A fusion-defective mutant of the vesicular stomatitis virus glycoprotein

Characterization of chicken interferon-inducible transmembrane protein-10

Interferon-inducible transmembrane protein (IFITM) family proteins are antivirus factors. In the present study, we examined the expression pattern of chicken IFITM10 using quantitative reverse transcription-polymerase chain reaction. In adult chickens, IFITM10 levels were markedly lower than those of IFITM3, which exhibits antivirus activity. On the other hand, IFITM10 was expressed in levels similar to those of IFITM3 in embryonic organs. Primordial germ cells in 2.5-d embryos expressed high levels of IFITM10, which gradually decreased with time. The interferon-α stimulation of embryonic fibroblast cells did not enhance the expression of IFITM10. The forced expression of IFITM10 slightly inhibited the infectivity of the VSV-G-pseudotyped lentiviral vector. Furthermore, cell fusion was inhibited by IFITM10 when HeLa cells transfected with the VSV-G expression vector were treated with low pH buffer. Although it remains unclear whether IFITM10 inhibits viral infections under physiological conditions, these results suggest that chicken IFITM10 exhibits antivirus activity.

A vesicular stomatitis virus glycoprotein epitope-incorporated oncolytic adenovirus overcomes CAR-dependency and shows markedly enhanced cancer cell killing and suppression of tumor growth

Utility of traditional oncolytic adenovirus (Ad) has been limited due to low expression of coxsackie and adenovirus receptor (CAR) in cancer cells which results in poor infectivity of Ads. Here with an aim of improving the efficiency of Ad's entry to the cell, we generated a novel tropism-expanded oncolytic Ad which contains the epitope of vesicular stomatitis virus glycoprotein (VSVG) at the HI-loop of Ad fiber. We generated 9 variants of oncolytic Ads with varying linkers and partial deletion to the fiber. Only one VSVG epitope-incorporated variant, RdB-1L-VSVG, which contains 1 linker and no deletion to fiber, was produced efficiently. Production of 3-dimensionaly stable fiber in RdB-1L-VSVG was confirmed by immunoblot analysis. RdB-1L-VSVG shows a remarkable improvement in cytotoxicity and total viral yield in cancer cells. RdB-1L-VSVG demonstrates enhanced cytotoxicity in cancer cells with subdued CAR-expression as it can be internalized by an alternate pathway. Competition assays with a CAR-specific antibody (Ab) or VSVG receptor, phosphatidyl serine (PS), reveals that cell internalization of RdB-1L-VSVG is mediated by both CAR and PS. Furthermore, treatment with RdB-1L-VSVG significantly enhanced anti-tumor effect in vivo. These studies demonstrate that the strategy to expand oncolytic Ad tropism may significantly improve therapeutic profile for cancer treatment.

Protein-Protein Interactions of Viroporins in Coronaviruses and Paramyxoviruses: New Targets for Antivirals?

Viroporins are members of a rapidly growing family of channel-forming small polypeptides found in viruses. The present review will be focused on recent structural and protein-protein interaction information involving two viroporins found in enveloped viruses that target the respiratory tract; (i) the envelope protein in coronaviruses and (ii) the small hydrophobic protein in paramyxoviruses. Deletion of these two viroporins leads to viral attenuation in vivo, whereas data from cell culture shows involvement in the regulation of stress and inflammation. The channel activity and structure of some representative members of these viroporins have been recently characterized in some detail. In addition, searches for protein-protein interactions using yeast-two hybrid techniques have shed light on possible functional roles for their exposed cytoplasmic domains. A deeper analysis of these interactions should not only provide a more complete overview of the multiple functions of these viroporins, but also suggest novel strategies that target protein-protein interactions as much needed antivirals. These should complement current efforts to block viroporin channel activity.

Alanine scanning of the rabies virus glycoprotein antigenic site III using recombinant rabies virus: implication for post-exposure treatment

The safety and availability of the human polyclonal sera that is currently utilized for post-exposure treatment (PET) of rabies virus (RABV) infection remain a concern. Recombinant monoclonal antibodies have been postulated as suitable alternatives by WHO. To this extent, CL184, the RABV human antibody combination comprising monoclonal antibodies (mAbs) CR57 and CR4098, has been developed and has delivered promising clinical data to support its use for RABV PET. For this fully human IgG1 cocktail, mAbs CR57 and CR4098 are produced in the PER.C6 human cell line and combined in equal amounts in the final product. During preclinical evaluation, CR57 was shown to bind to antigenic site I whereas CR4098 neutralization was influenced by a mutation of position 336 (N336) located within antigenic site III. Here, alanine scanning was used to analyze the influence of mutations within the potential binding site for CR4098, antigenic site III, in order to evaluate the possibility of mutated rabies viruses escaping neutralization. For this approach, twenty flanking amino acids (10 upstream and 10 downstream) of the RABV glycoprotein (G) asparagine (N336) were exchanged to alanine (or serine, if already alanine) by site-directed mutagenesis. Analysis of G expression revealed four of the twenty mutant Gs to be non-functional, as shown by their lack of cell surface expression, which is a requirement for the production of infectious RABV. Therefore, these mutants were excluded from further study. The remaining sixteen mutants were introduced in an infectious clone of RABV, and recombinant RABVs (rRABVs) were recovered and utilized for in vitro neutralization assays. All of the viruses were effectively neutralized by CR4098 as well as by CR57, indicating that single amino acid exchanges in this region does not affect the broad neutralizing capability of the CL184 mAb combination.

Posttranslational modification of vesicular stomatitis virus glycoprotein, but not JNK inhibition, is the antiviral mechanism of SP600125

Vesicular stomatitis virus (VSV), a negative-sense single-stranded-RNA rhabdovirus, is an extremely promising oncolytic agent for cancer treatment. Since oncolytic virotherapy is moving closer to clinical application, potentially synergistic combinations of oncolytic viruses and molecularly targeted antitumor agents are becoming a meaningful strategy for cancer treatment. Mitogen-activated protein kinase (MAPK) inhibitors have been shown to impair liver cell proliferation and tumor development, suggesting their potential use as therapeutic agents for hepatocellular carcinoma (HCC). In this work, we show that the impairment of MAPK in vitro did not interfere with the oncolytic properties of VSV in HCC cell lines. Moreover, the administration of MAPK inhibitors did not restore the responsiveness of HCC cells to alpha/beta interferon (IFN-α/β). In contrast to previous reports, we show that JNK inhibition by the inhibitor SP600125 is not responsible for VSV attenuation in HCC cells and that this compound acts by causing a posttranslational modification of the viral glycoprotein.

A recombinant vesicular stomatitis virus bearing a lethal mutation in the glycoprotein gene uncovers a second site suppressor that restores fusion

Vesicular stomatitis virus (VSV), a prototype of the Rhabdoviridae family, contains a single surface glycoprotein (G) that is responsible for attachment to cells and mediates membrane fusion. Working with the Indiana serotype of VSV, we employed a reverse genetic approach to produce fully authentic recombinant viral particles bearing lethal mutations in the G gene. By altering the hydrophobicity of the two fusion loops within G, we produced a panel of mutants, W72A, Y73A, Y116A, and A117F, that were nonfusogenic. Propagation of viruses bearing those lethal mutations in G completely depended on complementation by expression of the glycoprotein from the heterologous New Jersey serotype of VSV. The nonfusogenic G proteins oligomerize and are transported normally to the cell surface but fail to mediate acid pH-triggered membrane fusion. The nonfusogenic G proteins also interfered with the ability of wild-type G to mediate fusion, either by formation of mixed trimers or by inhibition of trimer function during fusion. Passage of one recombinant virus, A117F, identified a second site suppressor of the fusion block, E76K. When analyzed in the absence of the A117F substitution, E76K rendered G more sensitive to acid pH-triggered fusion, suggesting that this compensatory mutation is destabilizing. Our work provides a set of authentic recombinant VSV particles bearing lethal mutations in G, confirms that the hydrophobic fusion loops of VSV G protein are critical for membrane fusion, and underscores the importance of the sequence elements surrounding the hydrophobic tips of the fusion loops in driving fusion. This study has implications for understanding dominant targets for inhibition of G-mediated fusion. Moreover, the recombinant viral particles generated here will likely be useful in dissecting the mechanism of G-catalyzed fusion as well as study steps of viral assembly.

Ngaingan virus, a macropod-associated rhabdovirus, contains a second glycoprotein gene and seven novel open reading frames

Ngaingan virus (NGAV) was isolated from a pool of biting midges that were collected in the tropics of northern Australia. Reported here is the full-length sequence of the NGAV genome, which, at over 15.7 kb, is the largest in any rhabdovirus described to date and contains 13 genes, the highest number of genes observed in any (-) ssRNA virus. Seven of these putative genes show no significant homology to known proteins. Like viruses in the genus Ephemerovirus, NGAV possesses a second glycoprotein gene (G(NS)). Phylogenetic analyses, however, place NGAV within the yet to be classified "Hart Park" group containing Wongabel and Flanders viruses, which do not contain a second glycoprotein gene. Screening of various animal sera from northern Australia has indicated that NGAV is currently circulating in macropods (wallabies, wallaroos and kangaroos), highlighting the need for further studies to determine its potential to cause disease in these species.

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.

Enhancement of immunostimulatory properties of exosomal vaccines by incorporation of fusion-competent G protein of vesicular stomatitis virus

Exosomes have been proposed as candidates for therapeutic immunization. The present study demonstrates that incorporation of the G protein of vesicular stomatitis virus (VSV-G) into exosome-like vesicles (ELVs) enhances their uptake and induces the maturation of dendritic cells. Targeting of VSV-G and ovalbumin as a model antigen to the same ELVs increased the cross-presentation of ovalbumin via an endosomal acidification mechanism. Immunization of mice with VSV-G and ovalbumin containing ELVs led to an increased IgG2a antibody response, expansion of antigen-specific CD8 T cells, strong in vivo CTL responses, and protection from challenge with ovalbumin expressing tumor cells. Thus, incorporation of VSV-G and targeting of antigens to ELVs are attractive strategies to improve exosomal vaccines.

2-phenylimidazopyridines, a new series of Golgi compounds with potent antiviral activity

Drugs targeted to viral proteins are highly vulnerable to the development of resistant strains. We previously characterized a group of 2-phenylbenzimidazole compounds for their activity against allergy and asthma and more recently established the Golgi as their probable site of action. Herein we describe their activity against the propagation of several virus types through an action on the host cell. The most potent derivatives are the novel 2-phenylimidazopyridines, the lead compound of which is highly effective for blocking the spread of topical herpes infection in an animal model. These agents may provide an alternative antiviral approach, particularly for treating resistant strains.

Hydrophobic residues that form putative fusion loops of Epstein-Barr virus glycoprotein B are critical for fusion activity

To test the importance of the hydrophobic residues within the putative Epstein-Barr virus (EBV) glycoprotein B (gB) fusion loops in membrane fusion, WY(112-113) and WLIW(193-196) were mutated into alanine, glutamic acid, or the analogous residues from herpes simplex virus type 1 (HSV-1) gB (HR and RVEA). All gB variants exhibited cell surface expression, demonstrating that the substitutions did not perturb gB trafficking. None of six gB variants was, however, capable of mediating fusion with either epithelial or B cells. These data demonstrate that the bulky and hydrophobic EBV loop residues, which differ from the more hydrophilic HSV-1 residues and appear more compatible with membrane insertion, are essential for EBV gB-dependent fusion.

Mutagenesis of the La Crosse Virus glycoprotein supports a role for Gc (1066-1087) as the fusion peptide

The La Crosse Virus (LACV) M segment encodes two glycoproteins (Gn and Gc), and plays a critical role in the neuropathogenesis of LACV infection as the primary determinant of neuroinvasion. A recent study from our group demonstrated that the region comprising the membrane proximal two-thirds of Gc, amino acids 860-1442, is critical in mediating LACV fusion and entry. Furthermore, computational analysis identified structural similarities between a portion of this region, amino acids 970-1350, and the E1 fusion protein of two alphaviruses: Sindbis virus and Semliki Forrest virus (SFV). Within the region 970-1350, a 22-amino-acid hydrophobic segment (1066-1087) is predicted to correlate structurally with the fusion peptides of class II fusion proteins. We performed site-directed mutagenesis of key amino acids in this 22-amino acid segment and determined the functional consequences of these mutations on fusion and entry. Several mutations within this hydrophobic domain affected glycoprotein expression to some extent, but all mutations either shifted the pH threshold of fusion below that of the wild-type protein, reduced fusion efficiency, or abrogated cell-to-cell fusion and pseudotype entry altogether. These results, coupled with the aforementioned computational modeling, suggest that the LACV Gc functions as a class II fusion protein and support a role for the region Gc 1066-1087 as a fusion peptide.

Entry of hepatitis C virus pseudotypes into primary human hepatocytes by clathrin-dependent endocytosis

Hepatitis C virus (HCV) is a major cause of chronic hepatitis worldwide. Studies of the early steps of HCV infection have been hampered by the lack of convenient in vitro or in vivo models. Although several cell-surface molecules that mediate the binding of HCV envelope proteins to target cells have been identified, mechanisms of viral entry into human hepatocytes are still poorly understood. Vesicular stomatitis virus/HCV pseudotyped viruses expressing the HCV envelope glycoproteins on the viral envelope were generated and it was found that their entry into human hepatocytes required co-expression of E1 and E2 on the pseudotype surface. Neutralization of pseudotype infection by anti-HCV antibodies suggested that cellular entry was mediated by HCV envelope glycoproteins and by previously characterized cell-surface molecules, including CD81. An entry assay based on the release of a fluorochrome from labelled HCV pseudotypes provided evidence for a pH-dependent fusion of the pseudotype envelope with a cellular compartment. By using a panel of endocytosis inhibitors, it is postulated that penetration of HCV into primary cultures of hepatocytes takes place by clathrin-mediated endocytosis.

Immunogenicity and efficacy of immunodeficiency virus-like particles pseudotyped with the G protein of vesicular stomatitis virus

Vaccination with exogenous antigens such as recombinant viral proteins, immunodeficiency virus-derived whole inactivated virus particles, or virus-like particles (VLP) has generally failed to provide sufficient protection in animal models for AIDS. Pseudotyping VLPs with the vesicular stomatitis virus G protein (VSV-G), which is known to mediate entry into dendritic cells, might allow more efficient stimulation of immune responses. Therefore, we pseudotyped noninfectious immunodeficiency virus-like particles with VSV-G and carried out a preliminary screen of their immunogenicity and vaccination efficacy. Incorporation of VSV-G into HIV-1 VLPs led to hundred-fold higher antibody titers to HIV-1 Gag and enhancement of T cell responses in mice. Repeated vaccination of rhesus monkeys for 65 weeks with VSV-G pseudotyped simian immunodeficiency virus (SIV)-like particles (VLP[G]) provided initial evidence for efficient suppression of viral load after mucosal challenge with the SIVmac239 virus. Challenge of monkeys after a 28 week vaccination regimen with VLP[G] led to a reduction in peak viremia, but persistent suppression of viral load was not achieved. Due to limitations in the number of animals available for this study, improved efficacy of VSV-G pseudotyped VLPs in nonhuman primates could not be demonstrated. However, mouse experiments revealed that pseudotyping of VLPs with fusion-competent VSV-G clearly improves their immunogenicity. Additional strategies, particularly adjuvants, should be considered to provide greater protection against a challenge with pathogenic immunodeficiency virus.

Addition of N-glycans in the stalk of the Newcastle disease virus HN protein blocks its interaction with the F protein and prevents fusion

Most paramyxovirus fusion (F) proteins require the coexpression of the homologous attachment (HN) protein to promote membrane fusion, consistent with the existence of a virus-specific interaction between the two proteins. Analysis of the fusion activities of chimeric HN proteins indicates that the stalk region of the HN spike determines its F protein specificity, and analysis of a panel of site-directed mutants indicates that the F-interactive site resides in this region. Here, we use the addition of oligosaccharides to further explore the role of the HN stalk in the interaction with F. N-glycans were individually added at several positions in the stalk to determine their effects on the activities of HN, as well as its structure. N-glycan addition at positions 69 and 77 in the stalk specifically blocks fusion and the HN-F interaction without affecting either HN structure or its other activities. N-glycans added at other positions in the stalk modulate activities that reside in the globular head of HN. This correlates with an alteration of the tetrameric structure of the protein, as indicated by sucrose gradient sedimentation analyses. Finally, N-glycan addition in another region of HN (residues 124 to 152), predicted by a peptide-based analysis to mediate the interaction with F, does not significantly reduce the level of fusion, arguing strongly against this site being part of the F-interactive domain in HN. Our data support the idea that the F-interactive site on HN is defined by the stalk region of the protein.

Myristoylation, a protruding loop, and structural plasticity are essential features of a nonenveloped virus fusion peptide motif

Members of the fusion-associated small transmembrane (FAST) protein family are a distinct class of membrane fusion proteins encoded by nonenveloped fusogenic reoviruses. The 125-residue p14 FAST protein of reptilian reovirus has an approximately 38-residue myristoylated N-terminal ectodomain containing a moderately apolar N-proximal region, termed the hydrophobic patch. Mutagenic analysis indicated sequence-specific elements in the N-proximal portion of the p14 hydrophobic patch affected cell-cell fusion activity, independent of overall effects on the relative hydrophobicity of the motif. Circular dichroism (CD) of a myristoylated peptide representing the majority of the p14 ectodomain suggested this region is mostly disordered in solution but assumes increased structure in an apolar environment. From NMR spectroscopic data and simulated annealing, the soluble nonmyristoylated p14 ectodomain peptide consists of an N-proximal extended loop flanked by two proline hinges. The remaining two-thirds of the ectodomain peptide structure is disordered, consistent with predictions based on CD spectra of the myristoylated peptide. The myristoylated p14 ectodomain peptide, but not a nonmyristoylated version of the same peptide nor a myristoylated scrambled peptide, mediated extensive lipid mixing in a liposome fusion assay. Based on the lipid mixing activity, structural plasticity, environmentally induced conformational changes, and kinked structures predicted for the p14 ectodomain and hydrophobic patch (all features associated with fusion peptides), we propose that the majority of the p14 ectodomain is composed of a fusion peptide motif, the first such motif dependent on myristoylation for membrane fusion activity.

Prediction and identification of a permissive epitope insertion site in the vesicular stomatitis virus glycoprotein

We developed a rational approach to identify a site in the vesicular stomatitis virus (VSV) glycoprotein (G) that is exposed on the protein surface and tolerant of foreign epitope insertion. The foreign epitope inserted was the six-amino-acid sequence ELDKWA, a sequence in a neutralizing epitope from human immunodeficiency virus type 1. This sequence was inserted into six sites within the VSV G protein (Indiana serotype). Four sites were selected based on hydrophilicity and high sequence variability identified by sequence comparison with other vesiculovirus G proteins. The site showing the highest variability was fully tolerant of the foreign peptide insertion. G protein containing the insertion at this site folded correctly, was transported normally to the cell surface, had normal membrane fusion activity, and could reconstitute fully infectious VSV. The virus was neutralized by the human 2F5 monoclonal antibody that binds the ELDKWA epitope. Additional studies showed that this site in G protein tolerated insertion of at least 16 amino acids while retaining full infectivity. The three other insertions in somewhat less variable sequences interfered with VSV G folding and transport to the cell surface. Two additional insertions were made in a conserved sequence adjacent to a glycosylation site and near the transmembrane domain. The former blocked G-protein transport, while the latter allowed transport to the cell surface but blocked membrane fusion activity of G protein. Identification of an insertion-tolerant site in VSV G could be important in future vaccine and targeting studies, and the general principle might also be useful in other systems.

Molecular mechanisms of virus spread and virion components as tools of virulence. A review

Despite of differences in replication strategy among virus families, some basic principles have remained similar. Analogous mechanisms govern virus entry into cells and the use of enzymes which direct the replication of the virus genome. The function of many cell surface receptors (such as glycosoaminoglycans, glycoproteins, proteins) which interact with viral capsid proteins or envelope glycoproteins has recently been elucidated. The list of cellular receptors (Table I) is still far from being final. The capsid components, similarly as the envelope glycoproteins, may form specific pocket like sites, which interact with the cell surface receptors. Neutralizing antibodies usually react with antigenic domains adjacent to the receptor binding site(s) and hamper the close contact inevitable for virion attachment. In the case of more complex viruses, such as herpes simplex virus, different viral glycoproteins interact with several cellular receptors. At progressed phase of adsorption the virions are engulfed into endocytic vesicles and the virion fusion domain(s) become(s) activated. The outer capsid components of reoviruses which participate in adsorption and fusion may get activated already in the lumen of digestive tract, i.e. before their engulfment by resorptive epithelium cells. Activation of the hydrophobic fusion domain(s) is a further important step allowing to pass through the lipid bilayer when penetrating the cell membrane in order to reach the cytosol. Activation of the virion fusion domain is accomplished by a conformation change, which occurs at acid pH (influenza virus hemagglutinin, sigma 1 protein of the reovirus particle) and/or after protease treatment. The herpes simplex virus fusion factors (gD and gH) undergo conformation changes by a pH-independent mechanism triggered due to interaction with the cell surface receptor(s) and mediated by mutual interactions with the viral envelope glycoproteins. The virion capsid or envelope components participating in the entry and membrane fusion are not the only tools of virulence. The correct function of virus coded proteins, which participate in replication of the viral genome, and/or in the supply of necessary nucleotides, may be very essential. In the case of enteroviruses, which RNA interacts with ribosomes directly, the correct configuration of the non-coding viral RNA sequence is crucial for initiation of translation occurring in the absence of the classical "cap" structure.

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.

dl-VSVG-LacZ, a Vesicular Stomatitis Virus Glycoprotein Epitope-Incorporated Adenovirus, Exhibits Marked Enhancement in Gene Transduction Efficiency

Recombinant adenovirus (Ad) has emerged as the vector system of choice in cancer gene therapy. Its full utility, however, has been limited because of the low efficiency of adenovirus-mediated gene transfer to cancer cells - the main reason being that cancer cells in general express inherently low levels of the coxsackie and adenovirus receptor (CAR) on their surface. Development of novel strategies to achieve adenovirus infection in a CAR-independent manner may help to overcome this limitation. To this end, we have generated a novel recombinant Ad, dl-VSVG-LacZ, that contains a fiber knob with intact CAR entry capability and an additional phosphatidylserine (PS) entry capability. This was achieved by incorporating the vesicular stomatitis virus glycoprotein (VSV-G) epitope onto the C terminus of the fiber knob. VSV-G is an envelope protein that facilitates the specificity for binding of the virus to PS moieties on the cellular plasma membrane. The newly tropism-expanded adenovirus, dl-VSVG-LacZ, showed a remarkable improvement (3- to 20-fold) in the delivery of LacZ to a variety of mammalian cells including those that were CAR deficient. The greatest improvement in gene transfer was observed in cells that were difficult to transduce with an untargeted Ad (wildtype fiber). Furthermore, treatment with dl-VSVG-LacZ significantly enhanced gene transfer in vivo when compared with control adenovirus that lacked the VSV-G epitope. Taken together, these studies demonstrate that the strategy to extend adenovirus tropism may greatly improve the utility of adenovirus in gene therapy applications.

Viral fusion proteins: multiple regions contribute to membrane fusion

In recent years, the simple picture of a viral fusion protein interacting with the cell and/or viral membranes by means of only two localized segments (i.e. the fusion peptide and the transmembrane domain) has given way to a more complex picture in which multiple regions from the viral proteins interact with membranes. Indeed, possible roles in membrane binding and/or destabilization have been postulated for the N-terminal heptad repeats, pre-transmembrane segments, and other internal regions of fusion proteins from distant viruses (such as orthomyxo-, retro-, paramyxo-, or flaviviruses). This review focuses on the experimental evidence and functional models postulated so far about the role of these regions in the process of virus-induced membrane fusion.

Vesicular stomatitis virus glycoprotein is a determinant of pathogenesis in swine, a natural host

There are two major serotypes of vesicular stomatitis virus (VSV), Indiana (VSIV) and New Jersey (VSNJV). We recovered recombinant VSIVs from engineered cDNAs that contained either (i) one copy of the VSIV G gene (VSIV-G(I)); (ii) two copies of the G gene, one from each serotype (VSIV-G(NJ)G(I)); or (iii) a single copy of the G(NJ) gene instead of the G(I) gene (VSIV-G(NJ)). The recombinant viruses expressed the appropriate glycoproteins, incorporated them into virions, and were neutralized by antibodies specific for VSIV (VSIV-G(I)), VSNJV (VSIV-G(NJ)), or both (VSIV-G(NJ)G(I)), according to the glycoprotein(s) they expressed. All recombinant viruses grew to similar titers in cell culture. In mice, VSIV-G(NJ) and VSIV-G(NJ)G(I) were attenuated. However, in swine, a natural host for VSV, the G(NJ) glycoprotein-containing viruses caused more severe lesions and replicated to higher titers than the parental virus, VSIV-G(I). These observations implicate the glycoprotein as a determinant of VSV virulence in a natural host and emphasize the differences in VSV pathogenesis between mice and swine.

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.

Enhanced presentation of major histocompatibility complex class I-restricted human immunodeficiency virus type 1 (HIV-1) Gag-specific epitopes after DNA immunization with vectors coding for vesicular stomatitis virus glycoprotein-pseudotyped HIV-1 Gag particles

In vivo priming of cytotoxic T lymphocytes (CTL) by DNA injection predominantly occurs by antigen transfer from DNA-transfected cells to antigen-presenting cells. A rational strategy for increasing DNA vaccine potency would be to use a delivery system that facilitates antigen uptake by antigen-presenting cells. Exogenous antigen presentation through the major histocompatibility complex (MHC) class I-restricted pathway of some viral antigens is increased after adequate virus-receptor interaction and the fusion of viral and cellular membranes. We used DNA-based immunization with plasmids coding for human immunodeficiency virus type 1 (HIV-1) Gag particles pseudotyped with vesicular stomatitis virus glycoprotein (VSV-G) to generate Gag-specific CTL responses. The presence of the VSV-G-encoding plasmid not only increased the number of mice displaying anti-Gag-specific cytotoxic response but also increased the efficiency of specific lysis. In vitro analysis of processing confirmed that exogenous presentation of Gag epitopes occurred much more efficiently when Gag particles were pseudotyped with the VSV-G envelope. We show that the VSV-G-pseudotyped Gag particles not only entered the MHC class II processing pathway but also entered the MHC class I processing pathway. In contrast, naked Gag particles entered the MHC class II processing pathway only. Thus, the combined use of DNA-based immunization and nonreplicating pseudotyped virus to deliver HIV-1 antigen to the immune system in vivo could be considered in HIV-1 vaccine design.

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.

MHC-I-restricted presentation of HIV-1 virion antigens without viral replication

Dendritic cells and macrophages can process extracellular antigens for presentation by MHC-I molecules. This exogenous pathway may have a crucial role in the activation of CD8+ cytotoxic T lymphocytes during human viral infections. We show here that HIV-1 epitopes derived from incoming virions are presented through the exogenous MHC-I pathway in primary human dendritic cells, and to a lower extent in macrophages, leading to cytotoxic T-lymphocyte activation in the absence of viral protein synthesis. Exogenous antigen presentation required adequate virus-receptor interactions and fusion of viral and cellular membranes. These results provide new insights into how anti-HIV cytotoxic T lymphocytes can be activated and have implications for anti-HIV vaccine design.

Participation of two fusion peptides in measles virus-induced membrane fusion: emerging similarity with other paramyxoviruses

Paramyxoviruses penetrate into their host cells by fusing their membranes with the plasma membrane. The hydrophobic N terminus of their F1 protein, termed the 'fusion peptide', is thought to be responsible for this process. Recently, an additional internal fusion peptide, homologous in sequence to the N-terminal fusion peptide of HIV-1, was identified in the Sendai virus F1 protein. Here, we investigated whether the presence of an additional internal fusion peptide is a general feature of paramyxoviridae. To this end, we synthesized and structurally and functionally characterized three peptides: (i) MV-197, which corresponds to an internal segment of the F1 protein of the measles virus (amino acids 197-225), homologous in location but not in sequence to the internal fusion peptide of the Sendai virus, (ii) Mu-MV-197, a randomized version of MV-197, and (iii) the 33 amino acid N-terminal fusion peptide of the measles virus. Remarkably, only MV-197 was highly fusogenic toward large unilamellar vesicles composed of either zwitterionic (phosphatidylcholine or phosphatidylcholine/sphingomyelin/cholesterol, a composition similar to that of human cell membranes) or negatively charged phospholipids. Binding experiments, circular dichroism spectroscopy in phospholipid membranes, and homo energy-transfer studies with fluorescently labeled peptides revealed that MV-197 adopts a predominant alpha-helical structure and shares properties similar to those reported for known fusion peptides. These results suggest that the presence of two fusion peptides in the F1 protein is a general feature of paramyxoviruses.

Common properties of fusion peptides from diverse systems

Although membrane fusion occurs ubiquitously and continuously in all eukaroytic cells, little is known about the mechanism that governs lipid bilayer fusion associated with any intracellular fusion reactions. Recent studies of the fusion of enveloped viruses with host cell membranes have helped to define the fusion process. The identification and characterization of key proteins involved in fusion reactions have mainly driven recent advances in our understanding of membrane fusion. The most important denominator among the fusion proteins is the fusion peptide. In this review, work done in the last few years on the molecular mechanism of viral membrane fusion will be highlighted, focusing in particular on the role of the fusion peptide and the modification of the lipid bilayer structure. Much of what is known regarding the molecular mechanism of viral membrane fusion has been gained using liposomes as model systems in which the molecular components of the membrane and the environment are strictly controlled. Many amphilphilic peptides have a high affinity for lipid bilayers, but only a few sequences are able to induce membrane fusion. The presence of alpha-helical structure in at least part of the fusion peptide is strongly correlated with activity whereas, beta-structure tends to be less prevalent, associated with non-native experimental conditions, and more related to vesicle aggregation than fusion. The specific angle of insertion of the peptides into the membrane plane is also found to be an important characteristic for the fusion process. A shallow penetration, extending only to the central aliphatic core region, is likely responsible for the destabilization of the lipids required for coalescence of the apposing membranes and fusion.

Paramyxovirus F1 protein has two fusion peptides: implications for the mechanism of membrane fusion

Viral fusion proteins contain a highly hydrophobic segment, named the fusion peptide, which is thought to be responsible for the merging of the cellular and viral membranes. Paramyxoviruses are believed to contain a single fusion peptide at the N terminus of the F1 protein. However, here we identified an additional internal segment in the Sendai virus F1 protein (amino acids 214-226) highly homologous to the fusion peptides of HIV-1 and RSV. A synthetic peptide, which includes this region, was found to induce membrane fusion of large unilamellar vesicles, at concentrations where the known N-terminal fusion peptide is not effective. A scrambled peptide as well as several peptides from other regions of the F1 protein, which strongly bind to membranes, are not fusogenic. The functional and structural characterization of this active segment suggest that the F1 protein has an additional internal fusion peptide that could participate in the actual fusion event. The presence of homologous regions in other members of the same family suggests that the concerted action of two fusion peptides, one N-terminal and the other internal, is a general feature of paramyxoviruses.

The central proline of an internal viral fusion peptide serves two important roles

The fusion peptide of the avian sarcoma/leukosis virus (ASLV) envelope protein (Env) is internal, near the N terminus of its transmembrane (TM) subunit. As for most internal viral fusion peptides, there is a proline near the center of this sequence. Robson-Garnier structure predictions of the ASLV fusion peptide and immediate surrounding sequences indicate a region of order (beta-sheet), a tight reverse turn containing the proline, and a second region of order (alpha-helix). Similar motifs (order, turn or loop, order) are predicted for other internal fusion peptides. In this study, we made and analyzed 12 Env proteins with substitutions for the central proline of the fusion peptide. Env proteins were expressed in 293T cells and in murine leukemia virus pseudotyped virions. We found the following. (i) All mutant Envs form trimers, but when the bulky hydrophobic residues phenylalanine or leucine are substituted for proline, trimerization is weakened. (ii) Surprisingly, the proline is required for maximal processing of the Env precursor into its surface and TM subunits; the amount of processing correlates linearly with the propensity of the substituted residue to be found in a reverse turn. (iii) Nonetheless, proteolytically processed forms of all Envs are preferentially incorporated into pseudotyped virions. (iv) All Envs bind receptor with affinity greater than or equal to wild-type affinity. (v) Residues that support high infectivity cluster with proline at intermediate hydrophobicity. Infectivity is not supported by mutant Envs in which charged residues are substituted for proline, nor is it supported by the trimerization-defective phenylalanine and leucine mutants. Our findings suggest that the central proline in the ASLV fusion peptide is important for the formation of the native (metastable) Env structure as well as for membrane interactions that lead to fusion.

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.

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.

Interactions of a vesicular stomatitis virus G protein fragment with phosphatidylserine: NMR and fluorescence studies

The interaction of a 19 amino acid vesicular stomatitis virus G protein fragment (GTWLNPGFPPQSCGYATVT) with phosphatidylserine-containing model membranes was investigated using solution-phase 1d and 2d 1H NMR spectroscopy and intrinsic tryptophan fluorescence. Results of these studies show that this peptide interacts with model membranes containing negatively charged phospholipids. The interaction is modulated by both ionic and hydrophobic factors and appears to be dependent on the fluidity and lipid packing of the target bilayer. The data further suggest the existence of two isomeric forms of this peptide, which react differentially with model membranes. Upon binding, 2d 1H NOESY and tryptophan fluorescence data indicate penetration of the tryptophan residue into the bilayer. A model is proposed for the interaction of the peptide with model membranes, consistent with the experimental findings.

Inhibition of HIV type 1 infectivity by coexpression of a wild-type and a defective glycoprotein 120

An amino acid substitution (D --> K) in the C3 region of HIV-1 gp120 has previously been shown to inhibit binding of virions to CD4+ cells. We have introduced the same mutation into the HIV-1 isolate LAV-I(BRU), in which the mutation is denoted D373K. Here we show that the D373K envelope protein is processed and incorporated into virus particles, but that D373K virions have no detectable infectivity (below 0.1% relative to wild type). When D373K and the wild-type envelope gene were cotransfected in 293 cells at a 4:1 ratio, the resultant infectivity of the HIV-1 supernatant was reduced more than 100-fold. When the same ratio of plasmids was tested in COS-1 cells the inhibition of HIV-1 was an order of magnitude less than observed in 293 cells. COS-1 and 293 cells differed in that only 293 cells displayed saturation of virus production with respect to the envelope protein. Our data fit a simple model: when virion formation is saturated with envelope protein, expression and incorporation of a defective envelope protein imply a corresponding dilution of wild-type protein on the surface of virions. The cooperative function of wild-type envelope proteins is subsequently compromised, and a trans-dominant inhibition of virus infectivity is observed.

Phospholipid interactions of a peptide from the fusion-related domain of the glycoprotein of VHSV, a fish rhabdovirus

Previous studies mapped a p2 domain (aa 82-109) which binds phosphatidylserine (PS) (Estepa and Coll, 1996a) and contains three contiguous hydrophobic amino acid heptad repeats followed by a positively charged stretch (Coll, 1995b) in the glycoprotein G of the viral hemorrhagic septicemia virus (VHSV), a fish rhabdovirus. Anti-p2 antibodies inhibited low-pH VHSV-induced fusion (Estepa and Coll, 1997) and low-pH PS binding to VHSV (Estepa and Coll, 1996a). We report here further studies on the interaction of the synthetic peptide p2 with phospholipid vesicles. The synthetic p2 peptide was able to mediate aggregation, lipid mixing, and leakage of contents only with negatively charged phospholipid vesicles and in a concentration-dependent manner. As shown by its effect on lipid phase transitions deduced from data with fluorescence polarization and differential scanning calorimetry, the p2 peptide becomes inserted into the hydrophobic negatively charged phospholipid vesicle bilayers. In addition, data based on circular dichroism showed that the p2 peptide folds as a structure with a high content of beta-sheets stabilized by interaction with anionic phospholipids. These studies are potentially relevant to viral fusion in VHSV.

Mutational analysis of the candidate internal fusion peptide of the avian leukosis and sarcoma virus subgroup A envelope glycoprotein

The transmembrane subunit (TM) of the avian leukosis and sarcoma virus (ALSV) envelope glycoprotein (Env) contains a stretch of conserved hydrophobic amino acids internal to its amino terminus (residues 21 to 42). By analogy with similar sequences in other viral envelope glycoproteins, this region has been proposed to be a fusion peptide. We investigated the role of this region by changing each of three hydrophobic residues (Ile-21, Val-30, and Ile-39) to glutamatic acid and lysine in the ALSV subgroup A Env. Like wild-type (wt) Env, all six mutant Env proteins were proteolytically processed, oligomerized, and expressed at the cell surface in a form that bound Tva, the ALSV subgroup A receptor. Like wt Env, Ile21Glu, Ile21Lys, Va30Glu, and Val30Lys changed conformation upon binding Tva, as assayed by sensitivity to thermolysin. Ile39Glu and Ile39Lys were cleaved by thermolysin in both the absence and presence of Tva. Although incorporated into virus particles at approximately equal levels, all mutant Envs were compromised in their ability to support infection. The mutants at residues 21 and 30 showed levels of infection 2 to 3 orders of magnitude lower than that of wt Env. The mutants at residue 39 were noninfectious. Furthermore, none of the mutants displayed activity in a cell-cell fusion assay. Our results support the contention that residues 21 to 42 of ALSV subgroup A Env constitute its fusion peptide.

The transmembrane domain in viral fusion: essential role for a conserved glycine residue in vesicular stomatitis virus G protein

The transmembrane (TM) domains of viral fusion proteins are required for fusion, but their precise role is unknown. G protein, the fusion protein of vesicular stomatitis virus, was previously shown to lose syncytia-forming ability if six residues (GLIIGL) were deleted from its TM domain. The 20-residue TM domain of wild-type (TM20) G protein was thus changed into a TM domain of 14 residues (TM14). To assess possible sequence specificity for this loss of function, the two Gly residues in TM20 were replaced with either Ala or Leu. Both mutations resulted in complete loss of fusion activity, as measured by fusion-dependent reporter gene transfer. Single substitutions decreased activity by about half. TM14 was weakly active (15%) but reintroduction of a Gly residue into TM14 by a single Ile --> Gly substitution increased activity to 80%. All mutants retained normal hemifusion activity, i.e., lipid mixing between the outer leaflets of the reacting membranes. Thus, at least one TM Gly residue is required for a late step in fusion mediated by G protein. Gly residues were significantly (2.6-fold; P = 0.004) more abundant in the TM domains of viral fusion proteins than in those of nonfusion proteins and were distributed differently within the TM domain. Thus, Gly residues in the TM domain of other viral fusion proteins may also prove to be important for fusion activity.

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.

Attenuation of recombinant vesicular stomatitis viruses encoding mutant glycoproteins demonstrate a critical role for maintaining a high pH threshold for membrane fusion in viral fitness

A plasmid-based recovery system was used to generate four unique vesicular stomatitis virus (VSV) mutants that encode glycoproteins (G proteins) with single or double amino acid substitutions in two conserved acidic residues adjacent to the putative G protein fusion domain. Previously we demonstrated that three of the mutant G proteins (D137-L, E139-L, and DE-SS) have slightly reduced pH thresholds for membrane fusion activity. In this report we show that even though the viruses encoding D137-L, E139-L, and DE-SS were recovered with high efficiency, these mutants were attenuated for growth in cell culture. Plaque formation was significantly delayed with these mutants and the plaques were smaller and more diffuse than those produced by wild type VSV. In addition, cells infected with these mutants produced approximately 5- to 10-fold less infectious virus than cells infected with a similarly recovered VSV encoding the wild-type G protein. Using R18-labeled virus we found that the mutant G proteins had approximately 50% of the fusion activity of wild-type G at pH 6.3 and only 75% activity at pH 5.8. We also show that the mutant viruses were more sensitive to chloroquine inhibition of infection than either wild-type VSV or the mutant E139-T, which has a fusion phenotype similar to wild-type G protein. Reduced fusion activity and attenuation of infectivity was not due to differences in the amount of G protein incorporated into virions, nor to differences in the amount of virus binding to cells at physiological pH. Although infectivity was assayed at neutral pH, we observed an increase in virus binding with both mutant and wild-type virions as the pH was lowered, and the increase in binding occurred near the pH threshold for membrane fusion activity. From these data we propose a model in which VSV entry involves an increase in virus binding to the inner leaflet of the endosomal membrane during endosome acidification. Concomitant with this higher affinity binding, G protein becomes primed to initiate fusion of the viral envelope with the endosomal membrane. Viruses with mutations that delay the onset of increased binding and fusion lag behind wild-type VSV in their ability to initiate a productive infection, potentially because the location within the cytoplasm where these viruses ultimately fuse is not optimal for either virus uncoating or replication of the viral genome.

Influence of membrane anchoring and cytoplasmic domains on the fusogenic activity of vesicular stomatitis virus glycoprotein G

Chimeric proteins in which the transmembrane anchoring sequence (TM) or both the TM and the cytoplasmic tail (CT) of vesicular stomatitis virus glycoprotein G were replaced with corresponding domains of viral or cellular integral membrane proteins were used to examine the influence of these domains on acidic-pH-induced membrane fusion by G protein. The TM and CT of G were also replaced with the lipid anchor glycosylphosphatidylinositol. Hybrids containing foreign TM or TM and CT sequences were fusogenic at acidic pH but glycosylphosphatidylinositol-anchored G was nonfusogenic at acidic pH. The results suggest that the fusogenic activity of G protein requires membrane anchoring by a hydrophobic peptide sequence and the specific amino acid sequence of the TM has no influence on fusogenic activity.

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

The interaction of VSV glycoprotein (VSV G) with biological membranes was studied by photosensitized labeling. The method is based on photosensitized activation by the fluorescent lipid analog 3,3'-dioctadecyloxacarbocyanine (DiO) of a hydrophobic probe, [125I]iodonaphthyl azide (125INA), that rapidly partitions into the membrane bilayer of virus and cells. 125INA labeling of proteins and lipids can be confined to the site of chromophore localization by photosensitized labeling. Photoactivation using visible light of target membrane labeled with DiO and 125INA, to which unlabeled virions are bound, results in exclusive labeling of envelope glycoproteins inserted into the target membrane [Pak et al. (1994) J. Biol. Chem. 269, 14614]. In this study, we labeled lipid symmetric erythrocyte ghosts with 125INA and DiO. Photosensitized activation of VSV prebound to labeled ghosts with visible light resulted in VSV G labeling under fusogenic conditions. Photoactivation of 125INA by UV light, which is nonspecific, produced labeled VSV G at both acidic and neutral pH. Photosensitized labeling of VSV G by DiO-125INA-ghosts was also observed at pH 5.5, 4 degrees C, in the absence of mixing between viral and cellular lipids, suggesting insertion of the ectodomain of VSV G. Soluble VSV G lacking the transmembrane domain inserted into DiO-125INA-ghosts under the same conditions as intact VSV G. DiO inserted into intact VSV appeared to be a suitable fluorophore for continuous kinetic measurements of membrane fusion by fluorescence dequenching. Our photosensitized labeling results establish biochemical correlates for the three states of VSV G, which we had proposed based on kinetic data [Clague et al., Biochemistry 29, 1303]. In addition, we found that VSV G insertion into the target membrane is reversible, suggesting a "velcro"-like attachment of the fusogenic domain with the target membrane.

What studies of fusion peptides tell us about viral envelope glycoprotein-mediated membrane fusion (review)

This review describes the numerous and innovative methods used to study the structure and function of viral fusion peptides. The systems studied include both intact fusion proteins and synthetic peptides interacting with model membranes. The strategies and methods include dissecting the fusion process into intermediate stages, comparing the effects of sequence mutations, electrophysiological patch clamp methods, hydrophobic photolabelling, video microscopy of the redistribution of both aqueous and lipophilic fluorescent probes between cells, standard optical spectroscopy of peptides in solution (circular dichroism and fluorescence) and attenuated total reflection-Fourier transform infrared spectroscopy of peptides bound to planar bilayers. Although the goal of a detailed picture of the fusion pore has not been achieved for any of the intermediate stages, important properties useful for constraining the development of models are emerging. For example, the presence of alpha-helical structure in at least part of the fusion peptide is strongly correlated with activity; whereas, beta-structure tends to be less prevalent, associated with non-native experimental conditions, and more related to vesicle aggregation than fusion. The specific angle of insertion of the peptides into the membrane plane is also found to be an important characteristic for the fusion process. A shallow penetration, extending only to the central aliphatic core region, is likely responsible for the destabilization of the lipids required for coalescence of the apposing membranes and fusion. The functional role of the fusion peptides (which tend to be either nonpolar or aliphatic) is then to bind to and dehydrate the outer bilayers at a localized site; and thus reduce the energy barrier for the formation of highly curved, lipidic 'stalk' intermediates. In addition, the importance of the formation of specific, 'higher-order' fusion peptide complexes has also been shown. Recent crystallographic structures of core domains of two more fusion proteins (in addition to influenza haemagglutinin) has greatly facilitated the development of prototypic models of the fusion site. This latter effort will undoubtedly benefit from the insights and constraints gained from the studies of fusion peptides.

Characterization of two temperature-sensitive mutants of coronavirus mouse hepatitis virus strain A59 with maturation defects in the spike protein

Two temperature-sensitive (ts) mutants of mouse hepatitis virus strain A59, ts43 and ts379, have been described previously to be ts in infectivity but unaffected in RNA synthesis (M. J. M. Koolen, A. D. M. E. Osterhaus, G. van Steenis, M. C. Horzinek, and B. A. M. van der Zeijst, Virology 125:393-402, 1983). We present a detailed analysis of the protein synthesis of the mutant viruses at the permissive (31 degrees C) and nonpermissive (39.5 degrees C) temperatures. It was found that synthesis of the nucleocapsid protein N and the membrane protein M of both viruses was insensitive to temperature. However, the surface protein S of both viruses was retained in the endoplasmic reticulum at the nonpermissive temperature. This was shown first by analysis of endoglycosidase H-treated and immunoprecipitated labeled S proteins. The mature Golgi form of S was not present at the nonpermissive temperature for the ts viruses, in contrast to wild-type (wt) virus. Second, gradient purification of immunoprecipitated S after pulse-chase labeling showed that only wt virus S was oligomerized. We conclude that the lack of oligomerization causes the retention of the ts S proteins in the endoplasmic reticulum. As a result, ts virus particles that were devoid of S were produced at the nonpermissive temperature. This result could be confirmed by biochemical analysis of purified virus particles and by electron microscopy.

Phosphatidylserine binding to solid-phase rhabdoviral peptides: a new method to study phospholipid/viral protein interactions

A new method is described for the study of phosphatidylserine binding to rhabdoviral peptides by using solid-phase assays. This new assay could probably be extended to study the interactions between host membrane phospholipid and viral proteins in other viruses. By using labeled and hydrated phosphatidylserine (PS), PS-binding to solid-phase 15-mer peptides (pepscan) could map putative phospholipid-binding regions of the glycoprotein G of viral haemorrhagic septicaemia virus (VHSV), a salmonid rhabdovirus. The major PS-binding region of 27 aa (aa82-109, p2) did not only bind PS, but also phosphatidylethanolamine (PE) and phosphatidylcholine (PC). Extraction of the PS bound to solid-phase p2 by a variety of chemical compounds and competition experiments with several phospholipid-related compounds showed that PS-Binding to p2 was dependent on not only hydrophobic, but also ionic interactions, as suggested by prior work on phospholipid interactions in other rhabdoviruses. Saturation/competition experiments with labeled and cold PS, PE and PC also showed that the reaction probably takes place between high molecular weight aggregates of hydrated phospholipids and several molecules of solid-phase p2. This assay has been used previously to detect hydrophobic amino acid heptad-repeats in rhabdoviruses and when anti-p2 antibodies to VHSV were obtained they were capable of inhibiting VHSV-induced cell to cell fusion.

Photolabeling identifies a putative fusion domain in the envelope glycoprotein of rabies and vesicular stomatitis viruses

Vesicular stomatitis and rabies viruses enter cells through receptor-mediated endocytosis, followed by fusion of the viral with the endosomal membrane. The latter step is catalyzed by the viral envelope glycoprotein, which, in the low pH environment of the endosome, undergoes a conformational transition to a fusion-competent state. To investigate whether fusion competence involves the low pH exposure of a hydrophobic fusion region(s), we have applied hydrophobic photolabeling using the recently developed phospholipid analogue 1-O-hexadecanoyl-2-O-[9-[[[2-[125I]iodo-4-(trifluoromethyl-3H- diazirin-3-yl)benzyl]oxy]carbonyl] nonanoyl]-sn-glycero-3-phosphocholine ([125I]TID-PC/16) (Weber, T., and Brunner, J. (1995) J. Am. Chem. Soc. 117, 3084-3095). Rosettes of rabies virus glycoprotein, whole rabies virus, or vesicular stomatitis virus were incubated with large unilamellar vesicles containing [125I]TID-PC/16. Following reagent activation, the labeled glycoprotein was isolated and analyzed. In all cases, labeling of the glycoprotein strongly increased as the pH was lowered from 7.0 to 6.0, suggesting the exposure at acidic pH of a domain capable of interacting with membranes. To identify the labeled region(s), CNBr fragments were generated and analyzed by SDS-polyacrylamide followed by autoradiography. In rabies glycoprotein, the labeled segment was found to be contained within fragment RCr5 (residues 103-179). Glycoprotein from vesicular stomatitis virus was labeled within fragment VCr1 (residues 59-221). These results demonstrate that rhabdovirus glycoprotein contains a domain that at low pH is capable of interacting with a target membrane in a hydrophobic manner. This domain may play a role similar to that of the fusion peptide found in many other viral fusion proteins.