane in influenza virus fusion we chose erythrocyte membranes whose phospholipid arrangement can readily be modified. The phospholipids of normal erythrocytes are arranged asymmetrically across the plasma membrane; phosphatidylcholine (PC) and sphingomyelin are predominantly on the outer surface, whereas others such as phosphatidylserine (PS) and phosphatidylethanolamine (PE) are predominantly restricted to the inner leaflet. However, erythrocytes can be lyzed and resealed under conditions where the asymmetric distribution of phospholipids is lost or retained. Low pH-induced fusion of the A/PR 8 strain of influenza virus, monitored spectrofluorometrically by the octadecylrhodamine dequenching assay, was more rapid with lipid-symmetric erythrocyte ghosts than with lipid-asymmetric ghosts or intact erythrocytes. Neither conversion of PS in the lipid-symmetric ghost membrane to PE by means of the enzyme PS decarboxylaze, nor incorporation of spin-labeled phospholipid analogs with PS, PC or PE headgroups into the outer leaflet of lipid-asymmetric erythrocytes altered rates or extents of fusion of A/PR 8 with the modified target. These results indicate that effects on influenza virus fusion are not associated with any particular phospholipid headgroup, but rather related to the packing characteristics of the target membrane.

Dextran sulfate inhibits fusion of influenza virus and cells expressing influenza hemagglutinin with red blood cells

The influence of dextran sulfate with molecular weights of 500,000 and 8000 on binding and fusion of influenza virus (X31 strain) and of cells expressing influenza hemagglutinin (GP4F) with red blood cells (RBC) was investigated by spectrofluorimetry using virus and RBC labeled with the fluorescent dye octadecyl rhodamine B (R18). There was no significant inhibition of binding of virus and GP4F cells to red blood cells by dextran sulfate, but the polymer strongly inhibited the low pH induced fusion. Virus-RBC fusion was completely blocked by the high molecular weight dextran sulfate at concentrations as low as 0.5 mg/ml. Inhibition of RBC-GP4F cell fusion by dextran sulfate in the same concentration range was not as pronounced but the effect was potentiated by Ca2+. The polymer was only inhibitory when added at early steps of the fusion reaction, but the pH-induced conformational change of the hemagglutinin was not affected by dextran sulfate as measured by its susceptibility to proteolytic digestion. Removal of dextran sulfate after low pH-requiring steps allowed the system to fuse at neutral pH indicating that the inhibitory effect requires the continuous presence of dextran sulfate during the fusion reaction.

Intermediates and kinetics of membrane fusion

Recently, it has become clear that the influenza virus fusion protein, hemagglutinin (HA), produces membrane destabilization and fusion by a multistep process, which involves the aggregation of the HAs to form a fusion site. While the details of this process are under debate, it is important to recognize that proposing any sequence of "microscopic" fusion intermediates encumbers general "macroscopic" kinetic consequences, i.e., with respect to membrane mixing rates. Using a kinetic scheme which incorporates the essential elements of several recently proposed models, some of these measurable properties have been elucidated. First, a rigorous mathematical relationship between fusion intermediates and the fusion event itself is defined. Second, it is shown that what is measured as the macroscopic "fusion rate constant" is a simple function of all of the rate constants governing the transitions between intermediates, whether or not one of the microscopic steps is rate limiting. Third, while this kinetic scheme predicts a delay (or lag) time for fusion, as has been observed, it will be very difficult to extract reliable microscopic information from these data. Furthermore, it is predicted that the delay time can depend upon HA surface density even when the HA aggregation step is very rapid compared with fusion, i.e., the delay time need not be due to HA aggregation. Fourth, the inactivation process observed for influenza virions at low pH can be described within this kinetic scheme simply, yet rigorously, via the loss of the fusion intermediates. Fifth, predicted Arrhenius plots of fusion rates can be linear for this multistep scheme, even though there is no single rate-determining step and even when a branched step is introduced, i.e., where one pathway predominates at low temperature and the other pathway predominates at high temperature. Furthermore, the apparent activation energies obtained from these plots bear little or no quantitative resemblance to the microscopic activation energies used to simulate the data. Overall, these results clearly show that the intermediates of protein mediated fusion can be studied only by using assays sensitive to the formation of each proposed intermediate.