Conformational changes and fusion activity of vesicular stomatitis virus glycoprotein: [125I]iodonaphthyl azide photolabeling studies in biological membranes

Structural and cellular biology of rhabdovirus entry

Rhabdoviruses are enveloped viruses with a negative-sense single strand RNA genome and are widespread among a great variety of organisms. In their membrane, they have a single glycoprotein (G) that mediates both virus attachment to cellular receptors and fusion between viral and endosomal membranes allowing viral genome release in the cytoplasm. We present structural and cellular aspects of Rhabdovirus entry into their host cell with a focus on vesicular stomatitis virus (VSV) and rabies virus (RABV) for which the early events of the viral cycle have been extensively studied. Recent data have shown that the only VSV receptors are the members of the LDL-R family. This is in contrast with RABV for which multiple receptors belonging to unrelated families have been identified. Despite having different receptors, after attachment, rhabdovirus internalization occurs through clathrin-mediated endocytosis (CME) in an actin-dependent manner. There are still debates about the exact endocytic pathway of VSV in the cell and on RABV transport in the neuronal axon. In any case, fusion is triggered in the endosomal vesicle via a low-pH induced structural rearrangement of G from its pre- to its postfusion conformation. Vesiculovirus G is one of the best characterized fusion glycoproteins as the previously reported crystal structures of the pre- and postfusion states have been recently completed by those of intermediates during the structural transition. Understanding the entry pathway of rhabdoviruses may have strong impact in biotechnologies as, for example, VSV G is used for pseudotyping lentiviruses to promote efficient transduction, and VSV is a promising oncolytic virus.

Functional analysis of the Autographa californica multiple nucleopolyhedrovirus GP64 terminal fusion loops and interactions with membranes

The Autographa californica multiple nucleopolyhedrovirus (AcMNPV) glycoprotein GP64 is the major envelope protein of the budded virus (BV). GP64 is a class III fusion protein that mediates BV attachment to the cell surface and low-pH-triggered membrane fusion between the BV envelope and the endosome membrane during entry. Class III fusion proteins contain terminal looped structures that are believed to interact with membranes. To examine the functions of 3 loops found at the apex of the GP64 postfusion structure, we generated 2-alanine substitutions that scanned the two so-called fusion loops (loop 1 and loop 2) plus an adjacent loop structure (loop 3) that is closely attached to loop 2 and is also found at the apex of the GP64 postfusion structure. We identified essential residues from Y75 to T86 (loop 1) and N149 to H156 (loop 2) that are required for fusion activity, but no essential residues in loop 3. Further analysis revealed that critical fusion loop residues fall within two groups that are associated with either membrane merger (hemifusion) or fusion pore expansion. We next examined the interactions of soluble GP64 proteins and BV with membranes composed of various phospholipids. BV interacted directly with small unilamellar vesicles (SUVs) comprised of phospholipids phosphatidylcholine and phosphatidic acid (PC/PA) or phosphatidylcholine and phosphatidylserine (PC/PS) under neutral and acidic pH. We also examined the interactions of soluble GP64 constructs containing substitutions of the most hydrophobic residues within each of the two fusion loops. We found that a 2-residue substitution in either single loop (loop 1 [positions 81 and 82] or loop 2 [positions 153 and 154]) was not sufficient to substantially reduce the GP64-liposome interaction, but the same substitutions in both fusion loops severely reduced the GP64-liposome association at neutral pH. These results suggest that critical hydrophobic residues in both fusion loops may be involved in the interaction of GP64 with host cellular membranes and direct GP64-membrane interactions may represent a receptor-binding step prior to a low-pH-triggered conformational change.

Role of Abl kinase and the Wave2 signaling complex in HIV-1 entry at a post-hemifusion step

Entry of human immunodeficiency virus type 1 (HIV-1) commences with binding of the envelope glycoprotein (Env) to the receptor CD4, and one of two coreceptors, CXCR4 or CCR5. Env-mediated signaling through coreceptor results in Galphaq-mediated Rac activation and actin cytoskeleton rearrangements necessary for fusion. Guanine nucleotide exchange factors (GEFs) activate Rac and regulate its downstream protein effectors. In this study we show that Env-induced Rac activation is mediated by the Rac GEF Tiam-1, which associates with the adaptor protein IRSp53 to link Rac to the Wave2 complex. Rac and the tyrosine kinase Abl then activate the Wave2 complex and promote Arp2/3-dependent actin polymerization. Env-mediated cell-cell fusion, virus-cell fusion and HIV-1 infection are dependent on Tiam-1, Abl, IRSp53, Wave2, and Arp3 as shown by attenuation of fusion and infection in cells expressing siRNA targeted to these signaling components. HIV-1 Env-dependent cell-cell fusion, virus-cell fusion and infection were also inhibited by Abl kinase inhibitors, imatinib, nilotinib, and dasatinib. Treatment of cells with Abl kinase inhibitors did not affect cell viability or surface expression of CD4 and CCR5. Similar results with inhibitors and siRNAs were obtained when Env-dependent cell-cell fusion, virus-cell fusion or infection was measured, and when cell lines or primary cells were the target. Using membrane curving agents and fluorescence microscopy, we showed that inhibition of Abl kinase activity arrests fusion at the hemifusion (lipid mixing) step, suggesting a role for Abl-mediated actin remodeling in pore formation and expansion. These results suggest a potential utility of Abl kinase inhibitors to treat HIV-1 infected patients.

Photo-activation of the hydrophobic probe iodonaphthylazide in cells alters membrane protein function leading to cell death

Background

Photo-activation of the hydrophobic membrane probe 1, 5 iodonaphthylazide (INA) by irradiation with UV light (310–380 nm) results in the covalent modification of transmembrane anchors of membrane proteins. This unique selectivity of INA towards the transmembrane anchor has been exploited to specifically label proteins inserted in membranes. Previously, we have demonstrated that photo-activation of INA in enveloped viruses resulted in the inhibition of viral membrane protein-induced membrane fusion and viral entry into cells. In this study we show that photo-activation of INA in various cell lines, including those over-expressing the multi-drug resistance transporters MRP1 or Pgp, leads to cell death. We analyzed mechanisms of cell killing by INA-UV treatment. The effects of INA-UV treatment on signaling via various cell surface receptors, on the activity of the multi-drug resistance transporter MRP1 and on membrane protein lateral mobility were also investigated.

Results

INA treatment of various cell lines followed by irradiation with UV light (310–380 nm) resulted in loss of cell viability in a dose dependent manner. The mechanism of cell death appeared to be apoptosis as indicated by phosphatidylserine exposure, mitochondrial depolarization and DNA fragmentation. Inhibition by pan-caspase inhibitors and cleavage of caspase specific substrates indicated that at low concentrations of INA apoptosis was caspase dependent. The INA-UV treatment showed similar cell killing efficacy in cells over-expressing MRP1 function as control cells. Efflux of an MRP1 substrate was blocked by INA-UV treatment of the MRP1-overexpressing cells. Although INA-UV treatment resulted in inhibition of calcium mobilization triggered by chemokine receptor signaling, Akt phosphorylation triggered by IGF1 receptor signaling was enhanced. Furthermore, fluorescence recovery after photobleaching experiments indicated that INA-UV treatment resulted in reduced lateral mobility of a seven transmembrane G protein-coupled receptor.

Conclusion

INA is a photo-activable agent that induces apoptosis in various cancer cell lines. It reacts with membrane proteins to alter the normal physiological function resulting in apoptosis. This activity of INA maybe exploited for use as an anti-cancer agent.

Common principles and intermediates of viral protein-mediated fusion: the HIV-1 paradigm

Enveloped viruses encode specialized fusion proteins which promote the merger of viral and cell membranes, permitting the cytosolic release of the viral cores. Understanding the molecular details of this process is essential for antiviral strategies. Recent structural studies revealed a stunning diversity of viral fusion proteins in their native state. In spite of this diversity, the post-fusion structures of these proteins share a common trimeric hairpin motif in which the amino- and carboxy-terminal hydrophobic domains are positioned at the same end of a rod-shaped molecule. The converging hairpin motif, along with biochemical and functional data, implies that disparate viral proteins promote membrane merger via a universal "cast-and-fold" mechanism. According to this model, fusion proteins first anchor themselves to the target membrane through their hydrophobic segments and then fold back, bringing the viral and cellular membranes together and forcing their merger. However, the pathways of protein refolding and the mechanism by which this refolding is coupled to membrane rearrangements are still not understood. The availability of specific inhibitors targeting distinct steps of HIV-1 entry permitted the identification of key conformational states of its envelope glycoprotein en route to fusion. These studies provided functional evidence for the direct engagement of the target membrane by HIV-1 envelope glycoprotein prior to fusion and revealed the role of partially folded pre-hairpin conformations in promoting the pore formation.

The ESCRT-I subunit TSG101 controls endosome-to-cytosol release of viral RNA

Like other enveloped viruses, vesicular stomatitis virus infects cells through endosomes. There, the viral envelope undergoes fusion with endosomal membranes, thereby releasing the nucleocapsid into the cytoplasm and allowing infection to proceed. Previously, we reported that the viral envelope fuses preferentially with the membrane of vesicles present within multivesicular endosomes. Then, these intra-endosomal vesicles (containing nucleocapsids) are transported to late endosomes, where back-fusion with the endosome limiting membrane delivers the nucleocapsid into the cytoplasm. In this study, we show that the tumor susceptibility gene 101 (Tsg101) subunit of the endosomal sorting complexes required for transport (ESCRT)-I complex, which mediates receptor sorting into multivesicular endosomes, is dispensable for viral envelope fusion with endosomal membranes and viral RNA transport to late endosomes but is necessary for infection. Our data indicate that Tsg101, in contrast to the ESCRT-0 component Hrs, plays a direct role in nucleocapsid release from within multivesicular endosomes to the cytoplasm, presumably by controlling the back-fusion process. We conclude that Tsg101, through selective interactions with its partners including Hrs and Alix, may link receptor sorting and lysosome targeting to the back-fusion process involved in viral capsid release.

Photoinduced reactivity of the HIV-1 envelope glycoprotein with a membrane-embedded probe reveals insertion of portions of the HIV-1 Gp41 cytoplasmic tail into the viral membrane

The interactions of HIV-1 Env (gp120-gp41) with CD4 and coreceptors trigger a barrage of conformational changes in Env that drive the membrane fusion process. Various regions of gp41 have profound effects on HIV entry and budding. However, the precise interactions between gp41 and the membrane have not been elucidated. To examine portions of membrane proteins that are embedded in membrane lipids, we have studied photoinduced chemical reactions in membranes using the lipid bilayer specific probe iodonaphthyl azide (INA). Here we show that in addition to the transmembrane anchor, amphipatic sequences in the cytoplasmic tail (CT) of HIV-1 gp41 are labeled by INA. INA labeling of the HIV-1 gp41 CT was similar whether wild-type or a mutant HIV-1 was used with uncleaved p55 Gag, which does not allow entry. These results shed light on the disposition of the HIV-1 gp41 CT with respect to the membrane. Moreover, our data have general implications for topology of membrane proteins and their in situ interactions with the lipid bilayer.

HIV-1 envelope glycoprotein-mediated fusion and pathogenesis: implications for therapy and vaccine development

Our overall goal is to understand how viral envelope proteins mediate membrane fusion and pathogenesis. Membrane fusion is a crucial step in the delivery of the viral genome into the cell resulting in infection. On the other hand, fusion activity of viral envelope glycoproteins expressed in infected cells may cause the demise of uninfected bystander cells by apoptosis. Our general approach is to kinetically resolve steps in the pathway of viral envelope glycoprotein-mediated membrane fusion and to uncover physical parameters underlying those steps using a variety of biochemical, biophysical, virological, and molecular and cell biological techniques. Since HIV fusion involves a complex cascade of interactions of the envelope glycoprotein with two receptors, membrane organization plays an important role and interfering with it may modulate entry. To study this phenomenon, we have either examined cell lines with differential expression of sphingolipids (such as GM3), or altered membrane organization by modifying levels of cholesterol, ceramides, or glycosphingolipids. We show that the localized plasma membrane lipid microenvironment (and not the specific membrane lipids) in the vicinity of CD4 controls receptor mobility and HIV-1 fusion. The complex cascade of conformational changes that must occur to allow virus entry is also a very important target for therapy and vaccine development. We have recently designed and tested peptide analogs composed of chemical spacers and reactive moieties positioned strategically to promote permanent attachment. Using a temperature-arrested state in vitro assay we show evidence for the trapping of a pre-six-helix bundle fusion intermediate by a covalent reaction with the inhibitory reactive peptide. Also, using photo-reactive hydrophobic probes we have found ways to inactivate viral envelope glycoproteins while leaving their overall structures intact. Finally, in order to study the envelope glycoprotein effects on pathogenesis, we have used an in vitro model of co-culture of envelope-expressing cells as effectors and CD4+ T cells as targets. We delineated that apoptosis mediated by envelope glycoprotein in bystander cells correlates with transmembrane subunit (gp41)-induced hemifusion. The apoptotic pathway initiated by this interaction involves caspase-3-dependent mitochondrial depolarization and reactive oxygen species production, which depends on the phenotype of the envelope glycoprotein associated with the virus. Taken as a whole, our studies have many different important implications for antiviral therapies and vaccine development.

Molecular architecture of the bipartite fusion loops of vesicular stomatitis virus glycoprotein G, a class III viral fusion protein

The glycoprotein of vesicular stomatitis virus (VSV G) mediates fusion of the viral envelope with the host cell, with the conformational changes that mediate VSV G fusion activation occurring in a reversible, low pH-dependent manner. Based on its novel structure, VSV G has been classified as class III viral fusion protein, having a predicted bipartite fusion domain comprising residues Trp-72, Tyr-73, Tyr-116, and Ala-117 that interacts with the host cell membrane to initiate the fusion reaction. Here, we carried out a systematic mutagenesis study of the predicted VSV G fusion loops, to investigate the functional role of the fusion domain. Using assays of low pH-induced cell-cell fusion and infection studies of mutant VSV G incorporated into viral particles, we show a fundamental role for the bipartite fusion domain. We show that Trp-72 is a critical residue for VSV G-mediated membrane fusion. Trp-72 could only tolerate mutation to a phenylalanine residue, which allowed only limited fusion. Tyr-73 and Tyr-116 could be mutated to other aromatic residues without major effect but could not tolerate any other substitution. Ala-117 was a less critical residue, with only charged residues unable to allow fusion activation. These data represent a functional analysis of predicted bipartite fusion loops of VSV G, a founder member of the class III family of viral fusion proteins.

Aggregation and hemi-fusion of anionic vesicles induced by the antimicrobial peptide cryptdin-4

We show that cryptdin-4 (Crp4), an antimicrobial peptide found in mice, induces the aggregation and hemi-fusion of charged phospholipid vesicles constructed of the anionic lipid POPG and the zwitterionic lipid POPC. Hemi-fusion is confirmed with positive total lipid-mixing assay results, negative inner monolayer lipid-mixing assay results, and negative results from contents-mixing assays. Aggregation, as quantified by absorbance and dynamic light scattering, is self-limiting, creating finite-sized vesicle assemblies. The rate limiting step in the formation process is the mixing of juxtaposed membrane leaflets, which is regulated by bound peptide concentration as well as vesicle radius (with larger vesicles less prone to hemi-fusion). Bound peptide concentration is readily controlled by total peptide concentration and the fraction of anionic lipid in the vesicles. As little as 1% PEGylated lipid significantly reduces aggregate size by providing a steric barrier for membrane apposition. Finally, as stable hemi-fusion is a rare occurrence, we compare properties of Crp4 to those of many peptides known to induce complete fusion and lend insight into conditions necessary for this unusual type of membrane merger.

Structure of the prefusion form of the vesicular stomatitis virus glycoprotein G

Glycoprotein G of the vesicular stomatitis virus triggers membrane fusion via a low pH-induced structural rearrangement. Despite the equilibrium between the pre- and postfusion states, the structure of the prefusion form, determined to 3.0 angstrom resolution, shows that the fusogenic transition entails an extensive structural reorganization of G. Comparison with the structure of the postfusion form suggests a pathway for the conformational change. In the prefusion form, G has the shape of a tripod with the fusion loops exposed, which point toward the viral membrane, and with the antigenic sites located at the distal end of the molecule. A large number of G glycoproteins, perhaps organized as in the crystals, act cooperatively to induce membrane merging.

Charged residues are involved in membrane fusion mediated by a hydrophilic peptide located in vesicular stomatitis virus G protein

Membrane fusion is an essential step of the internalization process of the enveloped animal viruses. Vesicular stomatitis virus (VSV) infection is mediated by virus spike glycoprotein G, which induces membrane fusion at the acidic environment of the endosomal compartment. In a previous work, we identified a specific sequence in VSV G protein, comprising the residues 145 to 164, directly involved in membrane interaction and fusion. Unlike fusion peptides from other viruses, this sequence is very hydrophilic, containing six charged residues, but it was as efficient as the virus in catalyzing membrane fusion at pH 6.0. Using a carboxyl-modifying agent, dicyclohexylcarbodiimide (DCCD), and several synthetic mutant peptides, we demonstrated that the negative charges of peptide acidic residues, especially Asp153 and Glu158, participate in the formation of a hydrophobic domain at pH 6.0, which is necessary to the peptide-induced membrane fusion. The formation of the hydrophobic region and the membrane fusion itself were dependent on peptide concentration in a higher than linear fashion, suggesting the involvement of peptide oligomerization. His148 was also necessary to hydrophobicity and fusion, suggesting that peptide oligomerization occurs through intermolecular electrostatic interactions between the positively-charged His and a negatively-charged acidic residue of two peptide molecules. Oligomerization of hydrophilic peptides creates a hydrophobic region that is essential for the interaction with the membrane that results in fusion.

Identification of PhIL1, a novel cytoskeletal protein of the Toxoplasma gondii pellicle, through photosensitized labeling with 5-[125I]iodonaphthalene-1-azide

The pellicle of the protozoan parasite Toxoplasma gondii is a unique triple bilayer structure, consisting of the plasma membrane and two tightly apposed membranes of the underlying inner membrane complex. Integral membrane proteins of the pellicle are likely to play critical roles in host cell recognition, attachment, and invasion, but few such proteins have been identified. This is in large part because the parasite surface is dominated by a family of abundant and highly immunogenic glycosylphosphatidylinositol (GPI)-anchored proteins, which has made the identification of non-GPI-linked proteins difficult. To identify such proteins, we have developed a radiolabeling approach using the hydrophobic, photoactivatable compound 5-[(125)I]iodonaphthalene-1-azide (INA). INA can be activated by photosensitizing fluorochromes; by restricting these fluorochromes to the pellicle, [(125)I]INA labeling will selectively target non-GPI-anchored membrane-embedded proteins of the pellicle. We demonstrate here that three known membrane proteins of the pellicle can indeed be labeled by photosensitization with INA. In addition, this approach has identified a novel 22-kDa protein, named PhIL1 (photosensitized INA-labeled protein 1), with unexpected properties. While the INA labeling of PhIL1 is consistent with an integral membrane protein, the protein has neither a transmembrane domain nor predicted sites of lipid modification. PhIL1 is conserved in apicomplexan parasites and localizes to the parasite periphery, concentrated at the apical end just basal to the conoid. Detergent extraction and immunolocalization data suggest that PhIL1 associates with the parasite cytoskeleton.

Crystal structure of the low-pH form of the vesicular stomatitis virus glycoprotein G

The vesicular stomatitis virus has an atypical membrane fusion glycoprotein (G) exhibiting a pH-dependent equilibrium between two forms at the virus surface. Membrane fusion is triggered during the transition from the high- to low-pH form. The structure of G in its low-pH form shows the classic hairpin conformation observed in all other fusion proteins in their postfusion conformation, in spite of a novel fold combining features of fusion proteins from classes I and II. The structure provides a framework for understanding the reversibility of the G conformational change. Unexpectedly, G is homologous to gB of herpesviruses, which raises important questions on viral evolution.

pH-dependence of intermediate steps of membrane fusion induced by the influenza fusion peptide

Membrane fusion mediated by the influenza-virus fusion protein is activated by low pH via a cascade of reactions. Some processes among them are irreversible, such as helix hairpin formation of the ectodomain, whereas others are reversible, such as exposure of the fusion peptide. Using this property, we attempted to dissect, in temporal order, different stages of the fusion reaction involving the fusion peptide by an acidic-neutral-acidic pH cycle. The fluorescence-quenching data indicated that both insertion depth and self-assembly are pH-reversible. In addition, lipid mixing assay was demonstrated to be arrested by neutral pH. By contrast, membrane leakage was shown to be irreversible with respect to pH. Our results, along with those from other studies on the pH-dependence of membrane fusion, are used to build a model for the virus-mediated fusion event from the perspective of pH-reversibility.

Endocytosis and a low-pH step are required for productive entry of equine infectious anemia virus

Recently, it has become evident that entry of some retroviruses into host cells is dependent upon a vesicle-localized, low-pH step. The entry mechanism of equine infectious anemia virus (EIAV) has yet to be examined. Here, we demonstrate that wild-type strains of EIAV require a low-pH step for productive entry. Lysosomotropic agents that inhibit the acidification of internal vesicles inhibited productive entry of EIAV. The presence of ammonium chloride (30 mM), monensin (30 microM), or bafilomycin A (50 nM) in the medium dramatically decreased the number of EIAV antigen-positive cells. We found that a low-pH step was required for EIAV infection of tissue culture cell lines as well as primary cells, such as endothelial cells and monocyte-derived macrophages. The ammonium chloride treatment did not reduce virion stability, nor did the treatment prevent virion binding to cells. Consistent with a requirement for a low-pH step, virion infectivity was enhanced more than threefold by brief low-pH treatment following binding of viral particles to permissive cells. A superinfecting variant strain of EIAV, vMA-1c, did not require a low-pH step for productive infection of fibroblasts. However, lysosomotropic agents were inhibitory to vMA-1c infection in the other cell types that vMA-1c infected but did not superinfect, indicating that the entry pathway used by vMA-1c for superinfection abrogates the need for the low-pH step.

Probing the interaction between vesicular stomatitis virus and phosphatidylserine

The entry of enveloped animal viruses into their host cells always depends on membrane fusion triggered by conformational changes in viral envelope glycoproteins. Vesicular stomatitis virus (VSV) infection is mediated by virus spike glycoprotein G, which induces membrane fusion between the viral envelope and the endosomal membrane at the acidic environment of this compartment. In this work, we evaluated VSV interactions with membranes of different phospholipid compositions, at neutral and acidic pH, using atomic force microscopy (AFM) operating in the force spectroscopy mode, isothermal calorimetry (ITC) and molecular dynamics simulation. We found that the binding forces differed dramatically depending on the membrane phospholipid composition, revealing a high specificity of G protein binding to membranes containing phosphatidylserine (PS). In a previous work, we showed that the sequence corresponding amino acid 164 of VSV G protein was as efficient as the virus in catalyzing membrane fusion at pH 6.0. Here, we used this sequence to explore VSV-PS interaction using ITC. We found that peptide binding to membranes was exothermic, suggesting the participation of electrostatic interactions. Peptide-membrane interaction at pH 7.5 was shown to be specific to PS and dependent on the presence of His residues in the fusion peptide. The application of the simplified continuum Gouy-Chapman theory to our system predicted a pH of 5.0 at membrane surface, suggesting that the His residues should be protonated when located close to the membrane. Molecular dynamics simulations suggested that the peptide interacts with the lipid bilayer through its N-terminal residues, especially Val(145) and His(148).

Endosome-to-cytosol transport of viral nucleocapsids

During viral infection, fusion of the viral envelope with endosomal membranes and nucleocapsid release were thought to be concomitant events. We show here that for the vesicular stomatitis virus they occur sequentially, at two successive steps of the endocytic pathway. Fusion already occurs in transport intermediates between early and late endosomes, presumably releasing the nucleocapsid within the lumen of intra-endosomal vesicles, where it remains hidden. Transport to late endosomes is then required for the nucleocapsid to be delivered to the cytoplasm. This last step, which initiates infection, depends on the late endosomal lipid lysobisphosphatidic acid (LBPA) and its putative effector Alix/AIP1, and is regulated by phosphatidylinositol-3-phosphate (PtdIns3P) signalling via the PtdIns3P-binding protein Snx16. We conclude that the nucleocapsid is exported into the cytoplasm after the back-fusion of internal vesicles with the limiting membrane of late endosomes, and that this process is controlled by the phospholipids LBPA and PtdIns3P and their effectors.

Viral membrane fusion: is glycoprotein G of rhabdoviruses a representative of a new class of viral fusion proteins?

Enveloped viruses always gain entry into the cytoplasm by fusion of their lipid envelope with a cell membrane. Some enveloped viruses fuse directly with the host cell plasma membrane after virus binding to the cell receptor. Other enveloped viruses enter the cells by the endocytic pathway, and fusion depends on the acidification of the endosomal compartment. In both cases, virus-induced membrane fusion is triggered by conformational changes in viral envelope glycoproteins. Two different classes of viral fusion proteins have been described on the basis of their molecular architecture. Several structural data permitted the elucidation of the mechanisms of membrane fusion mediated by class I and class II fusion proteins. In this article, we review a number of results obtained by our laboratory and by others that suggest that the mechanisms involved in rhabdovirus fusion are different from those used by the two well-studied classes of viral glycoproteins. We focus our discussion on the electrostatic nature of virus binding and interaction with membranes, especially through phosphatidylserine, and on the reversibility of the conformational changes of the rhabdovirus glycoprotein involved in fusion. Taken together, these data suggest the existence of a third class of fusion proteins and support the idea that new insights should emerge from studies of membrane fusion mediated by the G protein of rhabdoviruses. In particular, the elucidation of the three-dimensional structure of the G protein or even of the fusion peptide at different pH's might provide valuable information for understanding the fusion mechanism of this new class of fusion proteins.

The energetics of membrane fusion from binding, through hemifusion, pore formation, and pore enlargement

The main steps of viral membrane fusion are local membrane approach, hemifusion, pore formation, and pore enlargement. Experiments and theoretical analyses have helped determine the relative energies required for each step. Key protein structures and conformational changes of the fusion process have been identified. The physical deformations of monolayer bending and lipid tilt have been applied to the steps of membrane fusion. Experiment and theory converge to strongly indicate that, contrary to former conceptions, the fusion process is progressively more energetically difficult: hemifusion has a relatively low energy barrier, pore formation is more energy-consuming, and pore enlargement is the most difficult to achieve.

Biological differences between vesicular stomatitis virus Indiana and New Jersey serotype glycoproteins: identification of amino acid residues modulating pH-dependent infectivity

We previously generated recombinant vesicular stomatitis viruses (VSV) based on the Indiana serotype genome which contained either the homologous glycoprotein gene from the Indiana serotype (VSIV-GI) or the heterologous glycoprotein gene from the New Jersey serotype (VSIV-GNJ). The virus expressing the GNJ gene was more pathogenic than the parental VSIV-GI virus in swine, a natural host (26). For the present study, we investigated the biological differences between the GI and GNJ proteins that may be related to the differences in pathogenesis between VSIV-GI and VSIV-GNJ. We show that the capacities of viruses with either the GNJ or GI glycoprotein to infect cultured cells differ depending on the pH. VSIV-GNJ could infect cells at acidic pHs, while the infectivity of VSIV-GI was severely reduced. VSIV-GNJ infection was also more sensitive to inhibition by ammonium chloride, indicating that the GNJ protein had a lower pH threshold for membrane fusion. We applied selective pressure to VSIV-GI by growing it at successively lower pH values and isolated variant viruses in which we identified amino acid changes that conferred low-pH-resistant infectivity. Repeated passage in cell culture at pH 6.8 resulted in the selection of a VSIV-GI variant (VSIV-6.8) that was similar to VSIV-GNJ regarding its pH- and ammonium chloride-dependent infectivity. Sequence analysis of VSIV-6.8 revealed that it had a single amino acid substitution in the amino-terminal region of the glycoprotein (F18L). This alteration was shown to be responsible for the observed phenotype by site-directed mutagenesis of a VSIV-GI full-length cDNA and analysis of the recovered engineered virus. A further adaptation of VSIV-6.8 to pHs 6.6 and 6.4 resulted in additional amino acid substitutions in areas of the glycoprotein that were not previously implicated in attachment or fusion.

Virus entry: molecular mechanisms and biomedical applications

Viruses have evolved to enter cells from all three domains of life--Bacteria, Archaea and Eukaryotes. Of more than 3,600 known viruses, hundreds can infect human cells and most of those are associated with disease. To gain access to the cell interior, animal viruses attach to host-cell receptors. Advances in our understanding of how viral entry proteins interact with their host-cell receptors and undergo conformational changes that lead to entry offer unprecedented opportunities for the development of novel therapeutics and vaccines.

The membrane-proximal region of vesicular stomatitis virus glycoprotein G ectodomain is critical for fusion and virus infectivity

The glycoprotein (G) of vesicular stomatitis virus (VSV) is responsible for binding of virus to cells and for mediating virus entry following endocytosis by inducing fusion of the viral envelope with the endosomal membrane. The fusion peptide of G is internal (residues 116 to 137) and exhibits characteristics similar to those of other internal fusion peptides, but recent studies have implicated the region adjacent to the transmembrane domain as also being important for G-mediated membrane fusion. Sequence alignment of the membrane-proximal region of G from several different vesiculoviruses revealed that this domain is highly conserved, suggesting that it is important for G function. Mutational analysis was used to show that this region is not essential for G protein oligomerization, transport to the cell surface, or incorporation into virus particles but that it is essential for acid-induced membrane fusion activity and for virus infectivity. Deletion of the 13 membrane-proximal amino acids (N449 to W461) dramatically reduced cell-cell fusion activity and reduced virus infectivity approximately 100-fold, but mutation of conserved aromatic residues (W457, F458, and W461) either singly or together had only modest effects on cell-cell fusion activity; recombinant virus encoding these mutants replicated as efficiently as wild-type (WT) VSV. Insertion of heterologous sequences in the juxtamembrane region completely abolished membrane fusion activity and virus infectivity, as did deletion of residues F440 to N449. The insertion mutants showed some changes in pH-dependent conformational changes and in virus binding, which could partially explain the defects in membrane fusion activity, but all the other mutants were similar to WT G with respect to conformational changes and virus binding. These data support the hypothesis that the membrane-proximal domain contributes to G-mediated membrane fusion activity, yet the conserved aromatic residues are not essential for membrane fusion or virus infectivity.

Further characterization of the rabies virus glycoproteins produced by virus-infected and G cDNA-transfected cells using a monoclonal antibody, #1-30-44, which recognizes an acid-sensitive epitope

Expression of rabies virus glycoprotein (G) by G cDNA-transfected mammalian cells resulted in the production of only a fusion-negative form. Low pH-dependent fusion activity, however, was seen when the expression was done under control of the T7 promoter with the help of recombinant vaccinia virus (RVV-T7) that provided T7 RNA polymerase. Fusion-inactive G proteins were transported to the cell surface as being detected by a conformational epitope-specific monoclonal antibody (mAb; #1-46-12). The fusion-inactive G proteins were recognized by most of our 13 conformation-specific mAbs, except for one mAb, #1-30-44, that recognized the low pH-sensitive conformational epitope. When the G gene expression was done with the help of RVV-T7, although most G proteins remained in the epitope-negative form, a small fraction of G gene products were 1-30-44 epitope-positive, and cell fusion activity could be seen when cells were exposed to low pH conditions. From these results, we conclude that acquisition of low pH-dependent fusion activity is closely related to structural maturation of the G protein to form the low pH-sensitive 1-30-44 epitope. Such maturation seems to be dependent on certain rabies virus-induced cellular conditions or functions, which might also be provided in part by the vaccinia virus infection. We further assume that expression of G cDNA alone mostly results in the production of mis-folded and/or differently folded forms of G protein, and only a small fraction is correctly folded even under RVV-T7-mediated expression conditions.

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.

Membrane fusion activity of vesicular stomatitis virus glycoprotein G is induced by low pH but not by heat or denaturant

The fusogenic envelope glycoprotein G of the rhabdovirus vesicular stomatitis virus (VSV) induces membrane fusion at acidic pH. At acidic pH the G protein undergoes a major structural reorganization leading to the fusogenic conformation. However, unlike other viral fusion proteins, the low-pH-induced conformational change of VSV G is completely reversible. As well, the presence of an alpha-helical coiled-coil motif required for fusion by a number of viral and cellular fusion proteins was not predicted in VSV G protein by using a number of algorithms. Results of pH dependence of the thermal stability of G protein as determined by intrinsic Trp fluorescence and circular dichroism (CD) spectroscopy show that the G protein is equally stable at neutral or acidic pH. Destabilization of G structure at neutral pH with either heat or urea did not induce membrane fusion or conformational change(s) leading to membrane fusion. Taken together, these data suggest that the mechanism of VSV G-induced fusion is distinct from the fusion mechanism of fusion proteins that involve a coiled-coil motif.

A photochemical approach to the lipid accessibility of engineered cysteinyl residues

Ordinary electrophilic reagents react too slowly in a nonpolar environment to be useful for the determination of the accessibility to lipid of continuous stretches of residues mutated to cysteine. By contrast, photoactivated 5-iodonaphthyl-1-azide (INA) reacted readily with 2-mercaptoethanol and dodecanethiol in nonpolar solvents and in liposomes. Continuous stretches of residues in the amphipathic N-terminal helix and first transmembrane helix of the bacterial potassium channel KcsA were replaced with cysteine, and the mutants were expressed in Escherichia coli and isolated in inner membranes. These membranes were dissolved in detergent and reconstituted into asolectin liposomes incorporating INA. The extent of light-induced reaction of INA with each cysteine was assayed by subsequent reaction with the gel-shifting, SH-specific methoxy-polyethylene glycol-2-pyridine disulfide. The pattern of apparent second-order rate constants for the photoreactions of eight substituted cysteines in the N-terminal helix conformed to other measures of lipid exposure. The pattern of the rate constants for the photoreactions of 15 cysteines in the first transmembrane helix had peaks every third residue, which partly conformed to other measures of lipid exposure.

Fluorescent lipid probes in the study of viral membrane fusion

Fluorescent lipid probes are widely used in the observation of viral membrane fusion, providing a sensitive method to study fusion mechanism(s). Due to the wealth of data concerning liposome fusion, a variety of fusion assays has been designed including fluorescent probe redistribution, fluorescence dequenching, fluorescence resonance energy transfer and photosensitized labeling. These methods can be tailored for different virus fusion assays. For instance, virions can be loaded with membrane dye which dequenches at the moment of membrane merger. This allows for continuous observation of fusion and therefore kinetic information can be acquired. In the case of cells expressing viral envelope proteins, dye redistribution studies of lipidic and water-soluble fluorophores yield information about fusion intermediates. Lipid probes can be metabolically incorporated into cell membranes, allowing observation of membrane fusion in vitro with minimal chance of flip flop, non-specific transfer and formation of microcrystals. Fluorescent lipid probes have been incorporated into liposomes and/or reconstituted viral envelopes, which provide a well-defined membrane environment for fusion to occur. Interactions of the viral fusion machinery with the membrane can be observed through the photosensitized labeling of the interacting segments of envelope proteins with a hydrophobic probe. Thus, fluorescent lipid probes provide a broad repertoire of fusion assays and powerful tools to produce precise, quantitative data in real time required for the elucidation of the complex process of viral fusion.

Characterization of the equilibrium between the native and fusion-inactive conformation of rabies virus glycoprotein indicates that the fusion complex is made of several trimers

Rabies virus-induced membrane fusion is triggered at low pH and is mediated by the trimeric viral glycoprotein (G). G assumes three conformations: the native state (N) detected above pH 7; the activated state (A), which initiates the fusion process; and the fusion-inactive conformation (I) observed after prolonged incubation at low pH. Differently from other viral fusogenic glycoproteins, G in the I state recovers its native conformation when reincubated above pH 7. Here, we demonstrate that there is a thermodynamic equilibrium between the different states of G between pH 6 and pH 7.5. The study of this equilibrium at various pH values indicated that the conformational change toward I is induced by the protonation of at least three residues per trimer. Finally, studies on the mechanism leading to low pH induced fusion inactivation indicated that a large number of G molecules is required for stable hydrophobic interaction of the virus with the target membrane.

Quantitative measurement of fusion of HIV-1 and SIV with cultured cells using photosensitized labeling

The fusion of HIV and SIV with biological membranes was studied by photosensitized activation of a hydrophobic probe, [(125)I]iodonaphthylazide ([(125)I]INA), by a fluorescent lipid which is situated in the target membrane. Photosensitized labeling of viral envelope-resident proteins occurs only upon their insertion into target membranes. Photosensitized labeling as a result of HIV-1 Env-mediated cell fusion showed the same kinetics as aqueous dye transfer. We have for the first time measured kinetics of HIV and SIV virus-cell fusion. HIV-1(MN) virions were about 10x less fusion active than SIVmne virions. SIV inactivated by aldrithiol-2 retained fusion activity similar to that seen with untreated virus. The relatively slow time course of SIV-cell fusion (t(1/2) = 19 min) indicates that the fusion events are stochastic. This feature provides a basis for understanding the mode of action of HIV/SIV entry inhibitors that target transition states.

Varicella-zoster Virus gB and gE coexpression, but not gB or gE alone, leads to abundant fusion and syncytium formation equivalent to those from gH and gL coexpression

Varicella-zoster virus (VZV) is distinguished from herpes simplex virus type 1 (HSV-1) by the fact that cell-to-cell fusion and syncytium formation require only gH and gL within a transient-expression system. In the HSV system, four glycoproteins, namely, gH, gL, gB, and gD, are required to induce a similar fusogenic event. VZV lacks a gD homologous protein. In this report, the role of VZV gB as a fusogen was investigated and compared to the gH-gL complex. First of all, the VZV gH-gL experiment was repeated under a different set of conditions; namely, gH and gL were cloned into the same vaccinia virus (VV) genome. Surprisingly, the new expression system demonstrated that a recombinant VV-gH+gL construct was even more fusogenic than seen in the prior experiment with two individual expression plasmids containing gH and gL (K. M. Duus and C. Grose, J. Virol. 70:8961-8971, 1996). Recombinant VV expressing VZV gB by itself, however, effected the formation of only small syncytia. When VZV gE and gB genes were cloned into one recombinant VV genome and another fusion assay was performed, extensive syncytium formation was observed. The degree of fusion with VZV gE-gB coexpression was comparable to that observed with VZV gH-gL: in both cases, >80% of the cells in a monolayer were fused. Thus, these studies established that VZV gE-gB coexpression greatly enhanced the fusogenic properties of gB. Control experiments documented that the fusion assay required a balance between the fusogenic potential of the VZV glycoproteins and the fusion-inhibitory effect of the VV infection itself.

Evolution of intermediates of influenza virus hemagglutinin-mediated fusion revealed by kinetic measurements of pore formation

Cells expressing wild-type influenza virus hemagglutinin (HA) or HA with a point mutation within the transmembrane domain (G520L) were bound to red blood cells and exposed to low pH for short times at suboptimal temperatures followed by reneutralization. This produced intermediate states of fusion. The ability of intermediate states to proceed on to fusion when temperature was raised was compared kinetically. In general, for wild-type HA, fusion occurred more quickly by directly lowering pH at 37 degrees C in the bound state than by raising temperature at the intermediate stage. When pH was lowered for 1-2 min, kinetics of fusion upon raising temperature of an intermediate slowed the longer the intermediate was maintained at neutral pH. But for a more sustained (10 min) acidification, kinetics was independent of the time the intermediate was held at neutral pH before triggering fusion by raising temperature. In contrast, generating intermediates in the same way with G520L yielded kinetics of fusion that did not depend on the time intermediates were maintained after reneutralization. For both HA and G520L, the extents of fusion did not depend on the temperature at which pH was lowered, but fusion from the intermediate was extremely sensitive to the temperature to which the cells were raised. The measured kinetics and temperature dependencies suggest that the rate-limiting step of fusion occurs subsequent to formation of any of the intermediates; the conformational change of HA into its final configuration may be the rate-limiting step.

Low pH-induced conformational changes in vesicular stomatitis virus glycoprotein involve dramatic structure reorganization

Membrane fusion is the key step in the entry of enveloped animal viruses into their host cells. Fusion of vesicular stomatitis virus with membranes occurs at acidic pH and is mediated by its envelope glycoprotein, the G protein. To study the structural transitions induced by acidic pH on G protein, we have extracted the protein from purified virus by incubation with nonionic detergent. At pH 6.0, purified G protein was able to mediate fusion of either phospholipid vesicles or Vero cells in culture. Intrinsic fluorescence studies revealed that changes in the environment of Trp residues occurred as pH decreases. In the absence of lipidic membranes, acidification led to G protein aggregation, whereas protein-protein interactions were substituted by protein-lipid interactions in the presence of liposomes. 1,1'-Bis(4-aniline-5-naphthalene sulfonate) (bis-ANS) binding was utilized to probe the degree of exposure of hydrophobic regions of G protein during acidification. Bis-ANS binding was maximal at pH 6.2, suggesting that a hydrophobic segment is exposed to the medium at this pH. At pH 6.0, a dramatic decrease in bis-ANS binding was observed, probably due to loss of tridimensional structure during the conformational rearrangement. This hypothesis was confirmed by circular dichroism analysis at different pH values, which showed a great decrease in alpha-helix content at pH values close to 6.0, suggesting that a reorganization of G protein secondary structure occurs during the fusion reaction. Our results indicate that G protein undergoes dramatic structural changes at acidic pH and acquires a conformational state able to interact with the target membrane.

Membrane fusion by an RGD-containing sequence from the core protein VP3 of hepatitis A virus and the RGA-analogue: implications for viral infection

The interaction of an RGD-containing epitope from the hepatitis A virus VP3 capsid protein and its RGA-analogue with lipid membranes was studied by biophysical methods. Two types of model membrane were used: vesicles and monolayers spread at the air/water interface, with a composition that closely resembles the lipid moiety of hepatocyte membranes: PC/SM/PE/PC (40:33:12:15; PC: 1-palmitoyl-2-oleoylglycero-sn-3-phosphocholine; SM: sphingomyelin from chicken egg yolk; PE, 1,2-dipalmitoyl-phosphatidylethanolamine; PS: L-alpha-phosphatidyl-L-serine from bovine brain). In addition, zwitterionic PC/SM/PE (47:39:14) and cationic PC/SM/PE/DOTAP (40:33:12:15; DOTAP: 1,2-dioleoyl-3-trimethylammonium-propane) membranes were also prepared in order to dissect the electrostatic and hydrophobic components in the interaction. Changes in tryptophan fluorescence, acrylamide quenching, and resonance energy transfer experiments in the presence of vesicles, as well as the kinetics of insertion in monolayers, indicate that both peptides bind to the three types of membrane at neutral and acidic pH; however, binding is irreversible only at low pH. Membrane-destabilizing and fusogenic activities are triggered by acidification at pH 4-6, characteristic of the endosome. Fluorescence experiments show that VP3-RGD and VP3-RGA induce mixing of lipids and leakage or mixing of aqueous contents in anionic and cationic vesicles at pH 4-6, indicating leaky fusion. Interaction with zwitterionic vesicles (PC/SM/PE) results in leakage without lipid mixing, indicating pore formation. Replacement of aspartic acid in the RGD motif by alanine maintains the membrane-destabilizing properties of the peptide at low pH, but not its antigenicity. Since the RGD tripeptide is related to receptor-mediated cell adhesion and antigenicity, results suggest that receptor binding is not a molecular requirement for fusion. The possible involvement of peptide-induced membrane destabilization in the mechanism of hepatitis A virus infection of hepatocytes by the endosomal route is discussed.

Rabies virus-induced membrane fusion pathway

Fusion of rabies virus with membranes is triggered at low pH and is mediated by the viral glycoprotein (G). The rabies virus-induced fusion pathway was studied by investigating the effects of exogenous lipids having various dynamic molecular shapes on the fusion process. Inverted cone-shaped lysophosphatidylcholines (LPCs) blocked fusion at a stage subsequent to fusion peptide insertion into the target membrane. Consistent with the stalk-hypothesis, LPC with shorter alkyl chains inhibited fusion at lower membrane concentrations and this inhibition was compensated by the presence of oleic acid. However, under suboptimal fusion conditions, short chain LPCs, which were translocated in the inner leaflet of the membranes, considerably reduced the lag time preceding membrane merging, resulting in faster kinetics of fusion. This indicated that the rate limiting step for fusion is the formation of a fusion pore in a diaphragm of restricted hemifusion. The previously described cold-stabilized prefusion complex was also characterized. This intermediate is at a well-advanced stage of the fusion process when the hemifusion diaphragm is destabilized, but lipid mixing is still restricted, probably by a ring-like complex of glycoproteins. I provide evidence that this state has a dynamic character and that its lipid organization can reverse back to two lipid bilayers.

P-glycoprotein-overexpressing multidrug-resistant cells are resistant to infection by enveloped viruses that enter via the plasma membrane

The multidrug resistance gene product P-glycoprotein confers drug resistance to tumor cells by acting as a transporter that blocks the entry into the cell of a great variety of drugs and hydrophobic peptides. In this study we find that in drug-resistant cells, the insertion of the influenza virus fusion protein (hemagglutinin-2) into the plasma membrane is blocked and that the fusion of the viral envelope with the plasma membrane of these cells is impaired. Multidrug-resistant cells display significant resistance to infection by envelope viruses that invade cells by fusion with the plasma membrane, but not to infection by pH-dependent viruses that penetrate cells by fusion with endocytic vesicles. These observations suggest that multidrug resistance phenomena may protect cells from infection by a large group of disease-causing viruses that includes human immunodeficiency virus, herpes simplex virus, and some cancer-inducing retroviruses.

Structures and mechanisms in flavivirus fusion

This chapter focuses on the work carried out with tick-borne encephalitis (TBE) virus, the structurally best characterized of the flaviviruses. The data is related to those obtained with other flaviviruses, which are assumed to have a conserved structural organization, and compare the characteristics of flavivirus fusion to those of other enveloped viruses. Fusion proteins from several different virus families, including Orthomyxoviridae , Paramyxoviridae , Retroviridae , and Filoviridae have been shown to exhibit striking structural similarities; they all use a common mechanism for inducing membrane fusion, and the same general model applies to all of these cases. The flavivirus genome is a positive-stranded RNA molecule consisting of a single, long open reading frame of more than 10,000 nucleotides flanked by noncoding regions at the 5′ and 3′ ends. The fusion properties of flaviviruses have been investigated using several different assay systems, including virus-induced cell–cell fusion and virus–liposome fusion. All of these studies indicate that flaviviruses require an acidic pH for fusion, consistent with their proposed mode of entry.

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).

Effects of double-site mutations of vesicular stomatitis virus glycoprotein G on membrane fusion activity

Site-directed mutagenesis of specific amino acids within a conserved amino-terminal region (H2) and a conserved carboxyl-terminal region (H10/A4) of the fusion protein G of vesicular stomatitis virus have previously identified these two segments as an internal fusion peptide and a region influencing low-pH induced conformational change, respectively. Here, we combined a number of the substitution mutants in the H2 and H10/A4 regions to produce a series of double-site mutants and determined the effect of these mutations on membrane fusion activity at acid pH and on pH-dependent conformational change. The results show that most of the double-site mutants have decreased cell-cell fusion activity and that the effects appeared to be additive in terms of inhibition of fusion, except for one mutant, which appeared to be a revertant. The double-site mutants also had pH optima for fusion that were lower than those observed with wild-type G but same as the pH optima for the parent fusion peptide (H2) mutants. The results suggest that although the H2 and H10/A4 sites may affect membrane fusion independently, a possible interaction between these two sites cannot be ruled out.

Rabies virus-induced membrane fusion

Rabies virus is a member of the rhabdovirus family. It enters cells by a process of receptor mediated endocytosis. Following this step, the viral envelope fuses with the endosomal membrane to allow release of the viral nucleocapsid into the cytoplasm. Fusion is induced by the low pH of the endosomal compartment and is mediated by the single viral glycoprotein G, a homotrimeric integral membrane protein. Rabies virus fusion properties are related to different conformational states of G. By different biochemical and biophysical approaches, it has been demonstrated that G can assume at least three different states: the native (N) state detected at the viral surface above pH 7, the activated (A) hydrophobic state which interacts with the target membrane as a first step of the fusion process, and the fusion inactive (I) conformation. Differently from other fusogenic viruses for which low pH-induced conformational changes are irreversible, there is a pH dependent equilibrium between these states, the equilibrium being shifted toward the I-state at low pH. The objective of this review is to detail recent findings on rhabdovirus-induced membrane fusion and to underline the differences that exist between this viral family and influenza virus which is the best known fusogenic virus. These differences have to be taken into consideration if one wants to have a global understanding of virus-induced membrane fusion.

Mutations in a carboxy-terminal region of vesicular stomatitis virus glycoprotein G that affect membrane fusion activity

The envelope glycoprotein G of vesicular stomatitis virus induces membrane fusion at acidic pH. A highly conserved amino terminal region spanning residues 123 to 137 has previously been identified as an internal fusion domain. Here we have substituted specific amino acids within a carboxy terminal region, conserved in five vesiculoviruses encompassing residues 395 to 418, and studied the effect of these mutations on membrane fusion at acid pH and pH-dependent conformational change. Substitution of conserved Gly 395, Gly 404, Gly 406, Asp 409, and Asp 411 with Glu, Ala, Ala, Asn, and Asn, respectively, decreased the cell-cell fusion efficiency, as well as reduced the pH threshold of membrane fusion. Mutation of Gly 404 and Asp 409 to Lys and Ala, respectively, abolished the fusion activity. Mutant Gly 404 Lys also showed markedly altered resistance to trypsin digestion at acidic pH. These results suggest that the region between amino acids 395 to 418 is important for the fusogenic activity of the G protein. The possible role of this domain in conformational changes involved in fusion activity of VSV G is also discussed.

Influenza hemagglutinin is spring-loaded by a metastable native conformation

Enveloped viruses enter cells by protein-mediated membrane fusion. For influenza virus, membrane fusion is regulated by the conformational state of the hemagglutinin (HA) protein, which switches from a native (nonfusogenic) structure to a fusion-active (fusogenic) conformation when exposed to the acidic environment of the cellular endosome. Here we demonstrate that destabilization of HA at neutral pH, with either heat or the denaturant urea, triggers a conformational change that is biochemically indistinguishable from the change triggered by low pH. In each case, the conformational change is coincident with induction of membrane-fusion activity, providing strong evidence that the fusogenic structure is formed. These results indicate that the native structure of HA is trapped in a metastable state and that the fusogenic conformation is released by destabilization of native structure. This strategy may be shared by other enveloped viruses, including those that enter the cell at neutral pH, and could have implications for understanding the membrane-fusion step of HIV infection.