Acid pH-induced fusion of cells by herpes simplex virus glycoproteins gB an gD

Newcastle Disease Virus Entry into Chicken Macrophages via a pH-Dependent, Dynamin and Caveola-Mediated Endocytic Pathway That Requires Rab5

The cellular entry pathways and the mechanisms of Newcastle disease virus (NDV) entry into cells are poorly characterized. In this study, we demonstrated that chicken interferon-induced transmembrane protein 1 (chIFITM1), which is located in the early endosomes, could limit the replication of NDV in chicken macrophage cell line HD11, suggesting the endocytic entry of NDV into chicken macrophages. Then, we presented a systematic study about the entry mechanism of NDV into chicken macrophages. First, we demonstrated that a low-pH condition and dynamin were required during NDV entry. However, NDV entry into chicken macrophages was independent of clathrin-mediated endocytosis. We also found that NDV entry was dependent on membrane cholesterol. The NDV entry and replication were significantly reduced by nystatin and phorbol 12-myristate 13-acetate treatment, overexpression of dominant-negative (DN) caveolin-1, or knockdown of caveolin-1, suggesting that NDV entry depends on caveola-mediated endocytosis. However, macropinocytosis did not play a role in NDV entry into chicken macrophages. In addition, we found that Rab5, rather than Rab7, was involved in the entry and traffic of NDV. The colocalization of NDV with Rab5 and early endosome suggested that NDV virion was transported to early endosomes in a Rab5-dependent manner after internalization. Of particular note, the caveola-mediated endocytosis was also utilized by NDV to enter primary chicken macrophages. Moreover, NDV entered different cell types using different pathways. Collectively, our findings demonstrate for the first time that NDV virion enters chicken macrophages via a pH-dependent, dynamin and caveola-mediated endocytosis pathway and that Rab5 is involved in the traffic and location of NDV. IMPORTANCE Although the pathogenesis of Newcastle disease virus (NDV) has been extensively studied, the detailed mechanism of NDV entry into host cells is largely unknown. Macrophages are the first-line defenders of host defense against infection of pathogens. Chicken macrophages are considered one of the main types of target cells during NDV infection. Here, we comprehensively investigated the entry mechanism of NDV in chicken macrophages. This is the first report to demonstrate that NDV enters chicken macrophages via a pH-dependent, dynamin and caveola-mediated endocytosis pathway that requires Rab5. The result is important for our understanding of the entry of NDV in chicken macrophages, which will further advance the knowledge of NDV pathogenesis and provide useful clues for the development of novel preventive or therapeutic strategies against NDV infection. In addition, this information will contribute to our further understanding of pathogenesis with regard to other members of the Avulavirus genus in the Paramyxoviridae family.

Common characteristics and unique features: A comparison of the fusion machinery of the alphaherpesviruses Pseudorabies virus and Herpes simplex virus

Membrane fusion is a fundamental biological process that allows different cellular compartments delimited by a lipid membrane to release or exchange their respective contents. Similarly, enveloped viruses such as alphaherpesviruses exploit membrane fusion to enter and infect their host cells. For infectious entry the prototypic human Herpes simplex viruses 1 and 2 (HSV-1 and -2, collectively termed HSVs) and the porcine Pseudorabies virus (PrV) utilize four different essential envelope glycoproteins (g): the bona fide fusion protein gB and the regulatory heterodimeric gH/gL complex that constitute the "core fusion machinery" conserved in all members of the Herpesviridae; and the subfamily specific receptor binding protein gD. These four components mediate attachment and fusion of the virion envelope with the host cell plasma membrane through a tightly regulated sequential activation process. Although PrV and the HSVs are closely related and employ the same set of glycoproteins for entry, they show remarkable differences in the requirements for fusion. Whereas the HSVs strictly require all four components for membrane fusion, PrV can mediate cell-cell fusion without gD. Moreover, in contrast to the HSVs, PrV provides a unique opportunity for reversion analyses of gL-negative mutants by serial cell culture passaging, due to a limited cell-cell spread capacity of gL-negative PrV not observed in the HSVs. This allows a more direct analysis of the function of gH/gL during membrane fusion. Unraveling the molecular mechanism of herpesvirus fusion has been a goal of fundamental research for years, and yet important mechanistic details remain to be uncovered. Nevertheless, the elucidation of the crystal structures of all key players involved in PrV and HSV membrane fusion, coupled with a wealth of functional data, has shed some light on this complex puzzle. In this review, we summarize and discuss the contemporary knowledge on the molecular mechanism of entry and membrane fusion utilized by the alphaherpesvirus PrV, and highlight similarities but also remarkable differences in the requirements for fusion between PrV and the HSVs.

Characterisation of the epitope for a herpes simplex virus glycoprotein B-specific monoclonal antibody with high protective capacity

Monoclonal antibody (MAb) 2c, specific for glycoprotein B of herpes simplex virus (HSV), had been shown to mediate clearance of infection from the mucous membranes of mice, thereby completely inhibiting mucocutaneous inflammation and lethality, even in mice depleted of both CD4(+) and CD8(+) cells. Additionally, ganglionic infection was highly restricted. In vitro, MAb 2c exhibits a potent complement-independent neutralising activity against HSV type 1 and 2, completely inhibits the viral cell-to-cell spread as well as the syncytium formation induced by syncytial HSV strains (Eis-Hübinger et al. in Intervirology 32:351-360, 1991; Eis-Hübinger et al. in J Gen Virol 74:379-385, 1993). Here, we describe the mapping of the epitope for MAb 2c. The antibody was found to recognise a discontinuous epitope comprised of the HSV type 1 glycoprotein B residues 299 to 305 and one or more additional discontinuous regions that can be mimicked by the sequence FEDF. Identification of the epitope was confirmed by loss of antibody binding to mutated glycoprotein B with replacement of the epitopic key residues, expressed in COS-1 cells. Similarly, MAb 2c was not able to neutralise HSV mutants with altered key residues, and MAb 2c was ineffective in mice inoculated with such mutants. Interestingly, identification and fine-mapping of the discontinuous epitope was not achieved by binding studies with truncated glycoprotein B variants expressed in COS cells but by peptide scanning with synthetic overlapping peptides and peptide key motif analysis. Reactivity of MAb 2c was immensely increased towards a peptide composed of the glycoprotein B residues 299 to 305, a glycine linker, and a C-terminal FEDF motif. If it could be demonstrated that antibodies of the specificity and bioactivity of MAb 2c can be induced by the epitope or a peptide mimicking the epitope, strategies for active immunisation might be conceivable.

Virus entry by endocytosis

Although viruses are simple in structure and composition, their interactions with host cells are complex. Merely to gain entry, animal viruses make use of a repertoire of cellular processes that involve hundreds of cellular proteins. Although some viruses have the capacity to penetrate into the cytosol directly through the plasma membrane, most depend on endocytic uptake, vesicular transport through the cytoplasm, and delivery to endosomes and other intracellular organelles. The internalization may involve clathrin-mediated endocytosis (CME), macropinocytosis, caveolar/lipid raft-mediated endocytosis, or a variety of other still poorly characterized mechanisms. This review focuses on the cell biology of virus entry and the different strategies and endocytic mechanisms used by animal viruses.

Low pH-induced conformational change in herpes simplex virus glycoprotein B

Herpesviruses can enter host cells using pH-dependent endocytosis pathways in a cell-specific manner. Envelope glycoprotein B (gB) is conserved among all herpesviruses and is a critical component of the complex that mediates membrane fusion and entry. Here we demonstrate that mildly acidic pH triggers specific conformational changes in herpes simplex virus (HSV) gB. The antigenic structure of gB was specifically altered by exposure to low pH both in vitro and during entry into host cells. The oligomeric conformation of gB was altered at a similar pH range. Exposure to acid pH appeared to convert virion gB into a lower-order oligomer. The detected conformational changes were reversible, similar to those in other class III fusion proteins. Exposure of purified, recombinant gB to mildly acidic pH resulted in similar changes in conformation and caused gB to become more hydrophobic, suggesting that low pH directly affects gB. We propose that intracellular low pH induces alterations in gB conformation that, together with additional triggers such as receptor binding, are essential for virion-cell fusion during herpesviral entry by endocytosis.

Human cytomegalovirus glycoproteins gB and gH/gL mediate epithelial cell-cell fusion when expressed either in cis or in trans

Herpesviruses use a cascade of interactions with different cell surface molecules to gain entry into cells. In many cases, this involves binding to abundant glycosaminoglycans or integrins followed by interactions with more limited cell surface proteins, leading to fusion with cellular membranes. Human cytomegalovirus (HCMV) has the ability to infect a wide variety of human cell types in vivo. However, very little is known about which HCMV glycoproteins mediate entry into various cell types, including relevant epithelial and endothelial cells. For other herpesviruses, studies of cell-cell fusion induced by viral proteins have provided substantial information about late stages of entry. In this report, we describe the fusion of epithelial, endothelial, microglial, and fibroblast cells in which HCMV gB and gH/gL were expressed from nonreplicating adenovirus vectors. Fusion frequently involved the majority of cells, and gB and gH/gL were both necessary and sufficient for fusion, whereas no fusion occurred when either glycoprotein was omitted. Coexpression of UL128, UL130, and UL131 did not enhance fusion. We concluded that the HCMV core fusion machinery consists of gB and gH/gL. Coimmunoprecipitation indicated that HCMV gB and gH/gL can interact. Importantly, expression of gB and gH/gL in trans (gB-expressing cells mixed with other gH/gL-expressing cells) resulted in substantial fusion. We believe that this is the first description of a multicomponent viral fusion machine that can be split between cells. If gB and gH/gL must interact for fusion, then these molecules must reach across the space between apposing cells. Expression of gB and gH/gL in trans with different cell types revealed surface molecules that are required for fusion on HCMV-permissive cells but not on nonpermissive cells.

Role for nectin-1 in herpes simplex virus 1 entry and spread in human retinal pigment epithelial cells

Herpes simplex virus 1 (HSV-1) demonstrates a unique ability to infect a variety of host cell types. Retinal pigment epithelial (RPE) cells form the outermost layer of the retina and provide a potential target for viral invasion and permanent vision impairment. Here we examine the initial cellular and molecular mechanisms that facilitate HSV-1 invasion of human RPE cells. High-resolution confocal microscopy demonstrated initial interaction of green fluorescent protein (GFP)-tagged virions with filopodia-like structures present on cell surfaces. Unidirectional movement of the virions on filopodia to the cell body was detected by live cell imaging of RPE cells, which demonstrated susceptibility to pH-dependent HSV-1 entry and replication. Use of RT-PCR indicated expression of nectin-1, herpes virus entry mediator (HVEM) and 3-O-sulfotransferase-3 (as a surrogate marker for 3-O-sulfated heparan sulfate). HVEM and nectin-1 expression was subsequently verified by flow cytometry. Nectin-1 expression in murine retinal tissue was also demonstrated by immunohistochemistry. Antibodies against nectin-1, but not HVEM, were able to block HSV-1 infection. Similar blocking effects were seen with a small interfering RNA construct specifically directed against nectin-1, which also blocked RPE cell fusion with HSV-1 glycoprotein-expressing Chinese hamster ovary (CHO-K1) cells. Anti-nectin-1 antibodies and F-actin depolymerizers were also successful in blocking the cytoskeletal changes that occur upon HSV-1 entry into cells. Our findings shed new light on the cellular and molecular mechanisms that help the virus to enter the cells of the inner eye.

A novel role for phagocytosis-like uptake in herpes simplex virus entry

It is becoming increasingly clear that herpesviruses can exploit the endocytic pathway to infect cells, yet several important features of this process remain poorly defined. Using herpes simplex virus-1 (HSV-1) as a model, we demonstrate that endocytosis of the virions mimic many features of phagocytosis. During entry, HSV-1 virions associated with plasma membrane protrusions followed by a phagocytosis-like uptake involving rearrangement of actin cytoskeleton and trafficking of the virions in large phagosome-like vesicles. RhoA GTPase was activated during this process and the mode of entry was cell type-specific. Clathrin-coated vesicles had no detectable role in virion trafficking as Eps15 dominant-negative mutants failed to affect HSV-1 uptake. Binding and fusion of the virion envelope with the phagosomal membrane is likely facilitated by clustering of nectin-1 (or HVEM) in phagosomes, which was observed in infected cells. Collectively, our data suggests a novel mode of uptake by which the virus can infect both professional and nonprofessional phagocytes.

Immobilized cobalt affinity chromatography provides a novel, efficient method for herpes simplex virus type 1 gene vector purification

Herpes simplex virus type 1 (HSV-1) is a promising vector for gene therapy applications, particularly at peripheral nerves, the natural site of virus latency. Many gene vectors require large particle numbers for even early-phase clinical trials, emphasizing the need for high-yield, scalable manufacturing processes that result in virus preparations that are nearly free of cellular DNA and protein contaminants. HSV-1 is an enveloped virus that requires the development of gentle purification methods. Ideally, such methods should avoid centrifugation and may employ selective purification processes that rely on the recognition of a unique envelope surface chemistry. Here we describe a novel method that fulfills these criteria. An immobilized metal affinity chromatography (IMAC) method was developed for the selective purification of vectors engineered to display a high-affinity binding peptide. Feasibility studies involving various transition metal ions (Cu2+, Zn2+, Ni2+, and Co2+) showed that cobalt had the most desirable features, which include a low level of interaction with either the normal virus envelope or contaminating DNA and proteins. The introduction of a cobalt-specific recognition element into the virus envelope may provide a suitable target for cobalt-dependent purification. To test this possibility, we engineered a peptide with affinity for immobilized cobalt in frame in the heparan sulfate binding domain of HSV-1 glycoprotein B, which is known to be exposed on the surface of the virion particle and recombined into the viral genome. By optimizing the IMAC loading conditions and reducing cobalt ion leakage, we recovered 78% of the tagged HSV-1 recombinant virus, with a >96% reduction in contaminating proteins and DNA.

Herpes simplex virus type 1 glycoprotein B sorting in hippocampal neurons

Herpes simplex virus type 1 (HSV-1) is a neuroinvasive human pathogen that spreads in the nervous system in functionally connected neurons. Determining how HSV-1 components are sorted in neurons is critical to elucidate the mechanisms of virus neuroinvasion. By using recombinant viruses expressing glycoprotein B (gB) tagged with green fluorescent protein (GFP), the subcellular localization of this envelope protein was visualized in infected hippocampal neurons in culture. Results obtained using a fully infectious recombinant virus containing GFP inserted into the ectodomain of gB support the view that capsids and gB are transported separately in neuron processes. Moreover, they show that during infection gB is sorted to the dendritic tree and the axons of polarized hippocampal neurons. However, GFP insertion into the cytoplasmic tail of gB impaired the maturation of the resulting fusion protein and caused its retention in the endoplasmic reticulum. The defective protein did not gain access to axons of infected neurons. These results suggest that the cytoplasmic tail of gB plays a role in maturation and transport and subsequently in axonal sorting in differentiated hippocampal neurons.

Roles for endocytosis and low pH in herpes simplex virus entry into HeLa and Chinese hamster ovary cells

Herpes simplex virus (HSV) infection of many cultured cells, e.g., Vero cells, can be initiated by receptor binding and pH-neutral fusion with the cell surface. Here we report that a major pathway for HSV entry into the HeLa and CHO-K1 cell lines is dependent on endocytosis and exposure to a low pH. Enveloped virions were readily detected in HeLa or receptor-expressing CHO cell vesicles by electron microscopy at <30 min postinfection. As expected, images of virus fusion with the Vero cell surface were prevalent. Treatment with energy depletion or hypertonic medium, which inhibits endocytosis, prevented uptake of HSV from the HeLa and CHO cell surface relative to uptake from the Vero cell surface. Incubation of HeLa and CHO cells with the weak base ammonium chloride or the ionophore monensin, which elevate the low pH of organelles, blocked HSV entry in a dose-dependent manner. Noncytotoxic concentrations of these agents acted at an early step during infection by HSV type 1 and 2 strains. Entry mediated by the HSV receptor HveA, nectin-1, or nectin-2 was also blocked. As analyzed by fluorescence microscopy, lysosomotropic agents such as the vacuolar H(+)-ATPase inhibitor bafilomycin A1 blocked the delivery of virus capsids to the nuclei of the HeLa and CHO cell lines but had no effect on capsid transport in Vero cells. The results suggest that HSV can utilize two distinct entry pathways, depending on the type of cell encountered.

Recovery of an unusual fusogenic herpes simplex virus type 2 strain from a clinical specimen

A highly unusual herpes simplex virus type 2 strain, strain Burr, was isolated from a female genital tract clinical specimen. This virus induced remarkably rapid and extensive syncytium formation in Vero cells involving hundreds of cells but was less fusion active in HEp-2 cells, MRC-5 cells, and mink lung cells. Virus-infected cells produced the glycoproteins gB, gC, gD and gE.

Characterization of cell-cell fusion mediated by herpes simplex virus 2 glycoproteins gB, gD, gH and gL in transfected cells

The mechanisms by which herpes simplex viruses (HSV) mediate fusion between their envelope and the plasma membrane during entry into cells, and between the plasma membranes of adjacent infected and uninfected cells to form multinucleated giant cells, are poorly understood. Four viral glycoproteins (gB, gD, gH and gL) are required for virus-cell fusion, whereas these plus several others are required for cell-cell fusion (syncytium formation). A better understanding would be aided by the availability of a model system, whereby fusion could be induced with a minimal set of proteins, in the absence of infection. A suitable system has now been developed for HSV-2, using transfected COS7, 293 or HEp-2 cells. Insofar as the minimal set of HSV-2 proteins required to cause cell-cell fusion in this system is gB, gD, gH and gL, it would appear to resemble virus-cell fusion rather than syncytium formation. However, the ability of a mutation in gB to enhance the fusion of both transfected cells and infected cells, while having no effect on virus-cell fusion, points to the opposite conclusion. The differential effects of a panel of anti-HSV antibodies, and of the fusion-inhibitor cyclosporin A, confirm that the fusion of transfected cells shares some properties with virus-cell fusion and others with syncytium formation. It may therefore prove useful for determining how these processes differ, and for testing the hypothesis that some viral proteins prevent membrane fusion until the appropriate point in the virus life-cycle, with other proteins then overcoming this block.

Pseudorabies virus glycoprotein M inhibits membrane fusion

A transient transfection-fusion assay was established to investigate membrane fusion mediated by pseudorabies virus (PrV) glycoproteins. Plasmids expressing PrV glycoproteins under control of the immediate-early 1 promoter-enhancer of human cytomegalovirus were transfected into rabbit kidney cells, and the extent of cell fusion was quantitated 27 to 42 h after transfection. Cotransfection of plasmids encoding PrV glycoproteins B (gB), gD, gH, and gL resulted in formation of polykaryocytes, as has been shown for homologous proteins of herpes simplex virus type 1 (HSV-1) (A. Turner, B. Bruun, T. Minson, and H. Browne, J. Virol. 72:873-875, 1998). However, in contrast to HSV-1, fusion was also observed when the gD-encoding plasmid was omitted, which indicates that PrV gB, gH, and gL are sufficient to mediate fusion. Fusogenic activity was enhanced when a carboxy-terminally truncated version of gB (gB-008) lacking the C-terminal 29 amino acids was used instead of wild-type gB. With gB-008, only gH was required in addition for fusion. A very rapid and extended fusion was observed after cotransfection of plasmids encoding gB-008 and gDH, a hybrid protein consisting of the N-terminal 271 amino acids of gD fused to the 590 C-terminal amino acids of gH. This protein has been shown to substitute for gH, gD, and gL function in the respective viral mutants (B. G. Klupp and T. C. Mettenleiter, J. Virol. 73:3014-3022, 1999). Cotransfection of plasmids encoding PrV gC, gE, gI, gK, and UL20 with gB-008 and gDH had no effect on fusion. However, inclusion of a gM-expressing plasmid strongly reduced the extent of fusion. An inhibitory effect was also observed after inclusion of plasmids encoding gM homologs of equine herpesvirus 1 or infectious laryngotracheitis virus but only in conjunction with expression of the gM complex partner, the gN homolog. Inhibition by PrV gM was not limited to PrV glycoprotein-mediated fusion but also affected fusion induced by the F protein of bovine respiratory syncytial virus, indicating a general mechanism of fusion inhibition by gM.

Pseudotyping of glycoprotein D-deficient herpes simplex virus type 1 with vesicular stomatitis virus glycoprotein G enables mutant virus attachment and entry

The use of herpes simplex virus (HSV) vectors for in vivo gene therapy will require the targeting of vector infection to specific cell types in certain in vivo applications. Because HSV glycoprotein D (gD) imparts a broad host range for viral infection through recognition of ubiquitous host cell receptors, vector targeting will require the manipulation of gD to provide new cell recognition specificities in a manner designed to preserve gD's essential role in virus entry. In this study, we have determined whether an entry-incompetent HSV mutant with deletions of all Us glycoproteins, including gD, can be complemented by a foreign attachment/entry protein with a different receptor-binding specificity, the vesicular stomatitis virus glycoprotein G (VSV-G). The results showed that transiently expressed VSV-G was incorporated into gD-deficient HSV envelopes and that the resulting pseudotyped virus formed plaques on gD-expressing VD60 cells, albeit at a 50-fold-reduced level compared to that of wild-type gD. This reduction may be related to differences in the entry pathways used by VSV and HSV or to the observed lower rate of incorporation of VSV-G into virus envelopes than that of gD. The rate of VSV-G incorporation was greatly improved by using recombinant molecules in which the transmembrane domain of HSV glycoprotein B or D was substituted for that of VSV-G, but these recombinant molecules failed to promote virus entry. These results show that foreign glycoproteins can be incorporated into the HSV envelope during replication and that gD can be dispensed with on the condition that a suitable attachment/entry function is provided.

Recombinant herpes simplex virus type 1 engineered for targeted binding to erythropoietin receptor-bearing cells

The utility of recombinant herpes simplex virus type 1 (HSV-1) vectors may be expanded by manipulation of the virus envelope to achieve cell-specific gene delivery. To this end, an HSV-1 mutant virus deleted for glycoprotein C (gC) and the heparan sulfate binding domain of gB (KgBpK-gC-) was engineered to encode different chimeric proteins composed of N-terminally truncated forms of gC and the full-length erythropoietin hormone (EPO). Biochemical analyses demonstrated that one gC-EPO chimeric molecule (gCEPO2) was posttranslationally processed, incorporated into recombinant HSV-1 virus (KgBpK-gCEPO2), and neutralized with antibodies directed against gC or EPO in a complement-dependent manner. Moreover, KgBpK-gCEPO2 recombinant virus was specifically retained on a soluble EPO receptor column, was neutralized by soluble EPO receptor, and stimulated proliferation of FD-EPO cells, an EPO growth-dependent cell line. FD-EPO cells were nevertheless refractory to productive infection by both wild-type HSV-1 and recombinant KgBpK-gCEPO2 virus. Transmission electron microscopy of FD-EPO cells infected with KgBpK-gCEPO2 showed virus endocytosis leading to aborted infection. Despite the lack of productive infection, these data provide the first evidence of targeted HSV-1 binding to a non-HSV-1 cell surface receptor.

A27L protein mediates vaccinia virus interaction with cell surface heparan sulfate

Vaccinia virus has a wide host range and infects mammalian cells of many different species. This suggests that the cell surface receptors for vaccinia virus are ubiquitously expressed and highly conserved. Alternatively, different receptors are used for vaccinia virus infection of different cell types. Here we report that vaccinia virus binds to heparan sulfate, a glycosaminoglycan (GAG) side chain of cell surface proteoglycans, during virus infection. Soluble heparin specifically inhibits vaccinia virus binding to cells, whereas other GAGs such as condroitin sulfate or dermantan sulfate have no effect. Heparin also blocks infections by cowpox virus, rabbitpox virus, myxoma virus, and Shope fibroma virus, suggesting that cell surface heparan sulfate could be a general mediator of the entry of poxviruses. The biochemical nature of the heparin-blocking effect was investigated. Heparin analogs that have acetyl groups instead of sulfate groups also abolish the inhibitory effect, suggesting that the negative charges on GAGs are important for virus infection. Furthermore, BSC40 cells treated with sodium chlorate to produce undersulfated GAGs are more refractory to vaccinia virus infection. Taken together, the data support the notion that cell surface heparan sulfate is important for vaccinia virus infection. Using heparin-Sepharose beads, we showed that vaccinia virus virions bind to heparin in vitro. In addition, we demonstrated that the recombinant A27L gene product binds to the heparin beads in vitro. This recombinant protein was further shown to bind to cells, and such interaction could be specifically inhibited by soluble heparin. All the data together indicated that A27L protein could be an attachment protein that mediates vaccinia virus binding to cell surface heparan sulfate during viral infection.

Glycoproteins gB, gD, and gHgL of herpes simplex virus type 1 are necessary and sufficient to mediate membrane fusion in a Cos cell transfection system

Herpes simplex virus type 1 glycoproteins gB, gD, and gHgL were expressed by transient transfection of Cos cells. Polykaryocyte formation above the background level seen in untransfected controls was observed only if all three components were expressed. Thus, gB, gD, and gHgL are necessary and sufficient to induce membrane fusion.

Mutational analysis of the vesicular stomatitis virus glycoprotein G for membrane fusion domains

The spike glycoprotein G of vesicular stomatitis virus (VSV) induces membrane fusion at low pH. We used linker insertion mutagenesis to characterize the domain(s) of G glycoprotein involved in low-pH-induced membrane fusion. Two or three amino acids were inserted in frame into various positions in the extracellular domain of G, and 14 mutants were isolated. All of the mutants expressed fully glycosylated proteins in COS cells. However, only seven mutant G glycoproteins were transported to the cell surface. Two of these mutants, D1 and A6, showed wild-type fusogenic properties. The mutant A2 had a temperature-sensitive defect in the transport of the mutant G glycoprotein to the cell surface. The other four mutants, H2, H5, H10, and A4, although present in cell surface, failed to induce cell fusion when cells expressing these mutant glycoproteins were exposed to acidic pH. These four mutant G proteins could form trimers, indicating that the defect in fusion was not due to defective oligomerization. One of these mutations, H2, is within a region of conserved, uncharged amino acids that has been proposed as a possible fusogenic sequence. The mutation in H5 was about 70 amino acids downstream of the mutation in H2, while mutations in H10 and A4 were about 300 amino acids downstream of the mutation in H2. Conserved sequences were also noted in the H10 and A4 segment. The results suggest that in the case of VSV G glycoprotein, the fusogenic activity may involve several spatially separated regions in the extracellular domain of the protein.

Immunoelectron microscopic localization of herpes simplex virus glycoprotein gB in the nuclear envelope of infected cells

Herpesvirus, such as herpes simplex type 1 (HSV-1) acquire their envelope by budding through a modified inner membrane of the nuclear envelope which forms thick and dense patches at the site of budding. This suggests that some of the viral envelope glycoproteins must be transported to the nuclear envelope in order to be incorporated into the virus. In an effort to establish the localization of the HSV-1 glycoprotein gB-1 in the nuclear envelope of HSV-1 infected cells directly, we have studied the distribution of the glycoprotein gB-1 by immunoelectron microscopy using a polyclonal anti gB-1 antibody. A specific accumulation of gB-1 in the nuclear envelope, which was five times more labeled than the plasma membrane was observed. The glycoprotein gB-1 was localized in both the outer and the inner membrane of the nuclear envelope. The labeling over the nuclear envelope was distributed evenly and no preferential concentration of gB-1 around or within the patches where the virus buds was detected. The nucleocapsids were found to be labeled only when they become associated with the nuclear envelope indicating that gB-1 is incorporated into the virus at this site.

GTP-induced fusion of isolated pancreatic microsomal vesicles is increased by acidification of the vesicle lumen

Using the 'fusogen' polyethyleneglycol (PEG), Dawson et al. have concluded that both guanosine triphosphate (GTP)-induced calcium efflux and the enhancement of IP3-promoted calcium release from rat liver microsomal vesicles could be attributed to a GTP-dependent vesicle fusion. We have studied GTP-induced fusion of microsomal vesicles from rat exocrine pancreas using light scatter and fluorescence dequenching methods. In the presence of PEG (3%), GTP (10 microM) induced a decrease in light scatter and an increase in fluorescence in the fluorescence dequenching assay (GTP-effect) indicating fusion of the vesicles. Guanosine 5'-O-(3-thiotriphosphate) (10 microM) had no effect on its own and inhibited the GTP-induced signals. Preincubation of the vesicles with adenosine triphosphate (ATP) (4 mM) increased the GTP-effect by 80%, whereas bafilomycin B1, a specific inhibitor of vacuolar type H(+)-ATPases, and the protonophore CCCP (10 microM) inhibited only the ATP-dependent part of the GTP-effect. Inhibitors of the vacuolar type H(+)-ATPase, which are also SH-alkylating reagents such as N-ethylmaleimid (100 microM) and the tyrosine-, cysteine- and lysine-reactive reagent 7-chloro-4-nitrobenz-2-exa-1,3-diazole (10 microM), abolished the GTP-effect in the absence or presence of ATP. We conclude that GTP induces fusion of pancreatic microsomes which is increased by an H+ gradient established by a vacuolar type H(+)-ATPase.

Fusion of intra- and extracellular forms of vaccinia virus with the cell membrane

The membrane fusion activities of the isolated single-envelope intracellular form of vaccinia virus (INV) and the double-envelope extracellular (EEV) form were studied by using a lipid-mixing assay based on the dilution of a fluorescent probe. Fluorescently labeled INV and EEV from both the IHD-J and WR strains of vaccinia virus fused with HeLa cells at neutral pH, suggesting that fusion occurs with the plasma membrane during virus entry. EEV fused more efficiently and with faster kinetics than INV: approximately 50% of bound EEV particles fused over the course of 1 h, compared with only 25% of the INV particles. Fusion of INV and EEV was strongly temperature dependent, being decreased by 50% at 34 degrees C and by 90% at 28 degrees C. A monoclonal antibody to a 14-kilodalton envelope protein of INV that has been implicated in the fusion reaction (J. F. Rodriguez, E. Paez, and M. Esteban, J. Virol. 61:395-404, 1987) completely suppressed the initial rate of fusion of INV but had no effect on the fusion activity of EEV, suggesting that vaccinia virus encodes two or more membrane fusion proteins. Finally, cells infected with the WR strain of vaccinia virus formed syncytia when briefly incubated at pH 6.4 or below, indicating that an acid-activated viral fusion protein is expressed on the cell surface. However, WR INV and EEV did not display increased fusion activity at acid pH, suggesting that the acid-dependent fusion factor is not incorporated into virions or that its activity there is masked.