Oligomer formation of the gB glycoprotein of herpes simplex virus type 1

Out of Sight, but Not Out of Mind: Aspects of the Avian Oncogenic Herpesvirus, Marek's Disease Virus

Marek's disease virus is an economically important avian herpesvirus that causes tumors and immunosuppression in chickens and turkeys. The virus, disease, and vaccines have been known for more than 50 years, but as knowledge gaps still exists, intensive research is still ongoing. The understanding of MDV complexity can provide scientific insight in topics that cannot be experimented in humans, providing a unique model that is dually useful for the benefit of the poultry industry and for studying general herpesvirology. The present review presents the following topics: the MDV biology, the vaccine's and virulent virus' peculiar presence in feathers, protection by vaccination. In addition, two relatively behind the scenes topics are reviewed; first, the meq MDV oncogene and its recent implication in molecular epidemiology and in the MDV virulence determination, and second, the functionality of conformational epitopes of the MDV immunodominant protein, glycoprotein B. Our studies were particular, as they were the only ones describing three-dimensional MDV gB oligomers. MDV gB (glycoprotein B) continuous and discontinuous epitopes were shown to possess distinctive neutralization activities. In contrast, the significance of oligomerization of the viral membrane proteins for the creation of discontinuous epitopes in other herpesviruses was explored extensively.

Acidic pH Mediates Changes in Antigenic and Oligomeric Conformation of Herpes Simplex Virus gB and Is a Determinant of Cell-Specific Entry

Herpes simplex virus (HSV) is an important human pathogen with a high worldwide seroprevalence. HSV enters epithelial cells, the primary site of infection, by a low-pH pathway. HSV glycoprotein B (gB) undergoes low pH-induced conformational changes, which are thought to drive membrane fusion. When neutralized back to physiological pH, these changes become reversible. Here, HSV-infected cells were subjected to short pulses of radiolabeling, followed by immunoprecipitation with a panel of gB monoclonal antibodies (MAbs), demonstrating that gB folds and oligomerizes rapidly and cotranslationally in the endoplasmic reticulum. Full-length gB from transfected cells underwent low-pH-triggered changes in oligomeric conformation in the absence of other viral proteins. MAbs to gB neutralized HSV entry into cells regardless of the pH dependence of the entry pathway, suggesting a conservation of gB function in distinct fusion mechanisms. The combination of heat and acidic pH triggered irreversible changes in the antigenic conformation of the gB fusion domain, while changes in the gB oligomer remained reversible. An elevated temperature alone was not sufficient to induce gB conformational change. Together, these results shed light on the conformation and function of the HSV-1 gB oligomer, which serves as part of the core fusion machinery during viral entry.IMPORTANCE Herpes simplex virus (HSV) causes infection of the mouth, skin, eyes, and genitals and establishes lifelong latency in humans. gB is conserved among all herpesviruses. HSV gB undergoes reversible conformational changes following exposure to acidic pH which are thought to mediate fusion and entry into epithelial cells. Here, we identified cotranslational folding and oligomerization of newly synthesized gB. A panel of antibodies to gB blocked both low-pH and pH-neutral entry of HSV, suggesting conserved conformational changes in gB regardless of cell entry route. Changes in HSV gB conformation were not triggered by increased temperature alone, in contrast to results with EBV gB. Acid pH-induced changes in the oligomeric conformation of gB are related but distinct from pH-triggered changes in gB antigenic conformation. These results highlight critical aspects of the class III fusion protein, gB, and inform strategies to block HSV infection at the level of fusion and entry.

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.

Multiple peptides homologous to herpes simplex virus type 1 glycoprotein B inhibit viral infection

The 773-residue ectodomain of the herpes simplex virus type 1 (HSV-1) glycoprotein B (gB) has been resistant to the use of mutagenic strategies because the majority of the induced mutations result in defective proteins. As an alternative strategy for the identification of functionally important regions and novel inhibitors of infection, we prepared a library of overlapping peptides homologous to the ectodomain of gB and screened for the ability of the peptides to block infection. Seven of 138 15-mer peptides inhibited infection by more than 50% at a concentration of 100 microM. Three peptides (gB94, gB122, and gB131) with 50% effective concentrations (EC(50)s) below 20 microM were selected for further studies. The gB131 peptide (residues 681 to 695 in HSV-1 gB [gB-1]) was a specific entry inhibitor (EC(50), approximately 12 microM). The gB122 peptide (residues 636 to 650 in gB-1) blocked viral entry (EC(50), approximately 18 microM), protected cells from infection (EC(50), approximately 72 microM), and inactivated virions in solution (EC(50), approximately 138 microM). We were unable to discern the step or steps inhibited by the gB94 peptide, which is homologous to residues 496 to 510 in gB-1. Substitution of a tyrosine in the gB122 peptide (Y640 in full-length gB-1) reduced the antiviral activity eightfold, suggesting that this residue is critical for inhibition. This peptide-based strategy could lead to the identification of functionally important regions of gB or other membrane proteins and identify novel inhibitors of HSV-1 entry.

Virucidal activity of a GT-rich oligonucleotide against herpes simplex virus mediated by glycoprotein B

We have investigated the antiviral mechanism of a phosphorothioate oligonucleotide, ISIS 5652, which has activity against herpes simplex virus (HSV) in the low micromolar range in plaque reduction assays. We isolated a mutant that is resistant to this compound. Marker rescue and sequencing experiments showed that resistance was due to at least one of three mutations in the UL27 gene which result in amino acid changes in glycoprotein B (gB). Because gB has a role in attachment and entry of HSV, we tested the effects of ISIS 5652 at these stages of infection. The oligonucleotide potently inhibited attachment of virus to cells at 4 degrees C; however, the resistant mutant did not exhibit resistance at this stage. Moreover, a different oligonucleotide with little activity in plaque reduction assays was as potent as ISIS 5652 in inhibiting attachment. Similarly, ISIS 5652 was able to inhibit entry of pre-attached virions into cells at 37 degrees C, but the mutant did not exhibit resistance in this assay. The mutant did not attach to or enter cells more quickly than did wild-type virus. Strikingly, incubation of wild-type virus with 1 to 2 microM ISIS 5652 at 37 degrees C led to a time-dependent, irreversible loss of infectivity (virucidal activity). No virucidal activity was detected at 4 degrees C or with an unrelated oligonucleotide at 37 degrees C. The resistant mutant and a marker-rescued derivative containing its gB mutations exhibited substantial resistance to this virucidal activity of ISIS 5652. We hypothesize that the GT-rich oligonucleotide induces a conformational change in gB that results in inactivation of infectivity.

Herpes simplex virus glycoprotein B binds to cell surfaces independently of heparan sulfate and blocks virus entry

Virion glycoproteins gB, gD, and gH/gL play essential roles for herpes simplex virus (HSV) entry. The function of gD is to interact with a cognate receptor, and soluble forms of gD block HSV entry by tying up cell surface receptors. Both gB and the nonessential gC interact with cell surface heparan sulfate proteoglycan (HSPG), promoting viral attachment. However, cells deficient in proteoglycan synthesis can still be infected by HSV. This suggests another function for gB. We found that a soluble truncated form of gB bound saturably to the surface of Vero, A431, HeLa, and BSC-1 cells, L-cells, and a mouse melanoma cell line expressing the gD receptor nectin-1. The HSPG analog heparin completely blocked attachment of the gC ectodomain to Vero cells. In contrast, heparin only partially blocked attachment of soluble gB, leaving 20% of the input gB still bound even at high concentrations of inhibitor. Moreover, heparin treatment removed soluble gC but not gB from the cell surface. These data suggest that a portion of gB binds to cells independently of HSPG. In addition, gB bound to two HSPG-deficient cell lines derived from L-cells. Gro2C cells are deficient in HSPG, and Sog9 cells are deficient in HSPG, as well as chondroitin sulfate proteoglycan (CSPG). To identify particular gB epitopes responsible for HSPG-independent binding, we used a panel of monoclonal antibodies (MAbs) to gB to block gB binding. Only those gB MAbs that neutralized virus blocked binding of soluble gB to the cells. HSV entry into Gro2C and Sog9 cells was reduced but still detectable relative to the parental L-cells, as previously reported. Importantly, entry into Gro2C cells was blocked by purified forms of either the gD or gB ectodomain. On a molar basis, the extent of inhibition by gB was similar to that seen with gD. Together, these results suggest that soluble gB binds specifically to the surface of different cell types independently of HSPG and CSPG and that by doing so, the protein inhibits entry. The results provide evidence for the existence of a cellular entry receptor for gB.

HSV trafficking and development of gene therapy vectors with applications in the nervous system

Herpes simplex virus type 1 (HSV-1) is a neurotropic double-stranded DNA virus that causes cold sores, keratitis, and rarely encephalitis in humans. Nonpathogenic HSV-1 gene transfer vectors have been generated by elimination of viral functions necessary for replication. The life cycle of the native virus includes replication in epithelial cells at the site of initial inoculation followed by retrograde axonal transport to the nuclei of sensory neurons innervating the area of cutaneous primary infection. In this review, we summarize the current understanding of the molecular basis for HSV cell entry, nuclear transport of the genome, virion egress following replication, and retrograde and anterograde axonal transport in neurons. We discuss how each of these properties has been exploited or modified to allow the generation of gene transfer vectors with particular utility for neurological applications. Recent advances in engineering virus entry have provided proof of principle that vector targeting is possible. Furthermore, significant and potentially therapeutic modifications to the pathological responses to various noxious insults have been demonstrated in models of peripheral nerve disease. These applications exploit the natural axonal transport mechanism of HSV, allowing transgene expression in the cell nucleus within the inaccessible trigeminal ganglion or dorsal root ganglion, following the noninvasive procedure of subcutaneous vector inoculation. These findings demonstrate the importance of understanding basic virology in the design of vector systems and the powerful approach of exploiting favorable properties of the parent virus in the generation of gene transfer vectors.

Herpes simplex virus type 1 gK is required for gB-mediated virus-induced cell fusion, while neither gB and gK nor gB and UL20p function redundantly in virion de-envelopment

Multiple amino acid changes within herpes simplex virus type 1 (HSV-1) gB and gK cause extensive virus-induced cell fusion and the formation of multinucleated cells (syncytia). Early reports established that syncytial mutations in gK could not cause cell-to-cell fusion in the absence of gB. To investigate the interdependence of gB, gK, and UL20p in virus-induced cell fusion and virion de-envelopment from perinuclear spaces as well as to compare the ultrastructural phenotypes of the different mutant viruses in a syngeneic HSV-1 (F) genetic background, gB-null, gK-null, UL20-null, gB/gK double-null, and gB/UL20 double-null viruses were constructed with the HSV-1 (F) bacterial artificial chromosome pYEBac102. The gK/gB double-null virus YEbacDeltagBDeltagK was used to isolate the recombinant viruses gBsyn3DeltagK and gBamb1511DeltagK, which lack the gK gene and carry the gBsyn3 or gBamb1511 syncytial mutation, respectively. Both viruses formed small nonsyncytial plaques on noncomplementing Vero cells and large syncytial plaques on gK-complementing cells, indicating that gK expression was necessary for gBsyn3- and gBamb1511-induced cell fusion. Lack of virus-induced cell fusion was not due to defects in virion egress, since recombinant viruses specifying the gBsyn3 or gKsyn20 mutation in the UL19/UL20 double-null genetic background caused extensive cell fusion on UL20-complementing cells. As expected, the gB-null virus failed to produce infectious virus, but enveloped virion particles egressed efficiently out of infected cells. The gK-null and UL20-null viruses exhibited cytoplasmic defects in virion morphogenesis like those of the corresponding HSV-1 (KOS) mutant viruses. Similarly, the gB/gK double-null and gB/UL20 double-null viruses accumulated capsids in the cytoplasm, indicating that gB, gK, and UL20p do not function redundantly in membrane fusion during virion de-envelopment at the outer nuclear lamellae.

Herpes simplex virus type 1 glycoprotein K and the UL20 protein are interdependent for intracellular trafficking and trans-Golgi network localization

Final envelopment of the cytoplasmic herpes simplex virus type 1 (HSV-1) nucleocapsid is thought to occur by budding into trans-Golgi network (TGN)-derived membranes. The highly membrane-associated proteins UL20p and glycoprotein K (gK) are required for cytoplasmic envelopment at the TGN and virion transport from the TGN to extracellular spaces. Furthermore, the UL20 protein is required for intracellular transport and cell surface expression of gK. Independently expressed gK or UL20p via transient expression in Vero cells failed to be transported from the endoplasmic reticulum (ER). Similarly, infection of Vero cells with either gK-null or UL20-null viruses resulted in ER entrapment of UL20p or gK, respectively. In HSV-1 wild-type virus infections and to a lesser extent in transient gK and UL20p coexpression experiments, both gK and UL20p localized to the Golgi apparatus. In wild-type, but not UL20-null, viral infections, gK was readily detected on cell surfaces. In contrast, transiently coexpressed gK and UL20p predominantly localized to the TGN and were not readily detected on cell surfaces. However, TGN-localized gK and UL20p originated from endocytosed gK and UL20p expressed at cell surfaces. Retention of UL20p to the ER through the addition of an ER retention motif forced total ER retention of gK, indicating that transport of gK is absolutely dependent on UL20p transport. In all experiments, gK and UL20p colocalized at intracellular sites, including the ER, Golgi, and TGN. These results are consistent with the hypothesis that gK and UL20p directly interact and that this interaction facilitates their TGN localization, an important prerequisite for cytoplasmic virion envelopment and egress.

Herpes simplex virus 1 (HSV-1) strain HSZP glycoprotein B gene: comparison of mutations among strains differing in virulence

The nonpathogenic HSZP strain of HSV-1 induces large polykaryocytes due to a syn3 mutation (His for Arg at residue 858) in the C-terminal endodomain of glycoprotein B (gB) (40). We determined the nucleotide (nt) sequence of the UL27 gene specifying the gB polypeptide of HSZP (gBHSZP) and found 3 mutations in its ectodomain at aminoacids (aa) 59, 79 and 108. The ANGpath virus, which also has a syn3 mutation in the C-terminal endodomain of gB (Val for Ala at residue 855) is pathogenic for adult mice (39), but can be made nonpathogenic by replacing the gBANGpath gene by the corresponding gBKOS sequence (21). The gBANGpath had three ectodomain mutations (at aa 62, 77 and 285), while gBKOS had at least four ectomain mutations (aa 59, 79, 313, and 553). Two mutations (aa 59 and 79) in the latter, located in the variable antigenic site IV/D1 were common for gBKOS and gBHSZP. These together with the gBANGpath mutations at aa 62 and 77 create a cluster of 4 mutations in diverse region of the N-terminal part of gB (between aa 59-79), in which the gBs of pathogenic ANGpath and 17 viruses differ from the gBs of nonpathogenic HSZP and KOS viruses. The lower pathogenicity of KOS as related to gBKOS, is furthermore associated with the change of Ser to Thr at aa 313 (locus III/D2). The possibility is discussed that mutations in both above mentioned antigenic loci could result in higher immunogenicity of the corresponding antigenic epitopes, which, in turn, would contribute to the decreased virulence of HSZP and KOS viruses.

Human herpesvirus-8 glycoprotein B interacts with Epstein-Barr virus (EBV) glycoprotein 110 but fails to complement the infectivity of EBV mutants

To characterize human herpesvirus 8 (HHV-8) gB, the open reading frame was PCR amplified from the HHV-8-infected cell line BCBL-1 and cloned into an expression vector. To facilitate detection of expressed HHV-8 gB, the cytoplasmic tail of the glycoprotein was tagged with the influenza hemagglutinin (HA) epitope. Expression of tagged HHV-8 gB (gB-HA), as well as the untagged form, was readily detected in CHO-K1 cells and several lymphoblastoid cell lines (LCLs). HHV-8 gB-HA was sensitive to endoglycosidase H treatment, and immunofluorescence revealed that HHV-8 gB-HA was detectable in the perinuclear region of CHO-K1 cells. These observations suggest that HHV-8 gB is not processed in the Golgi and localizes to the endoplasmic reticulum or nuclear membrane. Because both HHV-8 and EBV are gamma-herpesviruses, the ability of HHV-8 gB to interact with and functionally complement EBV gp110 was examined. HHV-8 gB-HA and EBV gp110 co-immunoprecipitated, indicating formation of hetero-oligomers. However, HHV-8 gB-HA and HHV-8 gB failed to restore the infectivity of gp110-negative EBV mutants. These findings indicate that although HHV-8 gB and EBV gp110 have similar patterns of intracellular localization and can interact, there is not sufficient functional homology to allow efficient complementation.

Heparan sulfate proteoglycan binding by herpes simplex virus type 1 glycoproteins B and C, which differ in their contributions to virus attachment, penetration, and cell-to-cell spread

Herpes simplex virus type 1 (HSV-1) mutants defective for envelope glycoprotein C (gC) and gB are highly impaired in the ability to attach to cell surface heparan sulfate (HS) moieties of proteoglycans, the initial virus receptor. Here we report studies aimed at defining the HS binding element of HSV-1 (strain KOS) gB and determining whether this structure is functionally independent of gB's role in extracellular virus penetration or intercellular virus spread. A mutant form of gB deleted for a putative HS binding lysine-rich (pK) sequence (residues 68 to 76) was transiently expressed in Vero cells and shown to be processed normally, leading to exposure on the cell surface. Solubilized gBpK- also had substantially lower affinity for heparin-acrylic beads than did wild-type gB, confirming that the HS binding domain had been inactivated. The gBpK- gene was used to rescue a KOS gB null mutant virus to produce the replication-competent mutant KgBpK-. Compared with wild-type virus, KgBpK- showed reduced binding to mouse L cells (ca. 20%), while a gC null mutant virus in which the gC coding sequence was replaced by the lacZ gene (KCZ) was substantially more impaired (ca. 65%-reduced binding), indicating that the contribution of gC to HS binding was greater than that of gB. The effect of combining both mutations into a single virus (KgBpK-gC-) was additive (ca. 80%-reduced binding to HS) and displayed a binding activity similar to that observed for KOS virus attachment to sog9 cells, a glycosaminoglycan-deficient L-cell line. Cell-adsorbed individual and double HS mutant viruses exhibited a lower rate of virus entry following attachment, suggesting that HS binding plays a role in the process of virus penetration. Moreover, the KgBpK- mutant virus produced small plaques on Vero cells in the presence of neutralizing antibody where plaque formation depended on cell-to-cell virus spread. These studies permitted the following conclusions: (i) the pK sequence is not essential for gB processing or function in virus infection, (ii) the lysine-rich sequence of gB is responsible for HS binding, and (iii) binding to HS is cooperatively linked to the process of efficient virus entry and lateral spread but is not absolutely required for virus infectivity.

Herpes simplex virus type 1 glycoprotein B requires a cysteine residue at position 633 for folding, processing, and incorporation into mature infectious virus particles

Herpes simplex virus type 1 (HSV-1) glycoprotein B (gB) resides in the virus envelope in an oligomeric form and plays an essential role in virus entry into susceptible host cells. The oligomerizing domain is a movable element consisting of amino acids 626 to 653 in the gB external domain. This domain contains a single cysteine residue at position 633 (Cys-633) that is predicted to form an intramolecular disulfide bridge with Cys-596. In this study, we examined gB oligomerization, processing, and incorporation into mature virus during infection by two mutant viruses in which either the gB Cys-633 [KgB(C633S)] or both Cys-633 and Cys-596 [KgB(C596S/C633S)] residues were mutated to serine. The result of immunofluorescence studies and analyses of released virus particles showed that the mutant gB molecules were not transported to the cell surface or incorporated into mature virus envelopes and thus infectious virus was not produced. Immunoprecipitation studies revealed that the mutant gB molecules were in an oligomeric configuration and that these mutants produced hetero-oligomers with a truncated form of gB consisting of residues 1 to 43 and 595 to 904, the latter containing the oligomerization domain. Pulse-chase experiments in combination with endoglycosidase H treatment determined that the mutant molecules were improperly processed, having been retained in the endoplasmic reticulum (ER). Coimmunoprecipitation experiments revealed that the cysteine mutations resulted in gB misfolding and retention by the molecular chaperones calnexin, calreticulin, and Grp78 in the ER. The altered conformation of the gB mutant glycoproteins was directly detected by a reduction in monoclonal antibody recognition of two previously defined distinct antigenic sites located within residues 381 to 441 and 595 to 737. The misfolded molecules were not transported to the cell surface as hetero-oligomers with wild-type gB, suggesting that the conformational change could not be corrected by intermolecular interactions with the wild-type molecule. Together, these experiments confirmed that a disulfide bridge involving Cys-633 and Cys-596 is not essential for oligomerization but rather is required for proper folding and maintenance of a gB domain essential to complete posttranslational modification, transport, and incorporation into mature virus particles.

Major immunogenic proteins of phocid herpes-viruses and their relationships to proteins of canine and feline herpesviruses

The immunogenic proteins of cells infected with the alpha- or the gamma-herpesvirus of seals, phocid herpesvirus-1 and -2 (PhHV-1, -2), were examined in radioimmunoprecipitation assays as a further step towards the development of a PhHV-1 vaccine. With sera obtained from convalescent seals of different species or murine monoclonal antibodies (Mabs), at least seven virus-induced glycoproteins were detected in lysates of PhHV-1-infected CrFK cells. A presumably disulphide-linked complex composed of glycoproteins of 59, 67 and 113/120 kDa, expressed on the surface of infected cells, was characterized as a major immunogenic infected cell protein of PhHV-1. This glycoprotein complex has previously been identified as the proteolytically cleavable glycoprotein B homologue of PhHV-1 (14). At least three distinct neutralization-relevant epitopes were operationally mapped, by using Mabs, on the glycoprotein B of PhHV-1. Among the infected cell proteins of the antigenically closely related feline and canine herpesvirus, the glycoprotein B equivalent proved to be the most highly conserved glycoprotein. Sera obtained from different seal species from Arctic, Antarctic, and European habitats did not precipitate uniform patterns of infected cell proteins from PhHV-1-infected cell lysates although similar titres of neutralizing antibodies were displayed. Thus, antigenic differences among the alphaherpesvirus species prevalent in the different pinniped populations cannot be excluded. PhHV-2 displayed a different pattern of infected cell proteins and only limited cross-reactivity to PhHV-1 at the protein level was detected, which is in line with its previous classification as a distinct species, based on nucleotide sequence analysis, of the gammaherpesvirus linenge. A Mab raised against PhHV-2 and specific for a major glycoprotein of 117 kDa, cross reacted with the glycoprotein B of PhHV-1. The 117-kDa glycoprotein could represent the uncleaved PhHV-2 glycoprotein B homologue.

Functional analysis of the transmembrane anchor region of bovine herpesvirus 1 glycoprotein gB

In herpesviruses, homologues of glycoprotein B (gB) are essential membrane proteins which are involved in fusion. However, there is no clear evidence regarding the location of the fusogenic domain on gB. By using bovine herpesvirus 1 (BHV-1) as a model, we studied the relationship between the structure and the fusogenic activity of gB. This was achieved by expressing genes of different gB derivatives containing specific truncations at the end of segments 2 or 3 of the transmembrane region in Madin-Darby bovine kidney cells under the control of the bovine heat-shock protein hsp70A gene promoter. All expressed gB products were structurally similar to authentic gB. One truncated form of gB, gBt, which contains residues 1-763, was efficiently secreted. However, gBtM (residues 1-807), which includes the first two segments at the carboxyl terminus, showed unstable retention on the cell surface, whereas gBtMA (residues 1 829), which contains all three membrane-spanning segments, was mostly intracellularly retained with some unstable surface anchorage. Another truncated gB, gBtDAF, which has gB residues 1-763 (gBt) and a human decay-accelerating factor (DAF) carboxyl tail, was also expressed. The DAF fragment provided a signal for the addition of a glycosyl phosphatidylinositol-based membrane anchor, which could target the gBt chimeric protein on the cell membrane. Immunofluorescence staining and pulse-chase kinetic studies support the theory that gBtM, gBtMA, and gBtDAF are retained on nuclear and cellular membranes via different segments of the transmembrane region or the DAF fragment, respectively. For the cells expressing gBt or gBtM, no cell fusion was observed, whereas cells expressing gBtMA clearly showed fusion. However, in gBtDAF cells, the overexpression and cellular accumulation of recombinant gB products did not cause fusion either, which supports our contention that the fusion phenomenon in gBtMA cells is caused by the fusogenic activity of the expressed gBtMA. With the help of sequence analysis, our results indicate that segment 2 of the transmembrane anchor region might be a fusogenic domain, whereas the real anchor is segment 3.

Intracellular compartmentalization of the glycoprotein B of herpesvirus Simian agent 8 expressed with a baculovirus vector in insect cells

The intracellular localization of the glycoprotein B of herpesvirus simian agent 8 expressed with a baculovirus system in insect cells was studied. Cell fractionation and immunoprecipitation revealed that gB is present in microsomal as well as in nuclear membranes. Both fractions contain oligomers, probably dimers, of gB with endoglycosidase-H sensitive, mannose-rich carbohydrates. Nuclear transport of gB was further analysed by immuno electron microscopy of recombinant baculovirus-infected cells. The glycoprotein is present both in the outer and the inner nuclear membrane as well as in cytoplasmic structures and at the cell surface. This study precludes the possibility that glycosylation and/or oligomerisation of SA8 gB are responsible for nuclear targeting.

Cross-linking of glycoprotein oligomers during herpes simplex virus type 1 entry

Herpes simplex virus (HSV) has 10 glycoproteins in its envelope. Glycoprotein B (gB), gC, gD, gH, and gL have been implicated in virus entry. We previously used chemical cross-linking to show that these five glycoproteins were close enough to each other to be cross-linked into homodimeric and hetero-oligomeric forms; hetero-oligomers of gB-gC, gC-gD, gD-gB, gH-gL, gC-gL and gD-gL were found in purified virions. To better understand the roles of these glycoproteins in viral entry, we have modified a standard HSV penetration assay to include cross-linkers. This allowed us to examine changes in associations of viral glycoproteins during the entry process. HSV-1(KOS) was adsorbed at 4 degrees C to human neuroblastoma cells (SY5Y). The temperature was raised to 37 degrees C and cells were treated with cross-linker at various times after the temperature shift. Cytoplasmic extracts were examined by Western blotting (immunoblotting) for viral glycoproteins. We found that (i) as in virus alone, the length and concentration of the cross-linking agent affected the number of specific complexes isolated; (ii) the same glycoprotein patterns found in purified virions were also present after attachment of virions to cells; and (iii) the ability to cross-link HSV glycoproteins changed as virus penetration proceeded, e.g., gB and gD complexes which were present during attachment disappeared with increasing time, and their disappearance paralleled the kinetics of penetration. However, this phenomenon appeared to be selective since it was not observed with gC oligomers. In addition, we examined the cross-linking patterns of gB and gD in null viruses K082 and KOSgD beta. Neither of these mutants, which attach but cannot penetrate, showed changes in glycoprotein cross-linking over time. We speculate that these changes are due to conformational changes which preclude cross-linking or spatial alterations which dissociate the glycoprotein interactions during the penetration events.

Deletion of the carboxy-terminus of herpes simplex virus type 1 (HSV-1) glycoprotein B does not affect oligomerization, heparin-binding activity, or its ability to protect against HSV challenge

A recombinant vaccinia virus designated VgBt which expresses a truncated secreted herpes simplex virus gB (gBt) was constructed and compared to V11gB, a vaccinia recombinant previously studied which expresses gB exclusively on the surface of infected cells. Indirect immunofluorescence assay (IFA) revealed that gBt was strongly associated with the surface of infected cells despite being released slowly into the cell culture medium. Both gB and gBt existed as oligomers, and both membrane bound and secreted forms of gBt exhibited heparin-binding activity. In protection studies VgBt and V11gB conferred equivalent protection against both homologous (HSV-1) and heterologous (HSV-2) challenge with HSV.

Glycoprotein B of herpes simplex virus type 1 oligomerizes through the intermolecular interaction of a 28-amino-acid domain

Herpes simplex virus type 1 glycoprotein B (gB) is an envelope component that plays an essential role in virus infection. The biologically active form of gB is an oligomer that contributes to the process of viral envelope fusion with the cell surface membrane, resulting in viral penetration and initiation of the replication cycle. In previous studies, two discontinuous sites for oligomer formation were identified: a nonessential upstream site located between residues 93 and 282 and an essential downstream site located between residues 596 and 711. In this study, in vitro-transcribed and -translated gB test molecules were used to characterize the more active essential membrane-proximal domain. A series of gB test polypeptides mutated in this downstream oligomerization domain were assayed for their abilities to form oligomers with a mutant gB capture polypeptide containing the analogous wild-type domain. Detection of oligomers was achieved by coimmunoprecipitation of two gB mutant molecules by using a monoclonal antibody specific for a hemagglutinin epitope tag introduced into the coding sequence of the capture polypeptide. Analysis of the immune-precipitated products by sodium dodecyl sulfate-polyacrylamide gel electrophoresis demonstrated that the downstream oligomerization domain resided within residues 626 to 676. This region was further resolved into two segments, residues 626 to 653 and 653 to 675, each of which was independently sufficient to form oligomers. However, residues 626 to 653 provided for a stronger interaction between gB monomers. Moreover, this stretch of 28 amino acids was shown to form oligomers when introduced into the carboxy-terminal region of gB monomers lacking this domain at the normal site, thus indicating that this domain was functionally independent of its natural location within the gB molecule. Further analysis of the sequence within residues 596 to 653 by using mutant test polypeptides altered in individual amino acids revealed that cysteines 9 and 10 located at positions 596 and 633, respectively, were not required for oligomer formation but contributed to dimer formation and/or stabilization. The results of this study suggest that oligomerization of gB monomers is induced by interactions between contiguous residues localized within the ectodomain near the site of molecule insertion into the viral envelope membrane.

Common antigenic epitopes are present on heat-labile oligomers of MDV glycoprotein B and on HSV glycoprotein B

The antigenic cross-reactivity between the Marek's disease virus glycoprotein B (MDV gB) and glycoprotein B (gB) of herpes simplex virus type 1 and 2 (HSV1 and HSV2) was analysed by the immunoblotting method. We studied cell lysates in both denatured and in undenatured form (i.e., unheated) and reacted them with convalescent sera from chickens infected with the RBIB MDV strain and with human anti-HSV1 gB. Both sera detected the heat-labile MDV gB and the HSV gB oligomers. In addition, monospecific antibodies to the MDV gB 230 kDa oligomer (strain CVI988) were immunoaffinity purified from both the chicken and the human sera. The chicken and human monospecific antibodies detected the homologous and the heterologous gB oligomers in native MDV- and HSV1-infected cell lysates. 15 human sera were tested by immunoblotting and by immunofluorescence on HSV1-, CVI988-and herpes virus of turkeys (HVT)-infected cells. By both assays about half of the human sera reacted with MDV-infected cells. This study demonstrates that the MDV gB heat-labile oligomers possess conformational epitopes shared with the human alpha-herpes virus HSV1 and HSV2 gB heat-labile oligomers.

The herpes simplex virus type 1 particle: structure and molecular functions. Review article

This review is a summary of our present knowledge with respect to the structure of the virion of herpes simplex virus type 1. The virion consists of a capsid into which the DNA is packaged, a tegument and an external envelope. The protein compositions of the structures outside the genome are described as well as the functions of individual proteins. Seven capsid proteins are identified, and two of them are mainly present in precursors of mature DNA-containing capsids. The protein components of the 150 hexamers and 12 pentamers in the icosahedral capsid are known. These capsomers all have a central channel and are connected by Y-shaped triplexes. In contrast to the capsid, the tegument has a less defined structure in which 11 proteins have been identified so far. Most of them are phosphorylated. Eleven virus-encoded glycoproteins are present in the envelope, and there may be a few more membrane proteins not yet identified. Functions of these glycoproteins include attachment to and penetration of the cellular membrane. The structural proteins, their functions, coding genes and localizations are listed in table form.

Proteolytic cleavage of bovine herpesvirus 1 (BHV-1) glycoprotein gB is not necessary for its function in BHV-1 or pseudorabies virus

Glycoprotein B homologs represent the most highly conserved group of herpesvirus glycoproteins. They exist in oligomeric forms based on a dimeric structure. Despite the high degree of sequence and structural conservation, differences in posttranslational processing are observed. Whereas gB of herpes simplex virus is not proteolytically processed after oligomerization, most other gB homologs are cleaved by a cellular protease into subunits that remain linked via disulfide bonds. Proteolytic cleavage is common for activation of viral fusion proteins, and it has been shown that herpesvirus gB homologs are essential for membrane fusion events during infection, e.g., virus penetration and direct viral cell-to-cell spread. To analyze the importance of proteolytic cleavage for the function of gB homologs, we isolated a mutant bovine herpesvirus 1 (BHV-1) expressing a BHV-1 gB that is no longer proteolytically processed because of a deletion of the proteolytic cleavage site and analyzed its phenotype in cell culture. We showed previously that BHV-1 gB can functionally substitute for the homologous glycoprotein in pseudorabies virus (PrV), based on the isolation of a PrV gB-negative PrV recombinant that expresses BHV-1 gB (A. Kopp and T. C. Mettenleiter, J. Virol, 66:2754-2762, 1992). Therefore, we also isolated a mutant PrV lacking PrV gB but expressing a noncleavable BHV-1 gB. Our results show that cleavage of BHV-1 gB is not essential for its function in either a BHV-1 or a PrV background. Compared with the PrV recombinant expressing cleavable BHV-1 gB, deletion of the cleavage site in the recombinant PrV did not detectably alter the viral phenotype, as analyzed by plaque assays, one-step growth kinetics, and penetration kinetics. In the BHV-1 mutant, the uncleaved BHV-1 gB was functionally equivalent to the wild-type protein with regard to penetration and showed only slightly delayed one-step growth kinetics compared with parental wild-type BHV-1. However, the resulting plaques were significantly smaller, indicating a role for proteolytic cleavage of BHV-1 gB in cell-to-cell spread of BHV-1.

Glycoprotein gB (gII) of pseudorabies virus can functionally substitute for glycoprotein gB in herpes simplex virus type 1

Glycoproteins homologous to gB of herpes simplex virus (HSV) constitute the most highly conserved family of herpesvirus glycoproteins. All gB homologs analyzed so far have been shown to play essential roles in penetration and direct viral cell-to-cell spread. In studies aimed at assessing whether the high sequence homology is also indicative of functional homology, we analyzed the ability of the gB-homologous glycoprotein (former designation gII) of pseudorabies virus (PrV) to complement a gB- HSV type 1 (HSV-1) mutant and vice versa. The results show that a PrV gB-expressing cell line phenotypically complemented the lethal defect in gB- HSV-1 whereas reciprocal complementation of a gB- PrV mutant by HSV-1 gB was not observed.

Effects of deletions in the carboxy-terminal hydrophobic region of herpes simplex virus glycoprotein gB on intracellular transport and membrane anchoring

The gB glycoprotein of herpes simplex virus type 1 is involved in viral entry and fusion and contains a predicted membrane-anchoring sequence of 69 hydrophobic amino acids, which can span the membrane three times, near the carboxy terminus. To define the membrane-anchoring sequence and the role of this hydrophobic stretch, we have constructed deletion mutants of gB-1, lacking one, two, or three predicted membrane-spanning segments within the 69 amino acids. Expression of the wild-type and mutant glycoproteins in COS-1 cells show that mutant glycoproteins lacking segment 3 (amino acids 774 to 795 of the gB-1 protein) were secreted from the cells. Protease digestion and alkaline extraction of microsomes containing labeled mutant proteins further showed that segment 3 was sufficient for stable membrane anchoring of the glycoproteins, indicating that this segment may specify the transmembrane domain of the gB glycoprotein. Also, the mutant glycoproteins containing segment 3 were localized in the nuclear envelop, which is the site of virus budding. Deletion of any of the hydrophobic segments, however, affected the intracellular transport and processing of the mutant glycoproteins. The mutant glycoproteins, although localized in the nuclear envelope, failed to complement the gB-null virus (K082). These results suggest that the carboxy-terminal hydrophobic region contains essential structural determinants of the functional gB glycoprotein.

Characterization of immunoreactive octapeptides of human-cytomegalovirus gp58

We have mapped continuous epitopes, for positions 591-673 of the human cytomegalovirus 58-kDa glycoprotein using overlapping synthetic peptides and human sera. This region contains a fragment previously described as including the dominant site for induction of human-cytomegalovirus antibodies. Since the selected sequence is highly conserved among herpes viruses, we have considered the possible presence of antigenic cross-reactivity, particularly with the Epstein-Barr virus. Several peptides in the studied region were antigenic and two main continuous epitopes have been identified. Serological cross-reactions observed with Epstein-Barr virus are discussed, focusing on the possible implications of structural features and sequence similarity between human-cytomegalovirus and Epstein-Barr-virus glycoproteins.

Syncytium-inducing mutations localize to two discrete regions within the cytoplasmic domain of herpes simplex virus type 1 glycoprotein B

Herpes simplex virus type 1 glycoprotein B (gB) is essential for virus entry, an event involving fusion of the virus envelope with the cell surface membrane, and virus-induced cell-cell fusion, resulting in polykaryocyte, or syncytium, formation. The experiments described in this report employed a random mutagenesis strategy to develop a more complete genetic map of mutations resulting in the syn mutant phenotype. The results indicate that syn mutations occur within two essential and highly conserved hydrophilic, alpha-helical regions of the gB cytoplasmic domain. Region I is immediately proximal to the transmembrane domain and includes residues R796 to E816/817. Region II is localized centrally in the cytoplasmic domain and includes residues A855 and R858. Positively charged residues were particularly affected in both regions, suggesting that charge interactions may be required to suppress the syn mutant phenotype. No syn mutations were identified within the transmembrane domain. A virus containing a rate of entry (roe) mutation at residue A851, either within or immediately proximal to syn region II, was isolated. Since roe mutations have also been discovered in the external domain of gB, it appears likely that the external and cytoplasmic domains cooperate in virus penetration. Moreover, the observation that both roe and syn mutations occur in the cytoplasmic domain further suggests that gB functions in an analogous manner in both membrane fusion events. It might be predicted from these observations that membrane fusion involves transduction of a fusion signal along the gB molecule through the transmembrane domain. Communication between the external and cytoplasmic domain may thus be required for gB-mediated membrane fusion events.