Alterations in glycoprotein gB specified by mutants and their partial revertants in herpes simplex virus type 1 and relationship to other mutant phenotypes

Opportunities, Technology, and the Joy of Discovery

My grandparents were immigrants. My paternal grandfather was illiterate. Yet my parents were able to complete college and to become teachers. I had a conventional upbringing in a small town in Florida, graduating from high school in 1960. I was fortunate enough to graduate cum laude from Florida State University and to earn other credentials leading to faculty positions at outstanding institutions of higher education: the University of Chicago and Northwestern University. At a time when women were rarely the leaders of research groups, I was able to establish a well-funded research program and to make contributions to our understanding of viral entry into cells. My best research was done after I became confident enough to seek productive interactions with collaborators. I am grateful for the collaborators and collaborations that moved our field forward and for my trainees who have gone on to successes in many different careers. Expected final online publication date for the Annual Review of Virology, Volume 9 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Cysteines and N-Glycosylation Sites Conserved among All Alphaherpesviruses Regulate Membrane Fusion in Herpes Simplex Virus 1 Infection

Neurotropism is a defining characteristic of alphaherpesvirus pathogenicity. Glycoprotein K (gK) is a conserved virion glycoprotein of all alphaherpesviruses that is not found in other herpesvirus subfamilies. The extracellular amino terminus of gK has been shown to be important to the ability of the prototypic alphaherpesvirus herpes simplex virus 1 (HSV-1) to enter neurons via axonal termini. Here, we determined the role of the two conserved N-linked glycosylation (N48 and N58) sites of gK in virus-induced cell fusion and replication. We found that N-linked glycosylation is important to the regulation of HSV-1-induced membrane fusion since mutating N58 to alanine (N58A) caused extensive virus-induced cell fusion. Due to the known contributions of N-linked glycosylation to protein processing and correct disulfide bond formation, we investigated whether the conserved extracellular cysteine residues within the amino terminus of gK contributed to the regulation of HSV-1-induced membrane fusion. We found that mutation of C37 and C114 residues led to a gK-null phenotype characterized by very small plaque formation and drastic reduction in infectious virus production, while mutation of C82 and C243 caused extensive virus-induced cell fusion. Comparison of N-linked glycosylation and cysteine mutant replication kinetics identified disparate effects on infectious virion egress from infected cells. Specifically, cysteine mutations caused defects in the accumulation of infectious virus in both the cellular and supernatant fractions, while glycosylation site mutants did not adversely affect virion egress from infected cells. These results demonstrate a critical role for the N glycosylation sites and cysteines for the structure and function of the amino terminus of gK.IMPORTANCE We have previously identified important entry and neurotropic determinants in the amino terminus of HSV-1 glycoprotein K (gK). Alphaherpesvirus-mediated membrane fusion is a complex and highly regulated process that is not clearly understood. gK and UL20, which are highly conserved across all alphaherpesviruses, play important roles in the regulation of HSV-1 fusion in the context of infection. A greater understanding of mechanisms governing alphaherpesvirus membrane fusion is expected to inform the rational design of therapeutic and prevention strategies to combat herpesviral infection and pathogenesis. This work adds to the growing reports regarding the importance of gK to alphaherpesvirus pathogenesis and details important structural features of gK that are involved in gK-mediated regulation of virus-induced membrane fusion.

Conservation of the glycoprotein B homologs of the Kaposi׳s sarcoma-associated herpesvirus (KSHV/HHV8) and old world primate rhadinoviruses of chimpanzees and macaques

The envelope-associated glycoprotein B (gB) is highly conserved within the Herpesviridae and plays a critical role in viral entry. We analyzed the evolutionary conservation of sequence and structural motifs within the Kaposi׳s sarcoma-associated herpesvirus (KSHV) gB and homologs of Old World primate rhadinoviruses belonging to the distinct RV1 and RV2 rhadinovirus lineages. In addition to gB homologs of rhadinoviruses infecting the pig-tailed and rhesus macaques, we cloned and sequenced gB homologs of RV1 and RV2 rhadinoviruses infecting chimpanzees. A structural model of the KSHV gB was determined, and functional motifs and sequence variants were mapped to the model structure. Conserved domains and motifs were identified, including an "RGD" motif that plays a critical role in KSHV binding and entry through the cellular integrin αVβ3. The RGD motif was only detected in RV1 rhadinoviruses suggesting an important difference in cell tropism between the two rhadinovirus lineages.

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.

Identification of syncytial mutations in a clinical isolate of herpes simplex virus 2

Small polykaryocytes resulting from cell fusion are found in herpes simplex virus (HSV) lesions in patients, but their significance for viral spread and pathogenesis is unclear. Although syncytial variants causing extensive fusion in tissue culture can be readily isolated from laboratory strains, they are rarely found in clinical isolates, suggesting that extensive cell fusion may be deleterious in vivo. Syncytial mutations have previously been identified for several laboratory strains, but not for clinical isolates of HSV type 2. To address this deficiency, we studied a recent syncytial clinical isolate, finding it to be a mixture of two syncytial and one nonsyncytial strain. The two syncytial strains have novel mutations in glycoprotein B, and in vitro cell fusion assays confirmed that they are responsible for syncytium formation. This panel of clinical strains may be ideal for examining the effect of increased cell fusion on pathogenesis.

Gene delivery using herpes simplex virus vectors

Herpes simplex virus (HSV) is a neurotropic DNA virus with many favorable properties as a gene delivery vector. HSV is highly infectious, so HSV vectors are efficient vehicles for the delivery of exogenous genetic material to cells. Viral replication is readily disrupted by null mutations in immediate early genes that in vitro can be complemented in trans, enabling straightforward production of high-titre pure preparations of non-pathogenic vector. The genome is large (152 Kb) and many of the viral genes are dispensable for replication in vitro, allowing their replacement with large or multiple transgenes. Latent infection with wild-type virus results in episomal viral persistence in sensory neuronal nuclei for the duration of the host lifetime. Transduction with replication-defective vectors causes a latent-like infection in both neural and non-neural tissue; the vectors are non-pathogenic, unable to reactivate and persist long-term. The latency active promoter complex can be exploited in vector design to achieve long-term stable transgene expression in the nervous system. HSV vectors transduce a broad range of tissues because of the wide expression pattern of the cellular receptors recognized by the virus. Increasing understanding of the processes involved in cellular entry has allowed preliminary steps to be taken towards targeting the tropism of HSV vectors. Using replication-defective HSV vectors, highly encouraging results have emerged from recent pre-clinical studies on models of neurological disease, including glioma, peripheral neuropathy, chronic pain and neurodegeneration. Consequently, HSV vectors encoding appropriate transgenes to tackle these pathogenic processes are poised to enter clinical trials.

Targeting gene expression using HSV vectors

Herpes simplex virus (HSV) is an encapsulated DNA virus, with many favourable properties for use as a gene transfer vector. For gene therapy applications, it may be desirable to restrict transgene expression to pre-defined subsets of cells. One potential method for achieving targeted transgene expression using the HSV vector system might involve dictating the cell types to which the vector will transfer the therapeutic transgene of interest. HSV delivers its genetic payload to cells directly through the plasmalemma; the mechanisms are complex and involve multiple viral and cell surface determinants. We have investigated several ways in which each component of the cell entry cascade may be manipulated in order to restrict viral DNA and transgene delivery to particular cellular populations. Our results indicate that targeted transduction may be a viable approach to achieving our goal of targeted HSV-mediated transgene expression.

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.

Disulfide bonds of herpes simplex virus type 2 glycoprotein gB

Glycoprotein B (gB) is the most highly conserved envelope glycoprotein of herpesviruses. The gB protein is required for virus infectivity and cell penetration. Recombinant forms of gB being used for the development of subunit vaccines are able to induce virus-neutralizing antibodies and protective efficacy in animal models. To gain structural information about the protein, we have determined the location of the disulfide bonds of a 696-amino-acid residue truncated, recombinant form of herpes simplex virus type 2 glycoprotein gB (HSV gB2t) produced by expression in Chinese hamster ovary cells. The purified protein, which contains virtually the entire extracellular domain of herpes simplex virus type 2 gB, was digested with trypsin under nonreducing conditions, and peptides were isolated by reversed-phase high-performance liquid chromatography (HPLC). The peptides were characterized by using mass spectrometry and amino acid sequence analysis. The conditions of cleavage (4 M urea, pH 7) induced partial carbamylation of the N termini of the peptides, and each disulfide peptide was found with two or three different HPLC retention times (peptides with and without carbamylation of either one or both N termini). The 10 cysteines of the molecule were found to be involved in disulfide bridges. These bonds were located between Cys-89 (C1) and Cys-548 (C8), Cys-106 (C2) and Cys-504 (C7), Cys-180 (C3) and Cys-244 (C4), Cys-337 (C5) and Cys-385 (C6), and Cys-571 (C9) and Cys-608 (C10). These disulfide bonds are anticipated to be similar in the corresponding gBs from other herpesviruses because the 10 cysteines listed above are always conserved in the corresponding protein sequences.

The syn3 strain HSZP of herpes simplex virus type 1 (HSV-1) is not pathogenic for mice and shows limited neural spread

Strain HSZP of the herpes simplex virus type 1 (HSV-1) forms large giant cells in vitro. This property was found associated with a mutation that alters the codon CGC (in the strain KOS or 17 sequence) to CAC (in the HSZP sequence), changing the amino acid 857 from arginine to histidine in the cytoplasmic domain of the glycoprotein B (gB) polypeptide chain. Giant cell formation by ANGpath was attributed to a mutation that alters the codon GCC (in KOS and strain 17 sequences) to GTC (in ANGpath sequence) changing the amino acid 854 in the same (syn3) region of the gB molecule. In contrast to the ANGpath virus, which is pathogenic (1 LD50 < 1 x 10(4) PFU) for adult DBA/2 mice after peripheral inoculation, strain HSZP was never found to be lethal for adult mice. Whereas ANGpath-infected mice which survived acute infection frequently (79%) developed latency in the regional sensory ganglion (as proved by virus reactivation during explantation), latent HSZP reactivated in ganglion culture at a considerably reduced rate (21%). Only 10-day-old DBA/2 mice were sensitive to HSZP infection. In these, HSZP spread from the site of peripheral administration mainly by hematogenous route. The neural spread of HSZP in suckling DBA/2 mice was manifested by the involvement of vegetative neurons in the wall of the small intestine and in the retroperitoneal vegetative ganglia. We conclude that HSZP, a polykaryocyte-forming strain with a mutation in the syn3 region II, shows limited neuroinvasity for mice after peripheral administration.

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.

Domains of herpes simplex virus I glycoprotein B that function in virus penetration, cell-to-cell spread, and cell fusion

Herpes simplex virus 1 glycoprotein B (gB) is one of 10 glycoproteins in the virion envelope and in the membranes of infected cells. It is required for infection of cells in culture and functions in penetration of the cell by fusing the virion envelope with the plasma membrane. In studies to map the functional domains on HSV-1 gB, we reported that epitopes of potent neutralizing antibodies cluster in three major antigenic domains, D1, D2, and D5a. D1 contains continuous epitopes in the very amino terminus of gB. D2 comprises discontinuous epitopes that are assembled on gB derivatives 457 amino acids in length. D5a contains discontinuous epitopes that map between amino acids 600 and 690. We have now analyzed the function of these domains in virion infectivity by a detailed examination of the effects of 16 neutralizing antibodies on virion adsorption, penetration, plaque development, and cell fusion. Our results are as follows. (i) Ten antibodies with complement-independent neutralizing activity blocked penetration of virions into cells but not their adsorption to the cell surface. Treating cell-bound, neutralized virus with the fusogenic agent polyethylene glycol promoted their entry into cells. (ii) Ten antibodies with complement-dependent and -independent neutralizing activity interfered with plaque development by preventing spread of virus from infected to neighboring uninfected cells. (iii) Nine neutralizing antibodies, all complement-independent, prevented cell fusion induced by strain HFEM syn. We conclude that domains mapping in three regions of gB function in penetration of virions into cells, and that most neutralizing antibodies to these domains also block cell-to-cell spread of virus and cell fusion. The findings that three complement-independent neutralizing antibodies that blocked penetration did not inhibit plaque development, and that only one of these blocked cell fusion, indicate that the cell-to-cell spread of virus and cell fusion are related processes, but not identical to the penetration function.

A mutation in the ectodomain of herpes simplex virus 1 glycoprotein B causes defective processing and retention in the endoplasmic reticulum

Herpes simplex virus 1 (HSV-1) glycoprotein B (gB) is one of several envelope glycoproteins required for virion infectivity and is the only one known to oligomerize into homodimers. To study the conformational constraints for translocation of HSV-1 gB to the surface of eukaryotic cells, we analyzed the transport through the exocytic pathway of the wild-type glycoprotein and of mutant forms with insertions in the ectodomain and intracellular carboxy terminus. Transient expression of the glycoproteins in COS-1 cells showed that an insertion at position 479 in the amino-terminal ectodomain of gB, shown previously by reactions with monoclonal antibodies to have altered the conformation of the molecule, also had a drastic effect on transport, precluding exit of the mutant from the endoplasmic reticulum (ER) and transport to the Golgi and the plasma membrane. The fact that the mutant, gB-(Lk479), formed dimers suggests that local changes in assembled regions caused the transport defect. Mutants containing insertions at residues 600 of the ectodomain and 810 in the intracellular domain were slightly retarded in their rate of transport from the ER to the Golgi. The glucose-regulated proteins GRP78 and GRP94, which are resident proteins of the ER, associated with partially glycosylated, faster-migrating forms of gB but not with the fully processed, more slowly migrating product. GRP78 and GRP94 formed complexes with the mutant gB-(Lk479), which was degraded in the ER. Our results indicate that GRP78, and perhaps also GRP94, acts as a chaperone in the assembly of native gB oligomers and also binds to aberrant forms of the molecule, arresting their transport from the ER and possibly serving as markers for protein degradation in this compartment of the exocytic pathway.

Mutations in conformation-dependent domains of herpes simplex virus 1 glycoprotein B affect the antigenic properties, dimerization, and transport of the molecule

Glycoprotein B (gB) is a component of the herpes simplex virus 1 envelope that is required for penetration of virions into cells. We constructed 11 mutants in the gB gene by deleting the carboxy terminus of the molecule, inserting linkers into the ectodomain and intracellular region, and creating point mutations in cysteine residues. To identify regions of the molecule that affect the formation of epitopes on gB, we cloned the mutated genes into a eukaryotic expression vector, transfected them in COS-1 cells, and reacted the gene products in immunofluorescence and immunoprecipitation tests with a panel of monoclonal antibodies. Our findings are as follows. (i) The ectodomain of gB between residues 600 and 690 is highly antigenic and contains residues that specify 8 continuous epitopes and affect the conformation of 12 discontinuous epitopes. Residues that form a novel neutralizing domain and affect the assembly of gB dimers are contained in this region. Dimerization of gB does not require the transmembrane region or the intracellular carboxy terminus. (ii) Transport of the insertion mutants was aberrant and depended on the site mutagenized. Insertions of linkers at residues 391, 413, and 479 of the ectodomain precluded the binding of neutralizing antibodies that recognize residues in four discontinuous-epitope domains; the latter mutant in intact gB was not translocated to the cell surface. In contrast, insertions at residue 600 of the ectodomain and 810 of the intracellular domain did not affect the conformation-dependent epitopes or gB transport. (iii) Substitution of serines for cysteine residues in a discontinuous-epitope domain in the midregion of gB altered the conformation of both proximal and distal sites. Seven epitopes were lost by mutagenesis of cysteine 382 and 4 epitopes by mutagenesis of cysteine 334. Together with previous findings, these results indicate that the ectodomain of gB contains three topographically distinct neutralizing regions, one of continuous and two of discontinuous epitopes. The continuous-epitope domains that map at the amino terminus are not altered by distal mutations. In contrast, the domains of discontinuous epitopes, assembled by juxtaposing residues on the surface of gB, are affected by proximal and distal mutations that alter the antigenic structure, processing, and surface transport of gB.

Identification of a site on herpes simplex virus type 1 glycoprotein D that is essential for infectivity

Herpes simplex virus glycoprotein D (gD) plays an essential role during penetration of the virus into cells. There is evidence that it recognizes a specific receptor after initial attachment of virions to cell surface heparan sulfate and also that gD-1, gD-2, and gp50 (the pseudorabies virus gD homolog) bind to the same receptor. Although the antigenic structure of gD has been studied intensively, little is known about functional regions of the protein. Antigenic site I is a major target for neutralizing antibodies and has been partially mapped by using deletion mutants and neutralization-resistant viruses. Working on the assumption that such a site may overlap with a functional region of gD, we showed previously that combining two or more amino acid substitutions within site I prevents gD-1 from functioning and is therefore lethal. We have now used a complementation assay to measure the functional activity of a panel of deletion mutants and compared the results with an antigenic analysis. Several mutations cause gross changes in protein folding and destroy functional activity, whereas deletions at the N and C termini have little or no effect on either. In contrast, deletion of residues 234 to 244 has only localized effects on antigenicity but completely abolishes functional activity. This region, which is part of antigenic site Ib, is therefore essential for gD-1 function. The complementation assay was also used to show that a gD-negative type 1 virus can be rescued by gD-2 and by two gD-1-gD-2 hybrids but not by gp50, providing some support for the existence of a common receptor for herpes simplex virus types 1 and 2 but not pseudorabies virus. Alternatively, gp50 may lack a signal for incorporation into herpes simplex virions.

Domain structure of herpes simplex virus 1 glycoprotein B: neutralizing epitopes map in regions of continuous and discontinuous residues

Herpes simplex virus 1 (HSV-1) glycoprotein B (gB) is a multifunctional glycoprotein required for infectivity; it is thought to promote fusion of the viral envelope with the cell membrane and entry of virions into cells. To map the antigenic and functional domains on gB, we constructed amino terminal derivatives lacking the entire carboxyl terminus and internal deletion mutants lacking defined regions of the extracellular and transmembrane domains. Transient expression of the mutants in COS-1 cells revealed that the amino terminal derivatives were released into the medium whereas those with deletions in the extracellular domain were mostly retained within the transfected cells. Analysis of intact gB and the amino terminal derivatives showed that the intact molecule formed dimers whereas the mutant derivatives did not. Reactions of the derivatives with a panel of well-characterized monoclonal antibodies to gB showed that the neutralizing epitopes cluster in two domains. The first maps in the amino terminal 190 residues and contains seven continuous epitopes, five of which are HSV-1-specific. Reactions of antibodies with a set of oligopeptides fine-mapped the epitopes between residues 1 and 47. The second domain is composed of discontinuous epitopes and was expressed by amino terminal derivatives that were at least 457 residues in length or longer. Eleven epitopes map in this region, including those of four potent neutralizing antibodies whose cognitive sites mapped between residues 273 and 298 in mapping studies using antibody-resistant mutants. Results of the present study indicate that the cognitive sites of these antibodies are assembled into the discontinuous domain by juxtaposing residues from the amino-terminal half of gB monomers.

Molecular pathogenesis of equine coital exanthema: identification and expression of infected cell polypeptides at the restricted temperature during equine herpesvirus 3 infection

Equine herpesvirus 3 (EHV-3)-infected equine cells display a kinetics of infected cell polypeptide (ICP) synthesis at 34 degrees C that is typical of coordinate cascade gene regulation of herpesviruses. In contrast, when infected cell cultures are incubated at the restricted temperature of 39 degrees C, the shift from early (beta) gene expression to late (gamma) gene expression is perturbed, i.e., there is an accumulation of early (beta) gene products and a decrease in, or absence of, late (gamma) gene products. Some of the affected late (gamma) gene products may be glycoproteins since these ICPs co-migrated with radiolabeled bands from infected cells incubated with [3H] glucosamine, separated by polyacrylamide gel electrophoresis. These findings are consistent with previous findings (Jacob, 1986), indicating that the growth restriction is in a late viral function(s) and possibly involves envelopment of nucleocapsids into infectious virions.

Constitutive expression in human cells of herpes simplex virus type 1 glycoprotein B gene cloned in an episomal eukaryotic vector

Expression of herpes simplex virus type 1 (HSV-1) glycoprotein B (gB-1) was obtained in human cells from the gB-1 gene cloned in the episomal replicating vector pBK-1, which contains the origin of replication and early region of the human papovavirus BK. Selective systems for the TK+ phenotype in TK-143B cells and for resistance to G418 in adenovirus 5-transformed 293 cells were used to obtain stable transformants that produced gB-1. While gB-1 expression in 143B cells required induction by HSV-1 early proteins, constitutive gB1 production was observed in 293 cells, where endogenous trans-acting factors probably replace the need for early viral products in the activation of the cloned gB-1 gene. The amount of recombinant gB-1 was comparable to that produced during HSV-1 lytic infection in human cells, due to amplification of the inserted gene in the replicating episomal vector. Expression of gB-1 was induced by cadmium and zinc when the promoter of the mouse metallothionein-I gene was placed upstream of gB1 structural sequences. The inducible system where the gB-1 gene is under the control of its own promoter could be employed to clarify the role of early viral products in induction of gB-1 synthesis. Constitutive expression of gB-1 in human cells could provide useful material for diagnostic purposes and for the preparation of a subunit vaccine against HSV infections.

Antibody-resistant mutations in cross-reactive and type-specific epitopes of herpes simplex virus 1 glycoprotein B map in separate domains

To characterize the domains of HSV-1 glycoprotein B (gB), we isolated mutants resistant to monoclonal antibodies with potent neutralizing activity. Partial nucleotide sequencing of the mutations revealed that gB contains two domains comprising discontinuous and continuous amino acids that bind cross-reactive and type-specific neutralizing antibodies. Four mutations in a discontinuous domain, R1435, R233, R1375, and R126, contained substitutions of Tyr278 for His278, His298 for Arg298, Gln274 for Arg274, and Asn273 for Tyr273, respectively. Two mutations in a continuous domain, R1392 and R1397, contained substitutions of Thr32 for Ala32 and Thr47 for Asn47, respectively, and overlapped two other type-specific epitopes. Analysis of the nucleotide sequence of strain KOS showed differences from strain F at four residues proximal to the R1392 mutation and one residue proximal to the R1397 mutation, which explains the failure of HSV-1(F)-specific antibodies to these epitopes to react with KOS. One target site for proteolytic cleavage of gB by cellular enzymes maps at the amino terminus, partially overlapping four HSV-1-specific epitopes.

Characterization of epitopes on native and denatured forms of herpes simplex virus glycoprotein B

Herpes simplex virus 1 glycoprotein B (gB) is an envelope glycoprotein which promotes fusion of virions with the cell membrane. This report characterizes the epitopes on native, disulfide-linked dimers of gB and monomeric forms of the glycoprotein using a panel of monoclonal antibodies. The antibodies were divided into groups, based on immune reactions with denatured or native forms of gB. The first group reacted with discontinuous epitopes assembled on gB dimers but failed to detect native or denatured monomers. In contrast, the second group reacted with denatured gB recognizing continuous epitopes on the parent oligomer and monomeric forms. Comparison of gB dimers formed by HFEM and tsB5 revealed that mutant forms specified altered immunological properties. Analysis of gB made in Vero cells showed that discontinuous epitopes were retained whereas a subset of continuous ones were lost on the cleavage products. Results of this study indicate that more than half of the epitopes on gB are generated by juxtaposing amino acids from one or more gB subunits and differ from continuous epitopes present on both forms of the molecule.

Abundant expression of herpes simplex virus glycoprotein gB using an adenovirus vector

Herpes simplex virus type 1 (HSV-1) glycoprotein B (gB) is a major component of infected cell membranes and virion envelopes. Glycoprotein B is known to be essential for entry of viruses into cells and may play important roles in virus-induced cell fusion and other alterations in cell morphology. In order to study the biochemical and immunological properties of gB in isolation from other HSV-1 polypeptides we have constructed human adenovirus vectors capable of expressing high levels of gB. The gB gene was coupled to the SV40 early promoter and inserted into the E3 region of two adenovirus vectors, one in which the E1 region was deleted (AdgB-1) and another which contained E1 sequences (AdgB-2). In AdgB-1 the orientation of the chimeric gB-SV40 gene was right to left, i.e., opposite to the direction of late and E3 mRNA transcription, whereas in AdgB-2 the orientation was left to right. Human 293 cells which express E1 functions supported replication of AdgB-1 and gB was expressed in these cells but not in mouse cells and only at very low levels in human cells other than 293. Replication of AdgB-2 was not limited to 293 cells and the virus was able to induce synthesis of gB at levels equal to or higher than those expressed in HSV-1-infected human or mouse cells. Microscopic examination of AdgB-2-infected cells revealed extensive vacuolization in a manner completely uncharacteristic of adenovirus-infected cells, and fluorescent antibody staining indicated that gB was not only present at the cell surface but also concentrated in the cytoplasmic vacuoles.

Identification of a herpes simplex virus 1 glycoprotein gene within a gene cluster dispensable for growth in cell culture

The genome of herpes simplex virus 1 consists of two components, L and S, each containing unique sequences flanked by inverted repeats. Current and earlier studies have shown that 11 of the 12 open reading frames contained in the unique sequences of the S component can be deleted and are dispensable for growth in cell culture. Analyses of one recombinant virus containing a deletion in the open reading frame US7 permitted the identification of a monoclonal antibody specific for the product of this gene. The protein encoded by this gene has a predicted translated molecular weight of 41,366 and an apparent molecular weight of approximately 65,000 in denaturing polyacrylamide gels. The electrophoretic mobility of the protein synthesized by cells in the presence of inhibitory concentrations of tunicamycin is faster than that of the protein accumulating in lysates of untreated infected cells. We conclude that the product of US7 is glycoprotein subject to N-linked glycosylation, and we have designated it glycoprotein I. These studies indicate that the unique sequences of the S component encode four glycoproteins (G, D, I, and E) of which at least three (G, I, and E) are dispensable for growth in continuous lines of primate cells.

The single base pair substitution responsible for the Syn phenotype of herpes simplex virus type 1, strain MP

Nucleotide sequences were determined for portions of the genomes of the syncytial (Syn) mutant of herpes simplex virus type 1, strain MP, and the related wild-type strain mP. Comparisons of the nucleotide sequences showed only 1 bp difference between the DNAs of strains MP and mP in the region to which the Syn mutation of MP had previously been mapped. This base pair substitution in MP (at map coordinate 0.737) eliminates a ThaI restriction endonuclease recognition site that is present in mP DNA. Analyses of MP X mP recombinant viruses showed that presence of the ThaI site correlates with the Syn+ phenotype and absence of the ThaI site correlates with the Syn phenotype as predicted. We conclude that the base pair substitution at map coordinate 0.737 is responsible for the Syn phenotype of MP. This mutation could alter translation in four of the six reading frames, causing amino acid substitutions. From only one of these reading frames is a product likely to be expressed. The 338-amino acid polypeptide that could be expressed has features characteristic of membrane-associated proteins, including hydrophobic domains, potential sites for the attachment of N-linked carbohydrate, and a potential cleavable signal sequence.

Expression of cell-associated and secreted forms of herpes simplex virus type 1 glycoprotein gB in mammalian cells

The gene for glycoprotein gB1 of herpes simplex virus type 1 strain Patton was expressed in stable Chinese hamster ovary cell lines. Expression vectors containing the dihydrofolate reductase (dhfr) cDNA plus the complete gB1 gene or a truncated gene lacking the 194 carboxyl-terminal amino acids of gB1 were transfected into CHO DHFR-deficient cells. Radioimmunoprecipitation demonstrated that the complete gB1 protein expressed in CHO cell lines was cell associated, whereas the truncated protein was secreted from the cells due to deletion of the transmembrane and C-terminal domains of gB1. Cells expressing the truncated gB1 protein were subjected to stepwise methotrexate selection, and a cell line was isolated in which the gB1 gene copy number had been amplified 10-fold and the level of expression of gB1 had increased over 60-fold. The truncated gB1 protein was purified from medium conditioned by the amplified cell line. N-terminal amino acid sequence analysis of this purified protein identified the signal peptide cleavage site and predicted the cleavage of a 30-amino-acid signal sequence from the primary protein. The immunogenicity of the truncated gB1 protein was also tested in mice, and high levels of antibody and protection from virus challenge were observed.

Kinetics of expression of herpes simplex virus type 1-specific glycoprotein species on the surfaces of infected murine, simian, and human cells: flow cytometric analysis

The kinetics of expression of the herpes simplex virus type 1-encoded major glycoprotein species gB, gC, gD, and gE on the surfaces of cells of murine, simian, and human origins were studied. Viable cells were stained with monoclonal antibodies specific for each species, and the levels expressed were determined by fluorescence flow cytometry. Differences were observed in both the kinetics and the levels of expression of individual glycoprotein species, depending upon the origin of the host cells. Glycoprotein gC was expressed early and at high levels in cells of murine and human origins, but late and at relatively low levels in simian cells. In contrast, gE was expressed at high levels in simian cells, but was not detectable until late in the infectious cycle in murine and human cells. The kinetics and levels of expression of gB were similar for all cells investigated, whereas gD, with high levels of expression in all cells late in infection, appeared on the surfaces of murine cells very early postinfection. This approach has allowed a simple quantitative method for comparing levels of glycoprotein expression.

Oligomerization of herpes simplex virus glycoprotein B

Glycoprotein B (gB) specified by herpes simplex virus can be extracted from virions or infected cells in the form of detergent-stable, heat-dissociable oligomers. The composition of the oligomers and requirements for their formation were investigated. Evidence is presented that the faster-migrating forms of the oligomers are homodimers of gB. Dimerization was shown to occur within minutes of polypeptide synthesis and did not depend on glycosylation, the expression of other viral proteins, or virion morphogenesis. The multiple, electrophoretically distinct forms of gB dimers differ in extent or rate of N-linked oligosaccharide processing and also have other differences that influence electrophoretic mobility.

Antigenic variation (mar mutations) in herpes simplex virus glycoprotein B can induce temperature-dependent alterations in gB processing and virus production

Monoclonal antibody-resistant (mar) mutants altered in the antigenic structure of glycoprotein B (gB) of herpes simplex virus type 1, strain KOS-321, were selected by neutralization with each of six independently derived gB-specific monoclonal antibodies. Analysis of the reactivity patterns of these mar mutants with a panel of 16 virus-neutralizing monoclonal antibodies identified at least five nonoverlapping epitopes on this antigen, designated groups I through V. Multiple mar mutations were also introduced into the gB structural gene by recombination and sequential antibody selection to produce a set of mar mutants with double, triple, and quadruple epitope alterations. Group II (B2) and group III (B4) antibodies were used to select the corresponding mutants, mar B2.1 and mar B4.1, which in addition to carrying the mar phenotype were temperature sensitive (ts) for processing of the major partially glycosylated precursor of gB, pgB (Mr = 107,000), to mature gB (Mr = 126,000) and showed reduced levels of gB on the cell surface at high temperature (39 degrees C). These mutants were not, however, ts for production of infectious progeny. A recombinant virus, mar B2/4.1, carrying both of these alterations was ts for virus production and failed to produce and transport any detectable mature gB to the cell surface at 39 degrees C. Rather, pgB accumulated in the infected cell. Revertants of the ts phenotype, isolated from virus plaques at 39 degrees C, regained the B2 but not the B4 epitope and were phenotypically indistinguishable from the mar B4.1 parent. Finally, it was shown that group II (B5) and group III (B4) antibodies failed to immunoprecipitate pgB (39 degrees C) produced by ts gB mutants of herpes simplex virus type 1 which were not selected with monoclonal antibodies. Taken together, our findings indicate that (i) mar mutations can alter antigenic as well as other functional domains of gB, namely, the domain(s) involved in processing and infectivity, and (ii) group II and group III epitopes lie within an essential functional domain of gB which is a target for ts gB mutations.

Pathogenicity in mice of herpes simplex virus type 2 mutants unable to express glycoprotein C

Herpes simplex virus type 2 (HSV-2) mutants that were unable to express glycoprotein C (gC-2) were isolated. Deletions were made in a cloned copy of the gC-2 gene, and recombinant viruses containing these deletions were screened by using an immunoreactive plaque selection protocol. The viruses did not display a syncytial phenotype. Intravaginal inoculation of BALB/cJ mice with one of the HSV-2 gC-2- viruses produced local inflammation followed by a lethal spread of the viral infection into the nervous system in a manner identical to that produced by parental HSV-2 strain 333. Similarly, intracerebral inoculation of DBA-2 mice with the gC-2- virus produced a lethal neurological disease paralleling that caused by HSV-2 strain 333. These results indicate that gC-2 is not required for the spread of HSV-2 infections in mice.

Anatomy of the herpes simplex virus 1 strain F glycoprotein B gene: primary sequence and predicted protein structure of the wild type and of monoclonal antibody-resistant mutants

In this paper we report the nucleotide sequence and predicted amino acid sequence of glycoprotein B of herpes simplex virus 1 strain F and the amino acid substitutions in the domains of the glycoprotein B gene of three mutants selected for resistance to monoclonal antibody H126-5 or H233 but not to both. Analyses of the amino acid sequence with respect to hydropathicity and secondary structure yielded a two-dimensional model of the protein. The model predicts an N-terminal, 29-amino-acid cleavable signal sequence, a 696-amino-acid hydrophilic surface domain containing six potential sites for N-linked glycosylation, a 69-amino-acid hydrophobic domain containing three segments traversing the membrane, and a charged 109-amino-acid domain projecting into the cytoplasm and previously shown to marker rescue glycoprotein B syn mutations. The nucleotide sequence of the mutant glycoprotein B DNA fragments previously shown to marker transfer or rescue the mutations revealed that the amino acid substitutions cluster in the hydrophilic surface domain between amino acids 273 and 305. Analyses of the secondary structure of these regions, coupled with the experimentally derived observation that the H126-5- and H233-antibody cognitive sites do not overlap, indicate the approximate locations of the epitopes of these neutralizing, surface-reacting, and immune-precipitating monoclonal antibodies. The predicted perturbations in the secondary structure introduced by the amino acid substitutions correlate with the extent of loss of reactivity with monoclonal antibodies in various immunoassays.

Characterization of a herpes simplex virus type 1 mutant resistant to benzhydrazone, a selective inhibitor of herpesvirus glycosylation

Benzhydrazone [BH; 1H-benz[f]indene-1,3(2H) -dionebis(amidinohydrazone)] significantly inhibits glycosylation of proteins, but only in cells infected with herpes simplex virus. We report on a herpes simplex virus type 1 (HSV-1) mutant resistant to BH. A syncytium-inducing mutant designated HSV-1(13)S11 was found to be biochemically resistant to BH in that [14C]glucosamine incorporation was not inhibited in infected HEp-2 cells exposed to the drug. Intertypic recombinants were obtained which showed that BH resistance is encoded in the DNA of the mutant virus and may be transferred into the genome of BH-sensitive HSV. In the recombinants the biochemical resistance marker segregated from the syncytial marker, suggesting that the two markers probably map in different loci. The BH-resistant mutant did not complement wild-type BH-sensitive HSV-1 and -2. Furthermore, resistance was apparent in HEp-2 but not in Vero cells. The paper discusses the hypothesis that inhibition of glycosylation of HSV proteins is the consequence of modification or selective transport of BH involving a HSV gene product.

Virus-specific glycoproteins associated with the nuclear fraction of herpes simplex virus type 1-infected cells

Monospecific antisera to herpes simplex virus type 1 (HSV-1) glycoproteins gB, gC, and gD were used to identify the HSV-1-specific glycoproteins associated with the nuclear fraction as compared with those associated with cytoplasmic fraction, whole-cell lysates, and purified virions. The results indicate that a predominance of HSV glycoprotein precursors pgC(105), pgB(110), and pgD(52) is associated with the nuclear fraction. Treatment of the nuclear fraction with the enzyme endo-beta-N-acetylglucosaminidase H indicated that the lower-molecular-weight glycoproteins are sensitive to this endoglycosidase. These results suggest that in the nuclear fraction of HSV-1-infected cells virus-specific glycoproteins gB, gC, and gD are predominately in the high-mannose precursor form; however, detectable amounts of the fully glycosylated forms of gC and gD were also found.

Processing of herpes simplex virus-1 glycans in cells defective in glycosyl transferases of the Golgi system: relationship to cell fusion and virion egress

We studied herpes simplex virus-1 (HSV-1) glycan structure and the expression of HSV-1 functions regulated by viral glycoproteins in Ric21 cells (P. VISCHER and R. C. HUGHES, Eur. J. Bioch. 117, 275-284, 1981). This is a line of ricin-resistant mutant BHK cells defective in the enzymes of the Golgi system which add terminal sugars to N-linked glycans. Two kinds of alterations were observed in the glycosylation of HSV glycoproteins in Ric21 cells. First, there was a defective processing of complex glycans leading to a reduction of biantennary and triantennary species and an increase of incompletely processed monosialylated oligosaccharides. Second, there was an overall reduction in the accumulation of HSV-1 glycoproteins. We found that (i) the release of herpesvirions from Ric21 cells was markedly lower than that from BHK cells, possibly reflecting reduced terminal sugar addition which, in turn, might affect the intracellular transport of glycoproteins. (ii) HSV-1 (MP)-infected Ric21 cells fused with a low efficiency. Furthermore, polycaryocytosis was reduced or abolished in BHK and in Ric21 cells exposed to neuraminidase, indicating that the presence of sialic acid residues in the cell surface glycans is essential for cells to interact in a fashion that brings cell fusion. (iii) Although capsid assembly was comparable, the rate of accumulation of infectious virus decreased in Ric21 cells. Infectivity of released virions from Ric21 and BHK cells was similar, in agreement with previous studies showing that complex-type glycans do not appear to be required for herpesvirion infectivity. The decrease in infectious HSV-1 yield seems to correlate with overall reduced ability to synthesize glycoproteins.

Anti-gD monoclonal antibodies inhibit cell fusion induced by herpes simplex virus type 1

Monoclonal antibodies directed against glycoprotein D of herpes simplex virus completely inhibited fusion of Vero cells infected with type 1 virus. In contrast, several monoclonal antibodies directed against other viral glycoproteins, including B, were ineffective or were only minimally inhibitory at the highest concentrations tested.

Physical mapping of the mutation in an antigenic variant of herpes simplex virus type 1 by use of an immunoreactive plaque assay

Two mutations affecting herpes simplex virus type 1 glycoprotein B were mapped by marker rescue using cloned sequences of wild-type herpes simplex virus type 1 strain KOS DNA. One mutant, tsB5, is a temperature-sensitive mutant which does not express mature, functional glycoprotein B at the nonpermissive temperature. The other mutant, marB1.1, expresses an antigenic variant of glycoprotein B and was selected for resistance to neutralization by a monoclonal antibody. The mutation in tsB5 mapped to a 1.2-kilobase segment of the herpes simplex virus type 1 genome between coordinates 0.361 and 0.368, whereas the mutation in marB1.1 mapped to a 1.6-kilobase segment between coordinates 0.350 and 0.361. An in situ enzyme immunoassay was used to detect plaques of recombinant wild-type virus among the progeny of transfections with mutant marB1.1 DNA and wild-type DNA fragments.

Membrane fusion proteins of enveloped animal viruses

In a living cell membrane-bound compartments are continuously either separated or united through fusion reactions, and literally thousands of such reactions take place every minute. The formation of membrane vesicles from pre-existing membranes, and their fusion with specific acceptor membranes, constitute a prerequisite for the transport of most impermeant molecules and macromolecules into the cell by endocytosis, out of the cell by exocytosis, and between the cellular organelles (Palade, 1975; Silverstein, 1978; Evered & Collins, 1982). Less frequent, but equally crucial, are fusion events in fertilization, cell division, polykaryon formation, enucleation, etc. (for reviews see Poste & Nicholson, 1978). Although a great deal is known about the properties and consequences of individual forms of membrane fusion in cellular systems, and about fusion in artificial lipid membranes, the molecular basis for the reactions remain largely unclear.

Inhibition of glycosylation of herpes simplex virus glycoproteins: identification of antigenic and immunogenic partially glycosylated glycopeptides on the cell surface membrane

The surface membranes of cells infected with herpes simplex virus type 1 (HSV-1), strain KOS, contain three principal glycoproteins, gC (apparent Mr 129k), gB (apparent Mr 120k), and gD (apparent Mr 58k). Infections carried out in the presence of the glycosylation inhibitor 2-deoxy-D-glucose result in the loss of the mature species with the concurrent appearance of lower-molecular-weight polypeptides which are presumably partially glycosylated forms of the fully processed glycoproteins. Specific immunoprecipitation of radiolabeled cytoplasmic extracts of 2-deoxy-D-glucose-inhibited infections identified partially glycosylated proteins designated DG92, DG88, and DG53, which are antigenically related to the corresponding mature forms gB, gC, and gD. Cell surface radioiodination, in combination with specific immunoprecipitation, revealed that DG88 and DG53 were the principal species transported to the cell surface in 2-deoxy-D-glucose-inhibited infections. DG92 was readily detected in the cytoplasm but not on the plasma membrane. Cells infected with the KOS mutant, syn LD70, did not synthesize glycoprotein gC. In glycosylation-inhibited syn LD70 infections, DG88 was not detected in either the cytoplasm or plasma membrane, demonstrating a genetic relationship between DG88 and gC. Polyclonal and monoclonal antibodies directed against the glycoproteins gC, gB, and gD sensitized infected cells to complement-mediated immune cytolysis. Cells infected in the presence of the inhibitor were sensitized to lysis only by antibody specific for gC and gD. The glycosylation-inhibited cells were insensitive to immunolysis by anti-gB monoclonal antibody. These findings confirm that the glycosylation-deficient forms of gC and gD, but not gB reach the cell surface in the presence of inhibitor and that the inhibitor-induced alterations in glycosylation do not cause a complete loss of antigenicity. Inoculation of mice with syngeneic 3T3 cells infected in the presence or absence of inhibitor-induced cytolytic and neutralizing antibody. A major portion of the cytolytic antibody was directed against gC, but anti-gC antibody appeared to play a minor role in virus neutralization. While the serum induced by the control infected cells contained precipitating antibodies for gC, gB, and gD, the serum derived from mice inoculated with inhibitor-treated infected cells had only weak immunoprecipitating activity against gB. Together, these findings have identified partially glycosylated forms of the major HSV glycoproteins and show that complete glycosylation is not required for transport of some of these partially glycosylated polypeptides to the cell surface. Moreover, complete glycosylation of the glycopeptides is not essential for maintenance of antigenicity or immunogenicity, indicating that at least some determinants recognized by antibodies directed against the mature glycoproteins are not affected by 2-deoxy-D-glucose-induced carbohydrate alterations.

Endo-beta-N-acetylglucosaminidase H sensitivity of precursors to herpes simplex virus type 1 glycoproteins gB and gC

The endoglycosidase endo-beta-N-acetylglucominidase H (endo H) was used to examine the nature of the oligosaccharides associated with the herpes simplex virus type 1 glycoproteins gA, gB, and gC. Immunoprecipitates from detergent extracts of infected cells, using monospecific antisera to gAB and gC, were treated with endo H. The low-molecular-weight precursor to gC, pgC(105), was found to be sensitive to endo H. Removal of the endo H-sensitive oligosaccharide chains from pgC(105) resulted in a protein with an apparent molecular weight of 75,000. In contrast, the fully glycosylated gC was not sensitive to endo H treatment. These results suggested that the oligosaccharide chains of pgC(105) were primarily of the simple high-mannose type. Both gA and gB were sensitive to endo H treatment; however, gB appeared to be only partially susceptible, whereas [3H]mannose-labeled gA was not detectable after endo H treatment. These results that gB contained both complex- and simple-type oligosaccharides, and gA contained only simple-type oligosaccharides. An accumulation of the high-mannose glycoproteins pgC(105) and gA was observed in monensin-treated infected cells with a concomitant inhibition of gB and gC. Glycoproteins gA and pgC(105) synthesized in the presence of monensin were also sensitive to endo H treatment.

Infectivity and glycoprotein processing of herpes simplex virus type 1 grown in a ricin-resistant cell line deficient in N-acetylglucosaminyl transferase I

We report on the replication of herpes simplex virus type 1 (HSV-1) and viral glycoprotein processing in RicR14 cells, a mutant ricin-resistant cell line defective in N-acetylglucosaminyl transferase I activity. In these cells HSV-1(MP) and (F) replicated to yields very similar to those in parental BHK cells. The kinetics of HSV-1 adsorption in mutant and in parent cells was also essentially identical. Progeny virions from ricin-resistant and wild-type cells displayed comparable specific infectivities. However, in the mutant cells the efficiency of plating of progeny virus from both RicR14 and BHK cells was reduced. HSV-1(MP) failed to induce syncytia in RicR14 cells either in a plaque assay or after a high-multiplicity infection. Moreover, the fully glycosylated forms of glycoproteins (gB, gC, and gD) were totally absent, and only the partially glycosylated precursors (pgC, pgD. and a triplet in the gB-gA region) accumulated in HSV-1-infected ricin-resistant cells and in herpesvirions made in these cells. Consistent with these results analysis of pronase glycopeptides from cells labeled with [14C]glucosamine showed a strong decrease of sialylated complex-type oligosaccharides and a dramatic accumulation of the neutral mannose-rich chains. The latter chains predominate in partially glycosylated precursors, whereas the complex acidic chains predominate in the fully processed forms of HSV glycoproteins. These results taken together indicate that (i) host-cell N-acetylglucosaminyl transferase I participates in the processing of HSV glycoproteins; and (ii) infectivity of herpesvirions does not necessarily require the mature form of gB. The absence of HSV-1(MP)-induced fusion in RicR14 cells is discussed.

Monensin inhibits the processing of herpes simplex virus glycoproteins, their transport to the cell surface, and the egress of virions from infected cells

HEp-2 cells or Vero cells infected with herpes simplex virus type 1 were exposed to the ionophore monensin, which is thought to block the transit of membrane vesicles from the Golgi apparatus to the cell surface. We found that yields of extracellular virus were reduced to less than 0.5% of control values by 0.2 microM monensin under conditions that permitted accumulation of cell-associated infectious virus at about 20% of control values. Viral protein synthesis was not inhibited by monensin, whereas late stages in the post-translational processing of the viral glycoproteins were blocked. The transport of viral glycoproteins to the cell surface was also blocked by monensin. Although the assembly of nucleocapsids appeared to be somewhat inhibited in monensin-treated cells, electron microscopy revealed that nucleocapsids were enveloped to yield virions, and electrophoretic analyses showed that the isolated virions contained immature forms of the envelope glycoproteins. Most of the virions which were assembled in monensin-treated cells accumulated in large intracytoplasmic vacuoles, whereas most of the virions produced by and associated with untreated cells were found attached to the cell surface. Our results implicate the Golgi apparatus in the egress of herpes simplex virus from infected cells and also suggest that complete processing of the viral envelope glycoproteins is not essential for nucleocapsid envelopment or for virion infectivity.

Characterization of an herpes simplex virus type 2 mutant, which is resistant to acycloguanosine and causes fusion of BSC1 cells

A mutant of herpes simplex virus type 2, which induces low levels of thymidine-kinase activity in infected BSC1 cells and consequently able to grow in the presence of acycloguanosine, was isolated. This mutant has also been shown to cause fusion of BSC1 cells. In BSC1 cells, co-infected with the wild-type strain and the mutant, the yield of each of the two viruses was normal but the rounding and aggregation of cells observed, resembled that found in wild-type infected cultures. When the mixed infection was performed in the presence of acycloguanosine (100 micrometers), the growth of the two virus strains was inhibited, as well as the cytopathic effect in the cultures. It is suggested that under these conditions, the thymidine-kinase which was induced in the infected cells by the wild-type strain, phosphorylated acycloguanosine and the activated drug formed, inhibited the growth of the two viruses by interference in their DNA syntheses.

Multimeric forms of herpes simplex virus type 2 glycoproteins

Herpes simplex virus type 2-specific glycoproteins present in detergent extracts of infected cells were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under conditions designed to permit detection of multimeric forms of these glycoproteins. Two high-molecular-weight glycosylated species were detected when samples were disrupted at lower temperatures or in the absence of any reducing agents. One multimer having an apparent molecular weight of 275,000 was identified as a multimer of the gA or gB glycoprotein or both. The second glycoprotein, having a molecular weight of approximately 230,000, was identified as a multimeric form of the gC glycoprotein. These data indicate that the gC as well as the gA and gB glycoproteins of herpes simplex virus type 2 may exist in a multimeric form.