O-glycosidic carbohydrate-peptide linkages of Herpes simplex virus glycoproteins

Global aspects of viral glycosylation

Enveloped viruses encompass some of the most common human pathogens causing infections of different severity, ranging from no or very few symptoms to lethal disease as seen with the viral hemorrhagic fevers. All enveloped viruses possess an envelope membrane derived from the host cell, modified with often heavily glycosylated virally encoded glycoproteins important for infectivity, viral particle formation and immune evasion. While N-linked glycosylation of viral envelope proteins is well characterized with respect to location, structure and site occupancy, information on mucin-type O-glycosylation of these proteins is less comprehensive. Studies on viral glycosylation are often limited to analysis of recombinant proteins that in most cases are produced in cell lines with a glycosylation capacity different from the capacity of the host cells. The glycosylation pattern of the produced recombinant glycoproteins might therefore be different from the pattern on native viral proteins. In this review, we provide a historical perspective on analysis of viral glycosylation, and summarize known roles of glycans in the biology of enveloped human viruses. In addition, we describe how to overcome the analytical limitations by using a global approach based on mass spectrometry to identify viral O-glycosylation in virus-infected cell lysates using the complex enveloped virus herpes simplex virus type 1 as a model. We underscore that glycans often pay important contributions to overall protein structure, function and immune recognition, and that glycans represent a crucial determinant for vaccine design. High throughput analysis of glycosylation on relevant glycoprotein formulations, as well as data compilation and sharing is therefore important to identify consensus glycosylation patterns for translational applications.

Involvement of viral glycoprotein gC-1 in expression of the selectin ligand sialyl-Lewis X induced after infection with herpes simplex virus type 1

Several herpesviruses induce expression of the selectin receptor sialyl-Lewis X (sLe(x) ) by activating transcription of one or more of silent host FUT genes, each one encoding a fucosyltransferase that catalyses the rate-limiting step of sLe(x) synthesis. The aim here was to identify the identity of the glycoconjugate associated with sLe(x) glycoepitope in herpes simplex virus type 1 (HSV-1) infected human diploid fibroblasts, using immunofluorescence confocal microscopy. Cells infected with all tested HSV-1 strains analysed demonstrated bright sLe(x) fluorescence, except for two mutant viruses that were unable to induce proper expression of viral glycoprotein gC-1: One gC-1 null mutant and another mutant expressing gC-1 devoid of its major O-glycan-containing region (aa 33-116). The sLe(x) reactivity of HSV-1 infected cells was abolished by mild alkali treatment. Altogether the results indicated that the detectable sLe(x) was associated with O-linked glycans, situated in the mucin region of gC-1. No evidence for sLe(x) (i) in other HSV-1 glycoproteins with mucin domains such as gI-1 or (ii) in host cell glycoproteins/glycolipids was found. Thus, the mucin domain of HSV-1 gC-1 may support expression of selectin ligands such as sLe(x) and other larger O-linked glycans in cell types lacking endogenous mucin domain-containing glycoproteins, optimized for O-glycan expression, provided that the adequate host glycosyltransferase genes are activated.

The Tn antigen-structural simplicity and biological complexity

Glycoproteins in animal cells contain a variety of glycan structures that are added co- and/or posttranslationally to proteins. Of over 20 different types of sugar-amino acid linkages known, the two major types are N-glycans (Asn-linked) and O-glycans (Ser/Thr-linked). An abnormal mucin-type O-glycan whose expression is associated with cancer and several human disorders is the Tn antigen. It has a relatively simple structure composed of N-acetyl-D-galactosamine with a glycosidic α linkage to serine/threonine residues in glycoproteins (GalNAcα1-O-Ser/Thr), and was one of the first glycoconjugates to be chemically synthesized. The Tn antigen is normally modified by a specific galactosyltransferase (T-synthase) in the Golgi apparatus of cells. Expression of active T-synthase is uniquely dependent on the molecular chaperone Cosmc, which is encoded by a gene on the X chromosome. Expression of the Tn antigen can arise as a consequence of mutations in the genes for T-synthase or Cosmc, or genes affecting other steps of O-glycosylation pathways. Because of the association of the Tn antigen with disease, there is much interest in the development of Tn-based vaccines and other therapeutic approaches based on Tn expression.

Early steps in O-linked glycosylation and clustered O-linked glycans of herpes simplex virus type 1 glycoprotein C: effects on glycoprotein properties

The pathogenesis of herpes simplex virus type 1 (HSV-1) implies the sequential infection of many cell types from mucosal cells to neurons, each having a unique pattern of protein glycosylation. The HSV-1 glycoprotein gC-1 is highly glycosylated and contains not only N-linked glycans but also a large number of O-linked glycans, some of which are clustered into two pronase-resistant arrays in the vicinity of the HSV-1 receptor-binding domain of gC-1. The aim of the present study was to characterize gC-1 signals for addition of clustered glycans, to determine the efficacy of synthetic peptides, representing putative O-glycosylation signals, as substrates for a panel of GalNAc transferases, and to identify possible effects of early O-linked glycosylation on the biological functions of gC-1. Gel filtration analysis of the pronase-resistant gC-1 O-glycan clusters from a glycoprotein mutant, lacking a site for N-linked glycosylation at Asn 73 in the vicinity of the O-glycosylation signal, suggested that one function of this N-linked glycan was to modulate the access for GalNAc transferases to one particular O-glycosylation peptide signal (aa 80-104). The ability of four GalNAc-transferase isoenzymes with different cell type expression patterns to initialize O-glycosylation of synthetic gC-1 derived peptides was analyzed. Two synthetic gC-1 peptides (aa 55-69 and aa 80-104) were excellent substrates for all four GalNAc-transferases, suggesting that cell types expressing less frequent GalNAc transferase species with unusual acceptor peptide sequence specificities may also produce a highly O-glycosylated gC-1 after HSV-1 infection. The O-linked glycans were not essential for cell surface expression of gC-1, but monoclonal antibody-assisted epitope analysis of N-acetylgalactosaminidase-treated gC-1 showed that the O-linked monosaccharide GalNAc contributed to expression of a three-dimensional epitope overlapping the heparan sulfate-binding domain of gC-1.

The blood group antigen-related glycoepitopes: key structural determinants in immunogenesis and AIDS pathogenesis

This overview will focus on the functional and pathophysiological aspects of blood group antigen (BGA)-related glycodeterminants with regard to immunogenesis and AIDS pathogenesis. It has been postulated that in a broad range of histogenetically different tissues and organs, BGA-related glycoepitopes are expressed on the cell surface at definite stages of cell differentiation. These glycoepitopes are expressed during embryogenesis, organogenesis, tissue repair, regeneration, remodelling and maturation when 'sorting-out' of one homotypic cell population from a heterotypic assemblage of cells occurs (1). In this event, the BGA-related glycoepitopes, if being expressed on the cell surface, play roles of key structural determinants in cell-cell recognition, association and aggregation. This mechanism will be discussed in relation to immunogenesis with regard to antigen presentation, self-non-self discrimination, and positive and negative selection during thymic education. It is postulated that the appearance of BGA-related glycoepitopes on the cell membrane is a consequence of the association of major histocompatibility complex antigens (MHC) and peptides, with the subsequent elimination of cells carrying a high density of BGA-related glycoepitopes on their surface. After human immunodeficiency virus (HIV) glycoproteins are glycosylated by host cell glycosyltransferases, the virus may use the BGA-related glycodeterminants as ligands and/or receptors for expansion to a spectrum of target cells during AIDS development and generalization of the infection throughout the body. We will review the experimental evidence that supports the concept that HIV uses an alternative to the gp120/CD4 ligand/receptor system, and that the alternative mechanism is probably carbohydrate-mediated in nature.

Identification and characterization of a novel herpes simplex virus glycoprotein, gK, involved in cell fusion

Antipeptide sera were used to identify a novel glycoprotein encoded by the UL53 gene of herpes simplex virus type 1 (HSV-1). The UL53 gene product is thought to play a central role in regulating membrane fusion because mutations giving rise to the syncytial phenotype, wherein cells are extensively fused, frequently map to this gene. A single 40-kDa protein, designated gK (the ninth HSV-1 glycoprotein to be described), was detected with antipeptide sera in cells infected with both wild-type and syncytial strains of HSV-1 which were labelled with [35S]methionine and [35S]cysteine or with [3H]glucosamine, and this protein was sensitive to treatment of cells with tunicamycin. With all other HSV glycoproteins studied to date, at least two glycosylated species, often differing substantially in electrophoretic mobility, have been observed in infected cells; thus, gK is unusual in this respect. The 40-kDa gK protein was also immunoprecipitated from cells infected with a recombinant adenovirus vector carrying the UL53 gene. Two glycosylated species of 39 and 41 kDa were produced when UL53 mRNA was translated in vitro in the presence of microsomes, and these proteins differed from gK produced in infected cells not only because they possessed different electrophoretic mobilities but also because they were unable to enter gels after being heated. In addition, a 36-kDa protein was detected in extracts from cells infected with HSV-2 with use of these sera.

Antigenic structure of the herpes simplex virus type 1 glycoprotein C: demonstration of a linear epitope situated in an environment of highly conformation-dependent epitopes

A continuous epitope, situated within or in close proximity to antigenic site II of the herpes simplex virus type 1-specified glycoprotein C (gC-1), was identified. The continuous linear nature of the epitope, defined by a monoclonal antibody C2H12, was established by three independent lines of evidence: (i) The epitope was detectable by immunoblot under denaturing and reducing conditions. (ii) The epitope was detectable by RIPA of extracts from TM-treated HSV-infected cells, despite the malfolding caused by this treatment. (iii) The epitope was detected in an approximately 5,000-dalton papain fragment of gC-1. A mapping analysis, primarily based on use of mutant virus, expressing truncated gC-1 molecules, suggested that the mapping position of the epitope was delimited by amino acids 120 and 230. Other epitopes of this region of gC-1 are highly conformation-dependent, and the existence of a linear epitope, accessible on native gC-1, may facilitate the elucidation of the functional anatomy of gC-1.

A novel herpes simplex virus glycoprotein, gL, forms a complex with glycoprotein H (gH) and affects normal folding and surface expression of gH

A glycoprotein encoded by the UL1 gene of herpes simplex virus type 1 (HSV-1) was detected in infected cells with antipeptide sera. The UL1 gene has previously been implicated in virus-induced cell fusion (S. Little and P. A. Schaffer, Virology 112:686-697, 1981). Two protein species, a 30-kDa precursor form and a 40-kDa mature form of the glycoprotein, both of which were modified with N-linked oligosaccharides, were observed. This novel glycoprotein is the 10th HSV-1 glycoprotein to be described and was named glycoprotein L (gL). A complex was formed between gL and gH, a glycoprotein known to be essential for entry of HSV-1 into cells and for virus-induced cell fusion. Previously, it had been reported that gH expressed in the absence of other viral proteins was antigenically abnormal, not processed, and not expressed at the cell surface (U.A. Gompels and A. C. Minson, J. Gen. Virol. 63:4744-4755, 1989; A. J. Forrester, V. Sullivan, A. Simmons, B. A. Blacklaws, G. L. Smith, A. A. Nash, and A. C. Minson, J. Gen. Virol. 72:369-375, 1991). However, gH coexpressed with gL by using vaccinia virus recombinants was antigenically normal, processed normally, and transported to the cell surface. Similarly, gL was dependent on gH for proper posttranslational processing and cell surface expression. These results suggest that it is a hetero-oligomer of gH and gL which is incorporated into virions and transported to the cell surface and which acts during entry of virus into cells.

An O-linked carbohydrate neutralization epitope of HIV-1 gp 120 is expressed by HIV-1 env gene recombinant vaccinia virus

Previous studies have disagreed about the presence of O-linked carbohydrate epitopes on gp 120 of HIV, although antibodies against short-chain O-linked glycans neutralize HIV infection and block syncytium formation in vitro. To settle this question, we analysed the O-linked glycans of gp 120 by chemical methods using purified HIV-1 gp 120 from cells infected with recombinant vaccinia virus solely expressing gp 160 or gp 120. Alkaline borohydride degradation of recombinant gp 120 released monosaccharides and also slightly larger structures (di/trisaccharides) by a beta-elimination, confirming the presence of simple O-linked oligosaccharides. The functional activity as neutralisation epitopes of the O-linked oligosaccharides expressed on recombinant gp 120 was preserved, since fusion between uninfected CD4+ cells and cells infected with recombinant vaccinia was blocked by monoclonal antibodies to the O-linked oligosaccharides of gp 120. Although the mechanism for HIV induction of O-linked oligosaccharide neoantigens is unknown, these results indicate that the O-linked neutralization epitopes are inherent to the glycoprotein itself, and that the unusual appearance of simple O-linked oligosaccharides on gp 120 is independent of any interaction between the host cell and retroviral genes other than env.

Heparan sulfate glycosaminoglycans as primary cell surface receptors for herpes simplex virus

Our current incomplete picture of the earliest events in HSV infection may be summarized as follows. The initial interaction of virus with cells is the binding of virion gC to heparan sulfate moieties of cell surface proteoglycans. Stable binding of virus to cells may require the interaction of other virion glycoproteins with other cell surface receptors as well (including the interaction of gB with heparan sulfate). Penetration of virus into the cell is mediated by fusion of the virion envelope with the cell plasma membrane. Events leading up to this fusion require the action of at least three viral glycoproteins (gB, gD and gH), one or more of which may interact with specific cell surface components. It seems likely that binding of gB to cell surface heparan sulfate may occur and may be important in the activation of some event required for virus penetration. Heparan sulfate is present not only as a constituent of cell surface proteoglycans but also as a component of the extracellular matrix and basement membranes in organized tissues. In addition, body fluids contain both heparin and heparin-binding proteins, either of which can prevent the binding of HSV to cells (WuDunn and Spear, 1989). As a consequence, the spread of HSV infection is probably influenced, not only by immune responses to the virus, but also by the probability that virus will be entrapped or inhibited from binding to cells by extracellular forms of heparin or heparan sulfate.

Analysis of bovine herpesvirus 4 (DN 599) major antigens with monoclonal antibodies and polyclonal immune serum

Monoclonal antibodies (MAbs) and polyclonal immune sera were produced and used to identify the major antigens of bovine herpesvirus type 4 (BHV-4). SDS-polyacrylamide gel electrophoresis of immunoprecipitates of radiolabeled lysates from infected cells resolved 24 peptide bands varying from 12 kDa to over 300 kDa. Six peptides were identified as major viral antigens by immunoprecipitation. Based on the pattern of radioimmunoprecipitation, MAbs were assigned into four groups. Group 1 precipitated a tunicamycin-sensitive glycoprotein complex which contained six components (245, 190, 152, 123, and 48/46 kDa). Deglycosylation with endoglycosidase F revealed two peptides with Mr of 93 and 38 kDa as the basic peptides of the glycoprotein complex. In addition, a 115 kDa glycopeptide containing glycan-peptide bonds of mixed type was identified. Group 2 precipitated a non-glycosylated protein complex consisting of three monomers (33/31/30 kDa). Groups 3 and 4 reacted with single monomeric non-glycosylated peptides with Mr of 48 and 14 kDa, respectively. Although none of the MAbs exhibited significant neutralizing activity, some reacted strongly in immunosorbent and/or immunohistochemical assays, suggesting they may be good candidates for use in diagnostic assays.

Inhibition of human immunodeficiency virus (HIV) infection in vitro by anticarbohydrate monoclonal antibodies: peripheral glycosylation of HIV envelope glycoprotein gp120 may be a target for virus neutralization

Carbohydrate structures are often involved in the initial adhesion of pathogens to target cells. In the present study, a panel of anticarbohydrate monoclonal antibodies (MAbs) was tested for their ability to inhibit in vitro human immunodeficiency virus infectivity. MAbs against three different N- and O-linked carbohydrate epitopes (LeY, A1, and sialyl-Tn) were able to block infection by cell-free virus as well as inhibit syncytium formation. Inhibition of virus infectivity was independent of virus strain (HTLVIIIB or patient isolate SSI-002), the cell line used for virus propagation (H9 or MT4), and the cell type used as the infection target (MT4, PMC, or selected T4 lymphocytes). Inhibition was observed when viruses were preincubated with MAbs but not when cells were preincubated with MAbs before inoculation, and the MAbs were shown to precipitate 125I-labeled gp120. The MAbs therefore define carbohydrate structures expressed by the viral envelope glycoprotein gp120, indicating that glycans of the viral envelope are possible targets for immunotherapy or vaccine development or both.

Compartment-specific immunolocalization of conserved epitopes of the glycoprotein gB of herpes simplex virus type 1 and bovine herpes virus type 2 in infected cells

Monoclonal antibodies directed against a surface glycoprotein of the bovine herpes virus type 2 (BHV-2, bovine herpes mammillitis virus) recognize also determinants of the major glycoprotein gB of the human herpes simplex virus type 1 (HSV-1). Cross-reacting antigens of the virions and in infected cells were localized with immunocytochemical methods, immunofluorescence as well as pre-embedding and cryoultramicrotomy immune electron microscopy. All antibodies stain to different degrees cell free BHV-2 and HSV-1 virions. In the cell two predominant staining patterns could be observed indicating that expression of epitopes is dependent upon the cell compartment: (i) staining of cytoplasmic membranes and enveloped particles within membrane systems and (ii) staining of intranuclear antigens. Antibodies tagging intranuclear antigens react with moderately dense material or with the periphery of nucleocapsids. This unexpected result is interpreted in terms of two hypotheses: (1) presence of common epitopes on two entirely different herpesvirus proteins conserved in HSV-1 and BHV-2 and (2) transport of gB or its precursor into the nucleus.

Isolation and characterization of a human cytomegalovirus glycoprotein containing a high content of O-linked oligosaccharides

Several disulfide linked glycoprotein complexes were extracted from human cytomegalovirus with a non-ionic detergent and separated by anion exchange high performance liquid chromatography (HPLC). One complex had a molecular weight of 93,000 and was classified as gCII-93. Another complex had a molecular weight greater than 200,000 and was classified as gCII-200. Both complexes immunoprecipitated with a monoclonal antibody (9E10). A third set of complexes (classified as gC-I) immunoprecipitated with another monoclonal antibody (41C2). Isolated complexes were reduced, alkylated, and individual glycoproteins separated by gel-filtration HPLC. Glycoproteins with molecular weights of 50-52,000 from gCII-93 and gCII-200 appeared to be the same glycoprotein since they could be immunoprecipitated by 9 E 10 and had identical peptide maps. The amino sugar content of these glycoproteins was compared to that of higher molecular weight glycoproteins obtained from gCII-200 and to a glycoprotein with a molecular weight of 93,000 (gp93 (I] from gCI. Glycoproteins with molecular weights of 50-52,000 from gCII-93 and gCII-200 contained similar amounts of galactosamine (GalN), glucosamine (GlcN) and sialic acid. However, they contained 2-3 times more GalN than any other glycoprotein from gCII-200 and 10 times more GalN than was detected in gp93 (I). All glycoproteins from gCII-93 or gCII-200 also contained more sialic acid when compared to gp93 (I). GalN in these glycoproteins was present in O-linked oligosaccharides. This was demonstrated by release of low molecular weight oligosaccharides from high molecular weight glycopeptides by mild base hydrolysis and the conversion of GalN to galactosaminitol. Thus, gp52(II) appears to have a unique phenotype marked by a high amount of O-linked oligosaccharides.

Expression of herpes simplex virus type 1 glycoprotein D deletion mutants in mammalian cells

Glycoprotein D (gD) is a viron envelope component of herpes simplex virus types 1 and 2. We have previously defined seven monoclonal antibody (MAb) groups which recognize distinct epitopes on the mature gD-1 protein of 369 amino acids. MAb groups VII, II, and V recognize continuous epitopes at residues 11-19, 272-279, and 340-356, respectively. MAb groups I, III, IV, and VI recognize discontinuous epitopes. Recent studies have focused on epitopes I, III, and VI. Using truncated forms of gD generated by recombinant DNA methods and proteolysis, epitopes III, IV, and VI were located within amino acids 1-233. A portion of discontinuous epitope I was located in a region within residues 233-275. For this study, we used recombinant DNA methods to create mutations in the gD-1 gene and studied the effects of those mutations on gD as expressed in mammalian cells. Plasmid pRE4, containing the coding sequence of gD-1 and the Rous sarcoma virus long terminal repeat promoter, was transfected into mammalian cells. The expressed protein, gD-1-(pRE4), was identical in size and antigenic properties to gD-1 from infected cells. Six in-frame deletion mutations were subsequently constructed by using restriction enzymes to excise portions of the gD-1 gene. Plasmids carrying these mutated forms were transfected into cells, and the corresponding proteins were examined at 48 h posttransfection for antigenicity and glycosylation patterns. Three deletions of varying size were located downstream of residue 233. Analysis of these mutants showed that amino acids within the region 234-244 were critical for binding of DL11 (group I), but not for other MAb groups. Three other deletion mutants lost all ability to bind MAbs which recognize discontinuous epitopes. In addition, much of the gD expressed by these mutants was observed to migrate as high-molecular-weight aggregated forms in nondenaturing gels. Each of these mutations involved the loss of a cysteine residue, suggesting that disulfide linkages play an essential role in the formation of discontinuous epitopes. The extent of glycosylation of the mutant gD molecules accumulated at 48 h posttransfection suggested altered carbohydrate processing. In one case, there was evidence for increased O-linked glycosylation. Those proteins which had lost a cysteine residue as part of the deletion did not accumulate molecules processed beyond the high-mannose stage. The results suggest that carbohydrate processing during synthesis of gD is very sensitive to alterations in structure, particularly changes involving cysteine residues.

Sialic acids on the surface of caprine arthritis-encephalitis virus define the biological properties of the virus

The lentivirus caprine arthritis-encephalitis virus (CAEV) is a pathogen of goats. It is transmitted in milk and causes a persistent infection in goats, which often fail to produce neutralizing antibodies to the virus. Native CAEV particles are remarkably resistant to digestion with proteinase K and are neutralized extremely slowly by immune sera. Our studies showed that the virus particles are heavily sialylated. Studies with highly specific sialyltransferase enzymes identified penultimate carbohydrate linkages typical of O- and N-linked oligosaccharides on the virus and suggested that the virus may be more heavily sialylated on O-linked than on N-linked oligosaccharides. Removal of sialic acids from the virus by neuraminidase treatment did not reduce infectivity of the particles. However, desialylation rendered the virus more susceptible to proteolysis by proteinase K. Desialylation also enhanced the kinetics of neutralization of the virus by goat antibodies. These results suggest that the carbohydrates on the viral surface are important both in protecting viral proteins from digestion by proteases and in protecting the virus from rapid neutralization by antibodies.

Role of oligosaccharides in the structure and function of respiratory syncytial virus glycoproteins

The contribution of oligosaccharides to the structural and functional make-up of respiratory syncytial (RS) virus G and F proteins was investigated by observing the effects of various oligosaccharide-specific enzymes on their molecular size as well as on virus infectivity. The N-linked oligosaccharides of the F protein were completely removed by endoglycosidase F and N-glycanase. Addition of oligosaccharides to F protein during synthesis was completely inhibited by the drug tunicamycin (TM), an inhibitor of N-linked glycosylation. Glycosylation of the G protein was partially resistant to TM resulting in an 80-kDa form designated GTM. The G protein was estimated to contain approximately 3% N-linked and 55% O-linked carbohydrates, based on migration of G and GTM in polyacrylamide gels. Furthermore, treatment of detergent-extracted G protein with endoglycosidase F and endo-alpha-N-acetylgalactosaminidase, enzymes that specifically cleave N-linked and O-linked oligosaccharides, respectively, generated a variety of partially unglycosylated species, ranging in molecular weight from approximately 80 to 40 kDa. Virus infectivity was sensitive to limited removal of N-linked or O-linked oligosaccharides by endoglycosidases under conditions which did not greatly alter the molecular weight of the G protein. Thus, G and F protein oligosaccharides readily accessible to enzymatic removal are presumed to play an important role in the infectious process.

O-linked glycosylation of retroviral envelope gene products

Treatment of [3H]glucosamine-labeled Friend mink cell focus-forming virus (FrMCF) gp70 with excess peptide:N-glycanase F (PNGase F) resulted in removal of the expected seven N-linked oligosaccharide chains; however, approximately 10% of the glucosamine label was retained in the resulting 49,000-Mr (49K) product. For [3H]mannose-labeled gp70, similar treatment led to removal of all the carbohydrate label from the protein. Prior digestion of the PNGase F-treated gp70 with neuraminidase resulted in an additional size shift, and treatment with O-glycanase led to the removal of almost all of the PNGase F-resistant sugars. These results indicate that gp70 possesses sialic acid-containing O-linked oligosaccharides. Analysis of intracellular env precursors demonstrated that O-linked sugars were present in gPr90env, the polyprotein intermediate which contains complex sugars, but not in the primary translation product, gPr80env, and proteolytic digestion studies allowed localization of the O-linked carbohydrates to a 10K region near the center of the gp70 molecule. Similar substituents were detected on the gp70s of ecotropic and xenotropic murine leukemia viruses and two subgroups of feline leukemia virus, indicating that O-linked glycosylation is a conserved feature of retroviral env proteins.

Amino-terminal sequence, synthesis, and membrane insertion of glycoprotein B of herpes simplex virus type 1

Glycoprotein B (gB) was purified from cells infected with two strains (KOS and F) of herpes simplex virus type 1. Determination of amino acid sequence at the NH2 termini revealed, by comparison with amino acid sequence deduced from previously published nucleotide sequence, that gB is made with a cleavable signal sequence of 29 or 30 amino acids, depending on the virus strain. Analysis of gB translated in vitro in the presence and absence of membranes showed that gB is inserted into membranes and glycosylated cotranslationally; a large portion of the gB polypeptide made in vitro is protected from proteolysis by membranes; the large protected fragment carries N-linked carbohydrate and is probably the NH2 terminus based on locations of signals for the addition of N-linked carbohydrate; and the size of the protected fragment is 93 kilodaltons (kDa) for gB made in vitro and associated with dog pancreas membranes, whereas both 93- and 98-kDa protected fragments can be detected for gB made in vivo. These last results are consistent with a previous proposal that gB may traverse the membrane three times.

The antiherpes drug (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVdU) interferes with formation of N-linked and of O-linked oligosaccharides of the herpes simplex virus type 1 glycoprotein C

In HSV-1 infected cell the antiherpes drug (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVdU) exerted at least three different effects on glycosylation of glycoprotein gC. First, an overall decrease of protein glycosylation occurred due to inhibition of synthesis of the lipid-linked oligosaccharides, precursors of N-linked oligosaccharides of gC. Second, an inhibition of processing of N-linked oligosaccharides occurred after the acquisition of endo H-resistance, and possibly due to inhibition of galactose incorporation. Third, a small inhibition of incorporation of glucosamine into O-linked oligosaccharides, and, may be associated with this, a change in the proportion of two different classes of O-linked oligosaccharides of gC, namely those with terminal N-acetylgalactosamine and those with terminal sialic acid.

Sialylated oligosaccharides O-glycosidically linked to glycoprotein C from herpes simplex virus type 1

Glycoprotein C (gC) was purified by immunoabsorbent from herpes simplex virus type-1-infected BHK cells labeled with [14C]glucosamine for 11 h and chased for 3 h. Glycopeptides obtained by pronase digestion of gC were fractionated by Bio-Gel filtration and concanavalin A-Sepharose chromatography. Each glycopeptide fraction was analyzed for amino sugar composition by thin-layer chromatography. The majority of radioactivity was recovered as N-acetylglucosamine, but a significant amount of labeled N-acetylgalactosamine was detected and recovered preferentially in some glycopeptide species. Mild alkaline borohydride treatment of the glycopeptides resulted in the release of small degradation products which contained N-acetylgalactosaminitol as the major labeled component and a drastic reduction of N-acetylgalactosamine in the residual glycopeptides. These results demonstrated that gC carries O-glycosidically linked oligosaccharides in addition to the N-linked di- and triantennary glycans previously described (F. Serafini-Cessi, F. Dall'Olio, L. Pereira, and G. Campadelli-Fiume, J. Virol. 51:838-844, 1984). Chromatographic behavior on DEAE-Sephacel chromatography and neuraminidase digestion of O-linked oligosaccharides indicated the presence of two major sialylated species carrying one and two sialic acid residues, respectively. The characterization of a peculiar glycopeptide species supported the notion that some of the O-linked oligosaccharides are bound to a cluster of hydroxyamino acids located near an N-glycosylation site which carries one N-linked diantennary oligosaccharide.

Intracellular transport of herpes simplex virus gD occurs more rapidly in uninfected cells than in infected cells

A mouse L cell line which expresses the herpex simplex virus type 1 immediate-early polypeptides ICP4 and ICP47 was cotransfected with a cloned copy of the BglII L fragment of herpes simplex virus type 2, which includes the gene for gD, and the plasmid pSV2neo, which contains the aminoglycosyl 3'-phosphotransferase (agpt) gene conferring resistance to the antibiotic G418. A G418-resistant transformed cell line was isolated which expressed herpes simplex virus type 2 gD at higher levels than were found in infected cells. The intracellular transport and processing of gD was compared in transformed and infected cells. In the transformed Z4/6 cells gD was rapidly processed and transported to the cell surface; in contrast, the processing and cell surface appearance of gD in infected parental Z4 cells occurred at a much slower rate, and gD accumulated in nuclear membrane to a greater extent. Thus, the movement of HSV-2 gD to the cell surface in infected cells is retarded as viral glycoproteins accumulate in the nuclear envelope, probably because they interact with other viral structural components.

Processing of virus-specific glycoproteins of varicella zoster virus

Monoclonal antibodies to varicella zoster virus (VZV) glycoproteins were used to study the processing of three glycoproteins with molecular weights of 83K-94K (gp 2), 64K (gp 3), and 55K (gp 5). Immunoprecipitation experiments performed with VZV-infected cells, pulse labeled with [3H]glucosamine in the presence of tunicamycin, suggest that O-linked oligosaccharide is present on the glycoprotein of gp 2. Use of the enzyme endo-beta-N-acetylglucosaminidase H revealed that the fully processed form of gp 3 had high-mannose type and that of gp 5 had only complex type of N-linked oligosaccharides. Experiments with monensin suggest that the precursor form (116K) of gp 3 is cleaved during the processing from Golgi apparatus to cell surface membrane. The extension of O-linked oligosaccharide chain and the complex type of N-linked oligosaccharide chains also occurs during this processing.

Effect of tunicamycin and monensin on biosynthesis, transport, and maturation of bovine herpesvirus type-1 glycoproteins

The effect of tunicamycin and monensin on the biosynthesis, intracellular transport, and maturation of bovine herpesvirus type-1 (BHV-1) glycoproteins was examined. Tunicamycin completely inhibited the production of infectious virus particles and significantly reduced the incorporation of [3H]glucosamine into viral glycoproteins. In the presence of monensin, reduced amounts of infectious virus particles were produced, which was mainly due to inhibition of virus release, rather than virus production. Monensin only slightly inhibited viral glycoprotein synthesis. The effects of these compounds on infectivity indicated that glycosylation is required for the production of infectious virus, though complete processing of the glycoproteins is not essential. In addition, egress of the virions from infected cells probably requires a functional Golgi complex. In the presence of tunicamycin or monensin various degrees of glycosylation of the major glycoproteins occurred, consequently their rates of migration differed from that of the normal glycoproteins. Tunicamycin completely blocked glycosylation of GVP 6/11a/16 and GVP 7. In contrast, GVP 3/9 and GVP 11b were partially glycosylated in the presence of tunicamycin. These results indicated that GVP 6/11a/16 and GVP 7 are N-linked glycoproteins, but GVP 3/9 and GVP 11b contain both N- and O-linked oligosaccharide side chains. Tunicamycin blocked the transport of all viral glycoproteins to the cell surface, suggesting that glycosylation is required for this process. In the presence of monensin, the viral glycoproteins were transported and expressed on the cell surface indicating that transport does not require complete processing of the glycoproteins and may occur via a Golgi-independent pathway. In addition, monensin-treated BHV-1 infected cells could act as target cells in an antibody-dependent cell cytotoxicity assay. Thus, complete glycosylation may not be essential for maintenance of antigenicity and participation in immune destruction.

Different populations of herpes simplex virus glycoprotein C discriminated by the carbohydrate-binding characteristics of N-acetylgalactosamine specific lectins (soybean and Helix pomatia). Brief report

From the herpes simplex virus specified glycoprotein C two fractions were isolated with affinity either for Helix pomatia lectin (HPA) or soybean lectin (SBA). The data indicated that HPA and SBA, despite their mutual main specificity for N-acetylgalactosamine, recognize structurally different gC populations.

Evidence for post-translational glycosylation of a nonglycosylated precursor protein of herpes simplex virus type 1

Incubation of herpes simplex virus type 1-infected Vero and HEp-2 cells at a reduced temperature (34 degrees C) enhanced the detection of the nonglycosylated precursors (pgB97 and pgC75) to the gB and gC glycoproteins in the cytoplasmic and nuclear fractions. Relative to the fully glycosylated and high-mannose forms detected, the nonglycosylated precursors were the predominant components associated with the nuclear fraction of infected cells. Furthermore, addition of protease inhibitors to the fractionation buffers did not affect the distribution or abundance of the nonglycosylated precursors, suggesting that the presence of pgB97 and pgC75 was not the result of proteolysis. When infected Vero or HEp-2 cells were harvested at various times postinfection, the nonglycosylated precursors were detected after the initial appearance of the high mannose components (pgB110 and pgC105). In Vero cells, pgB97 and pgC75 were detected simultaneously at 8 h postinfection, whereas detection was not apparent in HEp-2 cells until 20 h postinfection. Conditions which favored detection of appreciable amounts of nonglycosylated precursors provided an unique approach to probe possible post-translational modifications in the absence of inhibitors of glycosylation. In nuclear fractions isolated from cycloheximide-treated HEp-2 or Vero cells, numerous discrete gC-immunoreactive bands migrating with decreased electrophoretic mobility relative to the nonglycosylated precursor pgC75 were observed. This series of one to four additional bands was eliminated by digestion with endoglycosidase H, and the appearance of these bands was blocked by the addition of tunicamycin. Collectively, the data suggest that high-mannose core oligosaccharides may be added to the nonglycosylated precursor of the gC glycoprotein of herpes simplex virus type 1 in a post-translational fashion.

Varicella-zoster viral glycoproteins analyzed with monoclonal antibodies

Monoclonal antibodies to varicella-zoster virus were used to study viral glycoproteins by immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Based on the viral glycoproteins immunoprecipitated, the five monoclonal antibodies fell into three groups. Two antibodies, 4B7 and 8G9 (group 1), immunoprecipitated a single glycoprotein of molecular weight (MW) 118,000 (118K glycoprotein) and had high neutralizing activity in the absence of complement. One antibody, 3C7 (group 2), which lacked neutralizing activity, immunoprecipitated two glycoproteins of MWs 120,000 and 118,000 and a glycoprotein giving a diffuse band in the region of 64,000 to 65,000. Pulse-chase experiments and experiments with monensin as an inhibitor of glycosylation suggested that the 120K polypeptide was derived by glycosylation of the 118K polypeptide and that a 43K antigen was processed into the 64 to 65K glycoprotein. Two antibodies, 3G8 and 4E6 (group 3), both had neutralizing activity only in the presence of complement, and both immunoprecipitated at least five polypeptides, with MWs ranging from 50,000 to 90,000. Antibody 3G8 was isotype immunoglobulin G2b (IgG2b), and its immunoprecipitating activity was stronger than that of 4E6, which was isotype IgG1. Pulse-chase experiments with antibody 3G8 showed that lower-MW glycopeptides chased into three polypeptides of MWs 90,000, 80,000, and 60,000 by 24 h. Immunoprecipitation experiments with antibody 3G8 on infected cells treated with glycosylation inhibitors 2-deoxyglucose, monensin, and tunicamycin, suggested that a prominent, early-appearing 70K polypeptide may have been processed into the glycoproteins of higher MWs and that the 60K polypeptide may have been derived by glycosylation of polypeptides of lower MWs.

Processing of N-linked oligosaccharides from precursor- to mature-form herpes simplex virus type 1 glycoprotein gC

Immature and mature forms of glycoprotein gC were purified by immunoadsorbent from herpes simplex virus type 1-infected BHK cells labeled with [3H]mannose for a 20-min pulse or for 11 h followed by a 3-h chase. The nature of N-asparagine-linked oligosaccharides carried by the immature form, pgC (molecular weight = 92,000), and the mature gC (molecular weight = 120,000) has been investigated. All pronase-digested glycopeptides of pgC were susceptible to endo-beta-N-acetylglucosaminidase H treatment; thus they have a high-mannose structure. Using thin-layer chromatography to separate endo-beta-N-acetylglucosaminidase H-cleaved oligosaccharides, polymannosyl chains of different sizes, ranging from Man9GlcNAc to Man5GlcNAc, were separated. The major components were Man8GlcNAc and Man7GlcNAc, suggesting that pgC labeled in a 20-min pulse represents the form of glycoprotein already routed to the Golgi apparatus. Analysis of glycopeptides of mature gC showed that the majority (95%) of N-linked glycans were converted to complex-type glycans. Ion-exchange chromatography and affinity chromatography on concanavalin A-Sepharose and leucoagglutinin-agarose revealed that diantennary and triantennary glycans predominated, whereas tetrantennary chains were not present. Parts of the di- and triantennary chains were not fully sialylated. The high heterogeneity of complex-type chains found in mature gC may be related to the high number of N-glycosylation sites of the glycoprotein as predicted by DNA sequencing studies (Frink et al., J. Virol. 45:634-647, 1983).

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.

N-acetylgalactosaminyltransferase activity involved in O-glycosylation of herpes simplex virus type 1 glycoproteins

We report on N-acetylgalactosaminyltransferase (UDPacetylgalactosamine--protein acetylgalactosaminyltransferase; EC 2.4.1.41) activity in herpes simplex virus type 1 (HSV-1)-infected BHK and RicR14 cells, a line of ricin-resistant BHK cells defective in N-acetylglucosaminyltransferase I. The enzyme catalyzed the transfer of [14C]N-acetylgalactosamine (GalNAc) from UDP-[14C]GalNAc into HSV glycoproteins, as identified by immunoprecipitation. The sugar was selectively incorporated into the immature forms of herpesvirus glycoproteins pgC, pgD, and gA-pgB, which are known to contain N-linked glycans of the high-mannose type. The high incorporation of [14C]GalNAc into endogenous acceptors of HSV-1-infected RicR14 cells was consistent with the accumulation of immature forms of HSV glycoproteins which occurs in these cells. Mild alkaline borohydride treatment of glycoproteins labeled via GalNAc transferase showed that the transferred GalNAc was O-linked and represented the first sugar added to the peptide backbone.

Characterization of a herpes simplex virus type 2 75,000-molecular-weight glycoprotein antigenically related to herpes simplex virus type 1 glycoprotein C

Evidence is presented that the herpes simplex virus type 2 glycoprotein previously designated gF is antigenically related to herpes simplex virus type 1 gC (gC-1). An antiserum prepared against type 1 virion envelope proteins immunoprecipitated gF of type 2 (gF-2), and competition experiments revealed that the anti-gC-1 component of the antiserum was responsible for the anti-gF-2 cross-reactivity. An antiserum prepared against fully denatured purified gF-2, however, and three anti-gF-2 monoclonal antibodies failed to precipitate any type 1 antigen, indicating that the extent of cross-reactivity between gC-1 and gF-2 may be limited. Several aspects of gF-2 synthesis and processing were investigated. Use of the enzymes endo-beta-N-acetylglucosaminidase H and alpha-D-N-acetylgalactosaminyl oligosaccharidase revealed that the fully processed form of gF-2 (about 75,000 [75K] apparent molecular weight) had both complex-type N-linked and O-linked oligosaccharides, whereas newly synthesized forms (67K and 69K) had only high-mannose N-linked oligosaccharides. These last two forms were both reduced in size to 54K by treatment with endo-beta-N-acetylglucosaminidase H and therefore appear to differ only in the number of N-linked chains. Neutralization tests and radioiodination experiments revealed that gF-2 is exposed on the surfaces of virions and that the 75K form of gF-2 is exposed on cell surfaces. The similarities and differences of gF-2 and gC-1 are discussed in light of recent mapping results which suggest collinearity of their respective genes.

Inhibition of glycosylation of bovine herpesvirus 1 glycoproteins by the thymidine analog (E)-5-(2 Bromovinyl)-2'-deoxyuridine

(E)-5-(2-Bromovinyl)-2'-deoxyuridine (BVdU) was phosphorylated by the bovine herpesvirus 1 (BHV-1)-induced thymidine kinase and subsequently incorporated into viral DNA, resulting in DNA that was more dense than DNA from untreated cells. Incorporation of the drug did not result in the termination of replicating BHV-1 DNA molecules since radioactively labeled DNA synthesized in drug-treated and untreated cells sedimented at similar rates in alkaline sucrose gradients. No differences were observed in the electrophoretic mobility of [35S]methionine-labeled viral polypeptides synthesized in treated and untreated cells, although [3H]glucosamine-labeled viral glycoproteins synthesized in treated cells were of a lower molecular weight than those in untreated cells. In BVdU-treated cells, unlike untreated cells, immature neutral and basic precursors of the mature viral glycoproteins accumulated. Although BVdU-treated and untreated cells contained similar amounts of virus, very little virus was released into the culture supernatant from BVdU-treated cells. Our results suggest that BVdU partially inhibits the glycosylation of BHV-1 glycoproteins. BVdU-sensitive glycosylation, however, is not necessary for expression of these glycoproteins on the surface of infected cells since the glycoproteins could be labeled on intact cells with 125I and because BVdU-treated cells remained sensitive to antibody-dependent, cell-mediated cytotoxity mediated by anti-BHV-1 serum. The phosphorylation of BVdU was a prerequisite for its effect on glycosylation since the glycoproteins of a thymidine kinase-deficient mutant of BHV-1 were not affected.

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.

Glycosylation of herpes simplex virus type 1 gC in the presence of tunicamycin

The presence of O-glycosidic linkages on herpes simplex virus type 1 (HSV-1) glycoproteins was indicated by the synthesis and glycosylation of HSV-1 glycoproteins in the presence of tunicamycin. Monospecific antiserum to HSV-1 gC immunoprecipitated a 92,000-molecular-weight protein synthesized in the presence of tunicamycin and isotopically labeled with glucosamine or galactose. Anti-gAB did not immunoprecipitate a carbohydrate-labeled HSV-1 protein synthesized in the presence of tunicamycin. The purified glucosamine-labeled 92,000-molecular-weight protein synthesized in the presence of tunicamycin and the fully glycosylated forms of gAB and gC were tested for their sensitivity to mild alkaline hydrolysis. Purified gAB was resistant to mild alkaline hydrolysis, whereas gC and the 92,000-molecular-weight protein were both sensitive to mild alkaline hydrolysis. These results suggest that O-glycosidic linkages are associated with the HSV-1 gC glycoprotein.

O-linked oligosaccharides are acquired by herpes simplex virus glycoproteins in the Golgi apparatus

The O-linked oligosaccharides on mature forms of herpes simplex virus type 1 (HSV1) glycoproteins were characterized, and were found to account largely for the lower electrophoretic mobilities of these forms relative to the mobilities of immature forms. Other posttranslational modifications of HSV1 glycoproteins (designated gB, gC, gD and gE) were related temporally to the discrete shifts in electrophoretic mobilities that signal acquisition of the O-linked oligosaccharides. Fatty acid acylation (principally of gE) could be detected just prior to the shifts, whereas conversion of high-mannose-type N-linked oligosaccharides to the complex type occurred coincident with the shifts. The addition of O-linked oligosaccharides did not occur in cells treated with the ionophore monensin or in a ricin-resistant cell line defective in the processing of N-linked oligosaccharides. We conclude that extension of O-linked oligosaccharide chains on HSV1 glycoproteins, and probably also attachment of the first O-linked sugars, occurs as a late posttranslational modification in the Golgi apparatus.

Populations of herpes simplex virus glycoprotein gC with and without affinity for the N-acetyl-galactosamine specific lectin of Helix pomatia

Two fractions of herpes simplex virus glycoprotein gC were isolated and characterized by means of immunosorbent-purification with monoclonal antibodies against gC and Helix pomatia lectin (HPA) affinity chromatography. About 25 per cent of the glycoprotein gC population demonstrated affinity for the lectin, compatible with presence of N-acetylgalactosamine as terminal sugar of the oligosaccharide. The HPA-binding populations of gC appeared as two electrophoretic bands with lower molecular weights than the non-binding gC. The gC subfraction without affinity for the HPA was subjected to treatments aiming to desialylize the carbohydrate moiety. Only 5 per cent of the initially non-reactive fraction of gC became reactive to HPA after the treatments, suggesting that masking of penultimate N-acetylgalactosamine by sialic acid was not a main reason for lack of HPA affinity. Results of treatment with alkaline Na BH4 demonstrated presence of oligosaccharide-peptide linkages sensitive to beta-elimination suggesting O-glycosidic type of linkage. The subfraction of gC demonstrating affinity for HPA as well as gC devoid of HPA binding capacity both revealed affinity for Con A. Therefore N-glycosidically as well as O-glycosidically linked oligosaccharides seemed to be present on the one and same glycoprotein. On the basis of the results presented we assume that the glycosylation of HSV glycoprotein gC may lead to, at least, two populations of the glycoprotein gC, one with terminal N-acetylgalactosamine residues of oligosaccharides O-glycosidically linked to the polypeptide and the other without affinity for HPA. However, both populations of gC contain similar proportions of oligosaccharides of the high mannose or complex types with N-glycosidic carbohydrate-peptide linkages as indicated by their affinity for Con A.

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.