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

Animal and human RNA viruses: genetic variability and ability to overcome vaccines

RNA viruses, in general, exhibit high mutation rates; this is mainly due to the low fidelity displayed by the RNA-dependent polymerases required for their replication that lack the proofreading machinery to correct misincorporated nucleotides and produce high mutation rates. This lack of replication fidelity, together with the fact that RNA viruses can undergo spontaneous mutations, results in genetic variants displaying different viral morphogenesis, as well as variation on their surface glycoproteins that affect viral antigenicity. This diverse viral population, routinely containing a variety of mutants, is known as a viral 'quasispecies'. The mutability of their virions allows for fast evolution of RNA viruses that develop antiviral resistance and overcome vaccines much more rapidly than DNA viruses. This also translates into the fact that pathogenic RNA viruses, that cause many diseases and deaths in humans, represent the major viral group involved in zoonotic disease transmission, and are responsible for worldwide pandemics.

Internalization and fusion mechanism of vesicular stomatitis virus and related rhabdoviruses

Members of the Rhabdoviridae infect a wide variety of animals and plants, and are the causative agents of many important diseases. Rhabdoviruses enter host cells following internalization into endosomes, with the glycoprotein (G protein) mediating both receptor binding to host cells and fusion with the cellular membrane. The recently solved crystal structure of vesicular stomatitis virus G has allowed considerable insight into the mechanism of rhabdovirus entry, in particular the low pH-dependent conformational changes that lead to fusion activation. Rhabdovirus entry shows several distinct features compared with other enveloped viruses; first, the entry process appears to consist of two distinct fusion events, initial fusion into vesicles within endosomes followed by back-fusion into the cytosol; second, the conformational changes in the G protein that lead to fusion activation are reversible; and third, the G protein is structurally distinct from other viral fusion proteins and is not proteolytically cleaved. The internalization and fusion mechanisms of rhabdoviruses are discussed in this article, with a focus on viral systems where the G protein has been studied extensively: vesicular stomatitis virus and rabies virus, as well as viral hemorrhagic septicemia virus.

Structures and mechanisms of viral membrane fusion proteins: multiple variations on a common theme

Recent work has identified three distinct classes of viral membrane fusion proteins based on structural criteria. In addition, there are at least four distinct mechanisms by which viral fusion proteins can be triggered to undergo fusion-inducing conformational changes. Viral fusion proteins also contain different types of fusion peptides and vary in their reliance on accessory proteins. These differing features combine to yield a rich diversity of fusion proteins. Yet despite this staggering diversity, all characterized viral fusion proteins convert from a fusion-competent state (dimers or trimers, depending on the class) to a membrane-embedded homotrimeric prehairpin, and then to a trimer-of-hairpins that brings the fusion peptide, attached to the target membrane, and the transmembrane domain, attached to the viral membrane, into close proximity thereby facilitating the union of viral and target membranes. During these conformational conversions, the fusion proteins induce membranes to progress through stages of close apposition, hemifusion, and then the formation of small, and finally large, fusion pores. Clearly, highly divergent proteins have converged on the same overall strategy to mediate fusion, an essential step in the life cycle of every enveloped virus.

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).

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.

Conformation- and fusion-defective mutations in the hypothetical phospholipid-binding and fusion peptides of viral hemorrhagic septicemia salmonid rhabdovirus protein G

Fourteen single and two double point mutants in the highly conserved region (positions 56 to 159) of the G gene of viral hemorrhagic septicaemia virus (VHSV), a salmonid rhabdovirus, were selected and obtained in plasmids by site-directed mutagenesis. Fish cell monolayers transfected with the mutant plasmids were then assayed for protein G (pG) expression, conformation-dependent monoclonal antibody (MAb) reactivity, and cell-cell fusion. Some mutations located in the phospholipid-binding p2 peptide (positions 82 to 110; mutants P86A, A96E, G98A, and R107A) abolished both MAb recognition and fusion activity, while others (P79A, L85S, and R103A) abolished MAb recognition but retained fusion at similar or lower pHs compared to those for the wild type. Phospholipid-binding assays of p2-derived synthetic peptides suggested that phosphatidylserine binding was not affected by the mutations studied. On the other hand, three (P79A, L85S, and T135E) of the four mutants retaining fusion activity mapped around two locations showing amino acid variation in 22 VHSV isolates and in neutralizing MAb-resistant mutants described previously. Mutations located in the hypothetical fusion peptide (positions 142 to 159; mutants F147K, P148K, and W154K) abolished both MAb recognition and fusion activity. The existence of mutants with altered conformation and defective fusion in both p2 and fusion peptides provides further evidence in favor of the participation of these and adjacent regions in some of the steps of the VHSV fusion processes, as suggested by previous studies. In addition, because the studied region induced strong immunological responses in trout, some of the mutants described here might be used to design attenuated VHSV vaccines.

Improvement of transfection efficiency of epithelioma papulosum cyprini carp cells by modification of cell cycle and use of an optimal promoter

Several methods to improve transfection of epithelioma papulosum cyprini (EPC) carp cells have been tested and are reported here. By modifying the cell cycle state of EPC cell monolayers and selecting the best promoter for the plasmid to be transfected, we increased transfection efficiency from 12.8% to 55.1% and decreased the coefficient of variation among different experiments from 54.1% to 11.8%. Thus 2- to 3-fold higher transfection efficiencies were obtained when the EPC monolayers were treated with colchicine or thymidine before transfection. In addition, the plasmids pMOKbetagal and its shorter derivative pMVC1.4betagal, both containing 218 bp of additional sequences upstream of the cytomegalovirus promoter contained in plasmid pCMVbeta, consistently produced higher transfection efficiencies than pCMVbeta. Combination of the two methods resulted in an improvement of both efficiency and reproducibility. These results should facilitate transfection of EPC cells to use as a model to obtain transgenics, to conduct quantitative transfected-cell fusion assays, to improve DNA-immersion-vaccination methods, or to obtain infectious cDNA from fish RNA viruses.

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.

Fusion peptides and the mechanism of viral fusion

Segments of viral fusion proteins play an important role in viral fusion. They are defined by a number of criteria, including the sensitivity of this region of the viral fusion protein to loss of function as a consequence of mutation. In addition, small model peptides designed to mimic this segment of viral fusion proteins often have some membrane perturbing activity. The properties of viral fusion peptides are quite varied. Many are found at the amino terminus of viral fusion proteins. As isolated peptides, they have been found to form both alpha-helical as well as beta-structure. In addition, some viruses have internal fusion peptides. Just as there are several structural motifs for viral fusion peptides, there are also several mechanisms by which they accelerate the process of membrane fusion. These include the promotion of negative curvature, lowering the rupture tension of the lipid monolayer, acting as an anchor to join the fusion membranes, transmitting a force to the membrane or imparting energy to the system by other means. It is not likely that the fusion peptide can fulfill all of these diverse roles and future studies will elucidate which of these mechanisms is most important for the action of individual viral fusion peptides.

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.

The membrane-proximal domain of vesicular stomatitis virus G protein functions as a membrane fusion potentiator and can induce hemifusion

Recently we showed that the membrane-proximal stem region of the vesicular stomatitis virus (VSV) G protein ectodomain (G stem [GS]), together with the transmembrane and cytoplasmic domains, was sufficient to mediate efficient VSV budding (C. S. Robison and M. A. Whitt, J. Virol. 74:2239-2246, 2000). Here, we show that GS can also potentiate the membrane fusion activity of heterologous viral fusion proteins when GS is coexpressed with those proteins. For some fusion proteins, there was as much as a 40-fold increase in syncytium formation when GS was coexpressed compared to that seen when the fusion protein was expressed alone. Fusion potentiation by GS was not protein specific, since it occurred with both pH-dependent as well as pH-independent fusion proteins. Using a recombinant vesicular stomatitis virus encoding GS that contained an N-terminal hemagglutinin (HA) tag (GS(HA) virus), we found that the GS(HA) virus bound to cells as well as the wild-type virus did at pH 7.0; however, the GS(HA) virus was noninfectious. Analysis of cells expressing GS(HA) in a three-color membrane fusion assay revealed that GS(HA) could induce lipid mixing but not cytoplasmic mixing, indicating that GS can induce hemifusion. Treatment of GS(HA) virus-bound cells with the membrane-destabilizing drug chlorpromazine rescued the hemifusion block and allowed entry and subsequent replication of GS(HA) virus, demonstrating that GS-mediated hemifusion was a functional intermediate in the membrane fusion pathway. Using a series of truncation mutants, we also determined that only 14 residues of GS, together with the VSV G transmembrane and cytoplasmic tail, were sufficient for fusion potentiation. To our knowledge, this is the first report which shows that a small domain of one viral glycoprotein can promote the fusion activity of other, unrelated viral glycoproteins.

N-terminal domain of Borna disease virus G (p56) protein is sufficient for virus receptor recognition and cell entry

Borna disease virus (BDV) surface glycoprotein (GP) (p56) has a predicted molecular mass of 56 kDa. Due to extensive posttranslational glycosylation the protein migrates as a polypeptide of 84 kDa (gp84). The processing of gp84 by the cellular protease furin generates gp43, which corresponds to the C-terminal part of gp84. Both gp84 and gp43 have been implicated in viral entry involving receptor-mediated endocytosis and pH-dependent fusion. We have investigated the domains of BDV p56 involved in virus entry. For this, we used a pseudotype approach based on a recently developed recombinant vesicular stomatitis virus (VSV) in which the gene for green fluorescent protein was substituted for the VSV G protein gene (VSV Delta G*). Complementation of VSV Delta G* with BDV p56 resulted in infectious VSV Delta G* pseudotypes that contained both BDV gp84 and gp43. BDV-VSV chimeric GPs that contained the N-terminal 244 amino acids of BDV p56 and amino acids 421 to 511 of VSV G protein were efficiently incorporated into VSV Delta G* particles, and the resulting pseudotype virions were neutralized by BDV-specific antiserum. These findings indicate that the N-terminal part of BDV p56 is sufficient for receptor recognition and virus entry.

Vesicular stomatitis virus G protein acquires pH-independent fusion activity during transport in a polarized endometrial cell line

Entry of vesicular stomatitis virus (VSV), the prototype member of the rhabdovirus family, occurs by receptor-mediated endocytosis. Subsequently, during traversal through the endosomal compartments, the VSV G protein acquires a low-pH-induced fusion-competent form, allowing for fusion of the viral membrane with endosomal and lysosomal membranes. This fusion event releases genomic RNA into the cytoplasm of the cell. Here we provide evidence that the VSV G protein acquires a fusion-competent form during exocytosis in a polarized endometrial cell line, HEC-1A. VSV infection of HEC-1A cells results in high viral yields and giant cell formation. Syncytium formation is blocked in a concentration-dependent manner by treatment with the lysosomotropic weak base ammonium chloride, which raises intravesicular pH. Virus release is somewhat delayed by treatment with ammonium chloride, but virus yields gradually reach those of control cells. In addition, inhibition of vacuolar H(+)-ATPases by treatment with bafilomycin A1 also inhibited cell to cell fusion without altering virus yields. Virions released from infected HEC cells were themselves not fusion competent, since viral entry required an active H(+)-ATPase and a low-pH-induced conformational change in the viral G protein. Thus, the conformation change leading to fusion competence during exocytotic transport is reversible and reverts during or after release of the virion from the infected cell.