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

Étude comparée de trois souches du coronavirus de la gastroentérite transmissible: Conditions de la réplicationvirale et de la synthèse des antigènes structuraux

Purdue-115 and D-52 strains of TGEV were compared with the 188-SG strain, which was obtained by means of a survivor selection process in gastric juice of adult pig. The 188-SG strain was characterized by (a) low infectivity, (b) delayed and restricted growth associated with low and delayed RNA synthesis, and (c) a high content of structural antigens. In contrast, Purdue-115 and D-52 strains were characterized by (a) high infectivity, and (b) a normal pattern of virus replication and RNA and structural antigen synthesis. Tunicamycin induced the inhibition of synthesis of El and E2 glycoproteins (detected by the ELISA test using monoclonal and polyclonal antibodies) as well as a significant reduction in the NP protein. The inhibitory effect of tunicamycin was influenced by the cell type and virus strain.

Characterization of caprine herpesvirus 1 glycoprotein D gene and its translation product

Caprine herpesvirus 1 (CpHV-1) is responsible of systemic infection in neonatal kids as well as abortion and fertility disorders in adult goats. This virus is closely related to bovine herpesvirus 1 (BoHV-1) which causes infectious bovine rhinotracheitis. Glycoprotein D (gD) mediates important functions in alphaherpesviruses and is also a main immunogen. The sequence of CpHV-1 gD gene and the biochemical properties of its translation product were analyzed and compared to those of BoHV-1 and other alphaherpesviruses. A relatively high homology was found between CpHV-1 and BoHV-1 glycoproteins D amino acid sequences (similarity of 68.8%). Moreover, six cysteine residues are conserved by CpHV-1 gD and the other studied alphaherpesviruses. CpHV-1 gD has a molecular mass similar to BoHV-1 gD and contains complex N-linked oligosaccharides. In contrast to the BoHV-1 gD, CpHV-1 gD is expressed as a late protein. In spite of the observed differences which could influence its biological functions, CpHV-1 gD shares most characteristics with other alphaherpesviruses and especially BoHV-1.

Herpes simplex virus type 1 capsids transit by the trans-Golgi network, where viral glycoproteins accumulate independently of capsid egress

Egress of herpes capsids from the nucleus to the plasma membrane is a complex multistep transport event that is poorly understood. The current model proposes an initial envelopment at the inner nuclear membrane of capsids newly assembled in the nucleus. The capsids are then released in cytosol by fusion with the outer nuclear membrane. They are finally reenveloped at a downstream organelle before traveling to the plasma membrane for their extracellular release. Although the trans-Golgi network (TGN) is often cited as a potential site of reenvelopment, other organelles have also been proposed, including the Golgi, endoplasmic reticulum-Golgi intermediate compartment, aggresomes, tegusomes, and early or late endosomes. To clarify this important issue, we followed herpes simplex virus type 1 egress by immunofluorescence under conditions that slowed intracellular transport and promoted the accumulation of the otherwise transient reenvelopment intermediate. The data show that the capsids transit by the TGN and point to this compartment as the main reenvelopment site, although a contribution by endosomes cannot formally be excluded. Given that viral glycoproteins are expected to accumulate where capsids acquire their envelope, we examined this prediction and found that all tested could indeed be detected at the TGN. Moreover, this accumulation occurred independently of capsid egress. Surprisingly, capsids were often found immediately adjacent to the viral glycoproteins at the TGN.

Glycoprotein D homologs in herpes simplex virus type 1, pseudorabies virus, and bovine herpes virus type 1 bind directly to human HveC(nectin-1) with different affinities

Distinct subsets of human receptors for alphaherpesviruses mediate the entry of herpes simplex virus (HSV), pseudorabies virus (PrV), or bovine herpes virus type 1 (BHV-1) into cells. Glycoprotein D (gD) is essential for receptor-mediated entry of all three viruses into cells. However, the gD homologs of these viruses share only 22-33% amino acid identity. Several entry receptors for HSV have been identified. Two of these, HveA (HVEM) and HveC (nectin-1), mediate entry of most HSV-1 and HSV-2 strains and are bound directly by HSV gD. A third receptor, HveB (nectin-2), mediates entry of HSV-2 and only a limited number of HSV-1 strains. HveB and HveC can also serve as entry receptors for PrV, whereas only HveC can serve this function for BHV-1. We show here that gD from PrV and BHV-1 binds directly to the human receptors that mediate PrV and BHV-1 entry. We expressed soluble forms of PrV gD and BHV-1 gD using recombinant baculoviruses and purified each protein. Using ELISA, we detected direct binding of PrV gD to HveB and HveC and direct binding of BHV-1 gD to HveC. Biosensor analysis revealed that PrV gD had a 10-fold higher affinity than HSV-1 gD for human HveC. In contrast, the binding of BHV-1 gD to HveC was weak. PrV gD and HSV-1 gD competed for binding to the V domain of HveC and both inhibited entry of the homologous and heterologous viruses. These data suggest that the two forms of gD bind to a common region on human HveC despite their low amino acid similarity. Based on affinities for human HveC, we predict a porcine HveC homolog may be important for PrV infection in its natural host, whereas a BHV-1 infection in its natural host may be mediated by a receptor other than a bovine HveC homolog.

Identification and characterization of bovine herpesvirus type 5 glycoprotein H gene and gene products

Bovine herpesvirus type 5 (BHV-5) is the causative agent of a fatal meningo-encephalitis in calves and is closely related to BHV-1 which causes infectious bovine rhinotracheitis. The gene encoding BHV-5 glycoprotein gH was sequenced. A high degree of conservation was found between BHV-1 and BHV-5 deduced gH amino acid sequences (86. 4%), which is also observed for all alphaherpesvirus gH sequences. Transcriptional analysis revealed a 3.1 kb mRNA as the specific gH transcript which was detected 2 h post-infection (p.i.). Twelve out of twenty-one MAbs directed against BHV-1 gH immunoprecipitated a 108-110 kDa glycoprotein, which was then designated BHV-5 gH. Synthesis and intracellular processing of BHV- 5 gH was analysed in infected MDBK cells using gH cross-reacting MAbs. Glycoprotein gH was expressed as a beta-gamma protein, detected by radioimmunoprecipitation as early as 3 h p.i. Glycosylation studies indicated that BHV-5 gH contains N-linked carbohydrates which are essential for the recognition of the protein by the MAbs. This suggests that N-linked glycans are involved in protein folding or are targets for the gH cross-reacting MAbs. Plaque- reduction neutralization assays showed that at least one BHV-1 gH antigenic domain is lacking in BHV-5 which may possibly relate to in vivo differences in virus tropism.

Structural and functional analysis of bovine herpesvirus 1 minor glycoproteins

This paper focuses on the structure and functions of bovine herpesvirus 1 minor glycoproteins gH, gE, gG and gp42. It reviews the progress which has been made in their identification and characterization, in the study of their temporal expression and processing in infected cells, and finally in the understanding of their biological activities. In addition, aspects discussed include a comparison with two other alphaherpesviruses, namely herpes simplex virus and pseudorabies virus.

Synthesis and processing of bovine herpesvirus-1 glycoprotein H

The translation product of the bovine herpesvirus-1 (BHV-1) gH gene was identified and characterized. Synthetic peptides were used to generate specific antisera and a glycoprotein of 108K was precipitated by one of the antisera. Cross-immunoprecipitations with monoclonal antibodies to BHV-1 glycoprotein gp108 and the anti-gH peptide antiserum demonstrated that gp108 is the translation product of the gH open reading frame. Glycoprotein gH synthesis and intracellular processing was analyzed in infected Madin-Darby bovine kidney cells using anti-gp 108 monoclonal antibodies. Glycoprotein gH is expressed as a beta-gamma protein and could be detected by radioimmunoprecipitation as early as 2 hr postinfection. Cotranslational N-glycosylation of gH is essential for the recognition by monoclonal antibodies, suggesting that N-linked glycans are involved in protein folding or that they are targets for most of monoclonal antibodies used in this study.

Identification of 108K, 93K, and 42K glycoproteins of bovine herpesvirus-1 by monoclonal antibodies

Three glycoproteins of bovine herpesvirus-1 (BHV-1) other than glycoproteins gI, gIII, and gIV were identified by monoclonal antibody (MAb) analyses. Monoclonal antibodies were obtained by immunization of mice with either BHV-1 envelope or virus infected cells, from which the glycoproteins gI, gIII, and gIV were removed by immunoaffinity. In the latter immunization procedure mice were tolerized either against normal cellular antigens with or without glycoproteins gI, gIII, gIV, and nucleocapsid. From 154 anti-BHV-1 hybridomas isolated, 39 MAbs precipitated a 108K glycoprotein. Two other glycoproteins of respectively 42K and 93K were precipitated each by one MAb. These three glycoproteins were detected in infected cell lysate. Nine anti-108K glycoprotein MAbs neutralized BHV-1 infectivity and three non-neutralizing MAbs were able to reduce plaque development when virus was grown in the presence of these MAbs. It is therefore suggested that this glycoprotein is involved in viral entry into the cell and in cell-to-cell spread of the virus.

Role of N-linked glycans in antigenicity, processing, and cell surface expression of bovine herpesvirus 1 glycoprotein gIV

Glycoprotein gIV, a structural component of bovine herpesvirus type 1, stimulates high titers of virus-neutralizing antibody. The protein contains three potential sites for the addition of N-linked carbohydrates. Three mutants were constructed by oligonucleotide-directed mutagenesis, in each case changing one N-linked glycosylation site from Asn-X-Thr/Ser to Ser-X-Thr/Ser. A fourth mutant was altered at two sites. The altered forms of the gIV gene were cloned into a vaccinia virus transfer vector to generate recombinant vaccinia viruses expressing mutant proteins. Analysis of these mutants revealed that only two (residues 41 and 102) of the three (residues 41, 102, and 411) potential sites for the addition of N-linked glycans are actually utilized. Absence of glycans at residue 41 (gN1) showed no significant effect on the conformation of the protein or induction of a serum neutralizing antibody response. However, mutant proteins lacking glycans at residue 102 (gN2) or residues 41 and 102 (gN1N2) showed altered reactivity with conformation-dependent gIV-specific monoclonal antibodies. These mutants also induced significantly lower serum neutralizing antibody responses than wild-type gIV. Nonetheless, each of the mutant proteins were modified by the addition of O-glycans and transported to the cell surface. Our results demonstrate that absence of N-linked glycans at one (residue 102) or both (residues 41 and 102) utilized N-linked glycosylation sites alters the conformation but does not prevent processing and transport of gIV to the cell surface.

The N-terminal 22 amino acids encoded by the gene specifying the major secreted protein of vaccinia virus, strain Lister, can function as a signal sequence to direct the export of a foreign protein

Cells infected with vaccinia virus strain Lister secrete a polypeptide of approximate molecular weight 35,000 (35K) into the medium. Previous studies identified a cleavable, hydrophobic region of 17 amino acids in the 35K protein which could potentially function as a signal peptide to target the protein to the secretory pathway. Here we report the use of the expression-secretion signals derived from the 35K gene to direct export and secretion of a foreign protein. Vaccinia virus recombinants carrying the bacterial chloramphenicol acetyl transferase gene (cat) immediately downstream from the promoter and the N-terminal coding sequences of the 35K gene were constructed. Our studies show that the N-terminal 22 or 42 amino acids of the 35K protein direct efficient secretion of the CAT protein. However, due to a cryptic glycosylation site within CAT, glycosylated protein was secreted, which reduced enzymatic activity. Activity was restored in the presence of tunicamycin. Removal of the glycosylation site by site-directed mutagenesis abolished glycosylation with no effect on secretion, although CAT activity was again reduced, possibly due to an effect on the active site. The results presented here demonstrate the feasibility of using the promoter and the signal sequence of the 35K gene to generate recombinant viruses for overexpression and secretion of foreign proteins.

Post-translational processing of the glycoproteins of lymphocytic choriomeningitis virus

Intracellular events in the synthesis, glycosylation, and transport of the lymphocytic choriomeningitis virus (LCMV) glycoproteins have been examined. We have shown by N-glycanase digestion that LCMV strain Arm-4 bears five oligosaccharides on GP-1 and two on GP-2. By pulse-chase labeling experiments in the presence of drugs which inhibit N-linked oligosaccharide addition and processing we demonstrate that addition of high mannose precursor oligosaccharides is necessary for transport and cleavage of the viral GP-C glycoprotein. Moreover, in the presence of tunicamycin which inhibits en bloc addition of these mannose-rich side chains, virus budding was substantially decreased and infectious virions were reduced by more than 1000-fold in the supernatant medium. Incubation in the presence of castantospermine, which permits addition of oligomannosyl-rich chains but blocks further processing, restored transport and cleavage of GP-C and maturation of virions. Finally, by temperature block experiments we have determined that maturation of GP-C oligosaccharides to an endoglycosidase H resistant form precedes cleavage to GP-1 and GP-2. The latter process is most likely to occur in the Golgi or post-Golgi compartment.

Alteration of intracellular traffic by monensin; mechanism, specificity and relationship to toxicity

Monensin, a monovalent ion-selective ionophore, facilitates the transmembrane exchange of principally sodium ions for protons. The outer surface of the ionophore-ion complex is composed largely of nonpolar hydrocarbon, which imparts a high solubility to the complexes in nonpolar solvents. In biological systems, these complexes are freely soluble in the lipid components of membranes and, presumably, diffuse or shuttle through the membranes from one aqueous membrane interface to the other. The net effect for monensin is a trans-membrane exchange of sodium ions for protons. However, the interaction of an ionophore with biological membranes, and its ionophoric expression, is highly dependent on the biochemical configuration of the membrane itself. One apparent consequence of this exchange is the neutralization of acidic intracellular compartments such as the trans Golgi apparatus cisternae and associated elements, lysosomes, and certain endosomes. This is accompanied by a disruption of trans Golgi apparatus cisternae and of lysosome and acidic endosome function. At the same time, Golgi apparatus cisternae appear to swell, presumably due to osmotic uptake of water resulting from the inward movement of ions. Monensin effects on Golgi apparatus are observed in cells from a wide range of plant and animal species. The action of monensin is most often exerted on the trans half of the stacked cisternae, often near the point of exit of secretory vesicles at the trans face of the stacked cisternae, or, especially at low monensin concentrations or short exposure times, near the middle of the stacked cisternae. The effects of monensin are quite rapid in both animal and plant cells; i.e., changes in Golgi apparatus may be observed after only 2-5 min of exposure. It is implicit in these observations that the uptake of osmotically active cations is accompanied by a concomitant efflux of H+ and that a net influx of protons would be required to sustain the ionic exchange long enough to account for the swelling of cisternae observed in electron micrographs. In the Golgi apparatus, late processing events such as terminal glycosylation and proteolytic cleavages are most susceptible to inhibition by monensin. Yet, many incompletely processed molecules may still be secreted via yet poorly understood mechanisms that appear to bypass the Golgi apparatus. In endocytosis, monensin does not prevent internalization. However, intracellular degradation of internalized ligands may be prevented.(ABSTRACT TRUNCATED AT 400 WORDS)

Gene mapping of infectious bovine rhinotracheitis viral DNA genome

A bovine herpesvirus I (BHV-I) HindIII genomic bank spanning 89% of the entire genome was constructed and individual fragments analyzed for their capacity to select specific mRNAs which were then expressed by in vitro translation assays. This procedure allowed the mapping of more than 20 viral polypeptides to discrete regions of the DNA genome. Some polypeptides map in neighboring HindIII fragments while most seem encoded in single fragments. In particular, the coding sequences for an abundant 94 kDa polypeptide, which is the potential unglycosylated precursor of gII glycoprotein, have been assigned to the small 3.6 kbp HindIII genomic fragment M. The localization of structural and non-structural gene-coding sequences will help to characterize viral polypeptides and eventually, a better understanding of BHV-I infection will be gained.

Neutralizing epitopes of lymphocytic choriomeningitis virus are conformational and require both glycosylation and disulfide bonds for expression

Lymphocytic choriomeningitis virus (Armstrong strain) bears two overlapping epitopes, GP-1A (A) and GP-1D (D), recognized by neutralizing antibodies on the major surface glycoprotein GP-1. Both are discontinuous conformational epitopes that require prior formation of disulfide bridges and addition of N-linked oligosaccharides. Using monoclonal antibodies specific for each of these epitopes, as well as for conformation-independent epitopes, we have investigated the requirements for biosynthesis and folding of the epitopes. The carbohydrate residues themselves do not appear to comprise critical informational components of these epitopes, but are required for proper folding of the nascent glycopeptide chain within the rough endoplasmic reticulum. These epitopes differ in their resistance to denaturation; epitope D is retained when denatured with SDS under nonreducing conditions, whereas epitope A is lost. Monoclonal antibodies to epitope A cross-react with several strains of LCMV. However, epitope D is detected in only a subset of isolates derived from the Armstrong strain of LCMV. By RNA sequence analysis, we have mapped a single amino acid change distinguishing those virions containing epitope D. Acquisition of binding activity of the epitope D-specific monoclonal correlates with a Thr----Ala or Thr----Lys mutation at amino acid 173 of the GP-1 molecule and concomitant disruption of a consensus N-linked glycosylation site.

Synthesis, cellular location, and immunogenicity of bovine herpesvirus 1 glycoproteins gI and gIII expressed by recombinant vaccinia virus

Two of the major glycoproteins of bovine herpesvirus 1 (BHV-1) are gI, a polypeptide complex with apparent molecular weights of 130,000, 74,000, and 55,000, and gIII (a 91,000-molecular-weight [91K] glycoprotein), which also exists as a 180K dimer. Vaccinia virus (VAC) recombinants were constructed which carry full-length gI (VAC-I) or gIII (VAC-III) genes. The genes for gI and gIII were each placed under the control of the early VAC 7.5K gene promoter and inserted within the VAC gene for thymidine kinase. The recombinant viruses VAC-I and VAC-III retained infectivity and expressed both precursor and mature forms of glycoproteins gI and gIII. The polypeptide backbones, partially glycosylated precursors, and mature gI and gIII glycoproteins were indistinguishable from those produced in BHV-1-infected cells. Consequently, they were apparently cleaved, glycosylated, and transported in a manner similar to that seen during authentic BHV-1 infection, although the processing efficiencies of both gI and gIII were generally higher in recombinant-infected cells than in BHV-1-infected cells. Immunofluorescence studies further demonstrated that the mature gI and gIII glycoproteins were transported to and expressed on the surface of cells infected with the respective recombinants. Immunization of cattle with recombinant viruses VAC-I and VAC-III resulted in the induction of neutralizing antibodies to BHV-1, which were reactive with authentic gI and gIII. These data demonstrate the immunogenicity of VAC-expressed gI and gIII and indicate the potential of these recombinant glycoproteins as a vaccine against BHV-1.

Bovine naturally cytolytic cell activation against bovine herpes virus type 1-infected cells does not require late viral glycoproteins

Freshly isolated or overnight cultured bovine peripheral blood mononuclear (PBM) cells lysed bovine herpes virus 1 (BHV-1)-infected allogeneic and xenogeneic target cells but not non-infected target cells. To determine if late viral genes contribute to target cell lysis, phosphonoacetic acid (PAA), an inhibitor of DNA polymerase activity, was used to block DNA replication that is required for expression of late viral proteins. Both adherent and non-adherent (NA) cell populations mediated lysis against PAA-treated BHV-1-infected target cells in both 4- and 20-hr assays, indicating recognition and killing occurred in the absence of expression of late BHV-1 glycoproteins. Thus recognition of BHV-1 by bovine natural cytolytic effector cells does not require recognition of late BHV-1 glycoproteins for killing virally infected target cells.

Processing of the gp55-116 envelope glycoprotein complex (gB) of human cytomegalovirus

The processing pathway of the major envelope glycoprotein complex, gp55-116 (gB), of human cytomegalovirus was studied using inhibitors of glycosylation and endoglycosidases. The results of these studies indicated that the mature gp55-116 is synthesized by the addition of both simple and complex N-linked sugars to a nonglycosylated precursor of estimated Mr 105,000. In a rapid processing step, the Mr 105,000 precursor is glycosylated to a protein of Mr 150,000 (gp150) which contains only endoglycosidase H-sensitive sugar linkages. The gp150 is then processed relatively slowly to a Mr 165,000 to 170,000 species (gp165-170), which is then cleaved to yield the mature gp55-116. Monensin prevented the final processing steps of the gp150, including cleavage, suggesting that transport through the Golgi apparatus is required for complete processing. Digestion of the intracellular forms of this complex as well as the virion forms confirmed the above findings and indicated that the mature virion form of gp55 contains 8,000 daltons of N-linked sugars. The virion gp116 contains some 52,000 to 57,000 daltons of N-linked carbohydrates and approximately 5,000 daltons of O-linked sugars.

Effects of tunicamycin and monensin on the distribution of highly phosphorylated proteins in cells infected with herpes simplex virus type 1

New aspects of the distribution of highly phosphorylated proteins in cells infected with herpes simplex virus type 1 (HSV-1) were investigated at the ultrastructural level by the use of drugs which inhibit the glycosylation of viral proteins. The highly phosphorylated proteins were localized by the bismuth tartrate procedure applied on sections of glutaraldehyde-fixed cells embedded in Lowicryl. The drugs employed were tunicamycin, which alters the glycosylation activity of the rough endoplasmic reticulum (RER), and monensin, which blocks the migration of vesicles of the Golgi apparatus (GA) thereby impairing the glycosylation function of the GA. Tunicamycin induced proliferation of RER and the accumulation of highly phosphorylated proteins on its membranes and also impaired GA vesicle maturation and inhibited the usual accumulation of phosphorylated proteins within them. Monensin induced proliferation of the nuclear envelope, including both outer and inner membranes, with bismuth bound to staggered segments of the latter, and also affected the GA in that bismuth-binding proteins were accumulated on the external surface of the swollen vesicles instead of the lumen. These data suggest that an injury of one membrane system, RER or GA, engenders consequential effects on the other. This also supports evidence for an interrelationship between post-translational glycosylation and phosphorylation of proteins in HSV infection.

Expression of bovine herpesvirus 1 glycoproteins gI and gIII in transfected murine cells

Genes encoding two of the major glycoproteins of bovine herpesvirus 1 (BHV-1), gI and gIII, were cloned into the eucaryotic expression vectors pRSVcat and pSV2neo and transfected into murine LMTK- cells, and cloned cell lines were established. The relative amounts of gI or gIII expressed from the two vectors were similar. Expression of gI was cell associated and localized predominantly in the perinuclear region, but nuclear and plasma membrane staining was also observed. Expression of gI was additionally associated with cell fusion and the formation of polykaryons and giant cells. Expression of gIII was localized predominantly in the nuclear and plasma membranes. Radioimmunoprecipitation in the presence or absence of tunicamycin revealed that the recombinant glycoproteins were proteolytically processed and glycosylated and had molecular weights similar to those of the forms of gI and gIII expressed in BHV-1-infected bovine cells. However, both recombinant glycoproteins were glycosylated to a lesser extent than were the forms found in BHV-1-infected bovine cells. For gI, a deficiency in N-linked glycosylation of the amino-terminal half of the protein was identified; for gIII, a deficiency in O-linked glycosylation was implicated. The reactivity pattern of a panel of gI- and gIII-specific monoclonal antibodies, including six which recognize conformation-dependent epitopes, was found to be unaffected by the glycosylation differences and was identical for transfected or BHV-1-infected murine cells. Use of the transfected cells as targets in immune-mediated cytotoxicity assays demonstrated the functional recognition of recombinant gI and gIII by murine antibody and cytotoxic T lymphocytes. Immunization of mice with the transfected cells elicited BHV-1-specific virus-neutralizing antibody, thus verifying the antigenic authenticity of the recombinant glycoproteins and the important role of gI and gIII as targets of the immune response to BHV-1 in this murine model system.

Visualization of glycoproteins after tunicamycin and monensin treatment of herpes simplex virus infected cells

The effects of tunicamycin and monensin on the morphogenesis of herpes simplex virus type 1 and on the ultrastructure and function of host cell membranes was investigated by conventional technics of electron microscopy and cytochemical localization of glycoproteins with thiocarbohydrazide-SO2. Infected RS 537 rabbit fibroblasts were treated with tunicamycin, which inhibits the glycosylation of many glycoproteins, or monensin, which inhibits the transport of proteins to the cell surface, and were compared with untreated infected cells. Tunicamycin treatment almost entirely suppresses the perinuclear envelopment of viral capsids, induces the nuclear export of unusually numerous naked viral capsids, and prevents the proliferation of the Golgi apparatus. On the other hand, perinuclear envelopment of viral capsids still occurs following a monensin treatment; however, enveloped viral capsids are not released into the extracellular space; in addition this treatment induces the proliferation of the rough endoplasmic reticulum (RER). The number of structures stained for glycoproteins in tunicamycin-treated cells is markedly lower than that in nontreated infected cells, whereas an unusual additional staining of the entire outer nuclear membrane and of the RER occurs following monensin treatment.

Comparison of the bovine herpesvirus 1 gI gene and the herpes simplex virus type 1 gB gene

In a previous report, we localized the gene for a 130-kilodalton envelope glycoprotein (gI) of bovine herpesvirus 1 (BHV-1) to a 3.6-kilobase HpaI-KpnI restriction endonuclease fragment from the long unique region of the BHV-1 genome (map position 0.405 to 0.432) and showed that a herpes simplex virus 1 (HSV-1) glycoprotein B (gB) probe uniquely hybridized to this BHV-1 restriction fragment. Here we present the complete nucleotide sequence of the BHV-1 gI gene and the predicted 932-amino-acid sequence of the gI primary translation product. Comparison with the published nucleotide sequence of the HSV-1 (KOS) gB gene (D. J. Bzik, B. A. Fox, N. A. DeLuca, and S. Person, Virology 133:301-314, 1984) reveals a similarity of 56.3% at the nucleotide level and 45.9% at the amino acid level. Upstream of the proposed gI coding region are potential mRNA transcriptional promoter elements including a TATA box and multiple Sp1 binding sites (GC boxes). Downstream of the gI coding region are two sequence elements associated with mRNA cleavage and polyadenylation (AATAAA and a GT-rich region roughly 30 nucleotides further downstream). Like HSV-1 gB, the predicted gI amino acid sequence exhibits two broad hydrophobic regions likely to represent a transient amino-terminal signal sequence and a transmembrane anchor domain (near the carboxyl terminus). Additional features shared with gB include 6 potential N-linked glycosylation sites and 10 highly conserved cysteine residues in the gI extracellular domain. Two regions of nonsimilarity between gI and gB are a centrally located 22-amino-acid region of gI for which there is essentially no gB counterpart and the transient amino-terminal leaders which differ in both size and sequence. The hydrophobic signal sequence of the gI leader, unlike that of gB, is preceded by an unusually large region of predominantly hydrophilic amino acids. The unusual length of the gI leader may result from an overlap between that portion of the gI coding region and a potential upstream coding region.

Mode of action of a new type of UDP-glucose analog against herpesvirus replication

The mode of action of a new type of UDP-glucose analog against herpes simplex virus type 1 (HSV-1) replication was examined. The analog showed good selectivity and potent activity. At 10 micrograms/ml, P-536 inhibited the formation of infectious HSV-1 by more than 90%, whereas at 100 micrograms/ml it had no cytotoxic effects, as evidenced by phase-contrast microscopy. P-536 showed a wide spectrum of action and was active against HSV-1, adenovirus type 5, vaccinia virus, poliovirus type 1, encephalomyocarditis virus, vesicular stomatitis virus, influenza virus, and measles virus, irrespective of whether these viruses have lipidic envelopes or not. P-536 clearly inhibited protein glycosylation if added at the time when late viral proteins were being synthesized. Moreover, it also interfered with the synthesis of nucleic acids and the phosphorylation of nucleosides. If P-536 was present from the beginning of infection, HSV-1 replication was blocked at an early step and the infected cells continued to synthesize cellular proteins for long periods.

Monoclonal antibody analysis of bovine herpesvirus-1 glycoprotein antigenic areas relevant to natural infection

Neutralizing antigenic areas on the glycoproteins of bovine herpesvirus-1 (BHV-1) were identified by reciprocal competition radioimmunoassays using monoclonal antibodies. Three interrelated and two independent antigenic areas were identified on the 77-kDa (K) gIV envelope glycoprotein. Antigenic analysis of this protein has not been previously described. Four interrelated and one independent antigenic areas were found on the 97K gIII envelope glycoprotein. A third group of monoclonal antibodies reacting in Western blot with the 74K subunit of gI, a 130K disulfide-linked 74K/55K heterodimer, revealed four interrelated antigenic areas. All of the antigenic areas on all three glycoproteins were reactive with neutralizing monoclonal antibodies and all were targets for antibody-complement lysis. However, antibodies against gIV were the most efficient at neutralizing the virus and rendering infected cells susceptible to antibody-complement lysis. Convalescent sera from experimentally infected calves were used in a competitive radioimmunoassay to confirm that each antigenic area on the gI, gIII, or gIV glycoproteins was a target for bovine antibodies during primary infection with BHV-1.

Secretion of Escherichia coli chloramphenicol acetyltransferase by mammalian cells

We show here that expression of the Escherichia coli cat gene in mammalian cells results in accumulation of enzymatically active CAT in the culture media as well as in the cytoplasm. We call the extracellular product secreted CAT (sCAT). Three to four days after introduction of cat-expressing plasmids into mouse L cells by transient transfection, total extracellular sCAT activity exceeds total cytoplasmic CAT activity. As sCAT levels increase, substantially more CAT is found outside the cells than inside at later times. Comparison of different populations of cat-expressing cells shows that, at any given time, the level of sCAT is proportional to the level of intracellular CAT. Thus, assay of sCAT provides a convenient, non-invasive alternative to assay of intracellular CAT. The molecular sizes of sCAT and intracellular CAT are indistinguishable, suggesting that the protein is not cleaved or glycosylated during secretion. Several observations, including a lack of sensitivity to drugs which inhibit Golgi activity, suggest that CAT may be secreted via an unusual pathway.

Characterization of two different human cytomegalovirus glycoproteins which are targets for virus neutralizing antibody

In previous studies we have identified two viral polypeptides detected by murine monoclonal antibodies which neutralize the infectivity of human cytomegalovirus (CMV) AD169. One is an 86,000-Da polypeptide (p86) and the second is a complex of two major coimmunoprecipitating polypeptides of 130,000 and 55,000 Da (p130/55). In this study we have shown that the two viral polypeptides are immunologically unrelated and have distinct peptide cleavage patterns. We have characterized these polypeptides as glycoproteins and studied their biosynthesis in human embryonic lung cells. The oligosaccharides found on both the p86 and the p130/55 were characterized by endoglycosidase digestion as N-linked high-mannose carbohydrates. Inhibitors of glycosylation were used to further characterize the oligosaccharides. Tunicamycin, which inhibits the biosynthesis of N-linked oligosaccharides on the endoplasmic reticulum, inhibited both the infectivity and biosynthesis of the p86 and p130/55. The underglycosylated forms in tunicamycin-treated cultures could be detected only under conditions of pulse-labeling with L-[35S]methionine. Monensin, which inhibits the modification of glycoproteins from simple to complex forms in the Golgi, reduced viral infectivity at concentrations which had no effect on viral protein synthesis, but did not alter the apparent molecular weight of either the p86 or the p130/55. The oligosaccharides were critical for the in vitro immunologic reactivity of the p86 in immunoblots. However, endoglycosidase F-treated p86 was comparable to the native form in inducing virus neutralizing antibody in guinea pigs. Endoglycosidase F-treated p130/55 retained its ability to bind antibody in Western blots.

Release of a virus coded glycoprotein from herpes simplex virus type 1 infected cells

HEp-2 cells, which were infected with HSV-1, excrete besides other proteins a soluble glycoprotein (Mr 125,000-130,000) related to the virus protein gC. The excretion of the glycoprotein and the production of extracellular virus particles is reduced to a similar extent when the cells were treated with monensin. Possible consequences of the excretion of soluble viral proteins to a modulation of the immune response are discussed.

Temporal control of bovine herpesvirus 1 glycoprotein synthesis

The gI, gIII, and gIV glycoproteins are major bovine herpesvirus 1 antigens involved in virus neutralization. Results indicate that the gI and gIV glycoproteins were expressed as beta proteins, whereas the gIII glycoprotein was expressed strictly as a gamma protein. These findings suggest that gI and gIV may be superior to gIII as vaccine candidates.

Intracellular localization and transport of three different bovine herpesvirus type 1 glycoproteins involved in neutralization

Monoclonal antibodies against 3 different glycoproteins of bovine herpesvirus type 1 (BHV-1) involved in virus neutralization were used in indirect immunofluorescence (IIF) tests to characterize the appearance and transport to the plasma membrane of virus antigens in the infected cells. Antibodies against gp 117 and gp 71 glycoproteins first showed pronounced ring-like nuclear fluorescence at 4 hours post-infection (PI), followed by staining of the perinuclear region, presumably the Golgi apparatus. In contrast, antibody against gp 87 produced staining in cell-to-cell junctional areas at 3 hours PI before any staining close to the nucleus. The expression of the 3 glycoproteins at the surface of the infected cells was confirmed by the use of monoclonal antibodies having neutralizing activity, but not by non-neutralizing antibodies against gp 117 and gp 71. Non-neutralizing antibody against gp 87 detected the surface fluorescence only in those cells showing marked degeneration. Inhibition of glycosylation of the viral glycoproteins with tunicamycin (TM) was followed by interference with transport of gp 117 and gp 87 to the plasma membrane. On the other hand, gp 71 was incorporated into the plasma membrane despite the lack of N-linked glycosylation.

Inhibition by monensin of human cytomegalovirus DNA replication

Monensin, at concentrations which depended on the multiplicity of infection, was found to prevent DNA replication of human cytomegalovirus (HCMV) as well as production of viral progeny in human foreskin fibroblasts. The drug did not affect DNA replication of herpes simplex virus. Inhibition of consecutive HCMV DNA synthesis was also observed following delayed addition of the drug within 12-24 hours postinfection, but was fully reversible upon its removal. Viral replication proceeded, however, without impairment in cultures treated with monensin prior to infection. Induction of viral DNA polymerase activity was not impeded by the inhibitor. Analysis of protein- and glycoprotein synthesis revealed that monensin interfered with the production of a number of HCMV-specific polypeptides. Furthermore, evidence was obtained that the drug may hinder intracellular transport of a 135 kd glycopolypeptide.

Synthesis and processing of bovine herpesvirus 1 glycoproteins

Four unique glycoproteins or glycoprotein complexes were recognized by a panel of monoclonal antibodies to bovine herpesvirus 1 (BHV-1), i.e., GVP 6/11a/16 (130,000-molecular-weight glycoprotein [130K glycoprotein]/74K/55K), GVP 7 (108K), GVP 3/9 (180K/91K), and GVP 11b (71K). The absence of any antigenic or structural relationship between GVP 11a and GVP 11b, which were previously identified as one glycoprotein, GVP 11, demonstrated that these two GVP 11 species are unique glycoproteins. GVP 3 and GVP 9 showed complete sequence homology, as shown by the identity of their antigenic determinants and by partial peptide mapping. This observation, as well as the ratio of their apparent molecular weights, indicated that GVP 3 (180K) is a dimeric form of GVP 9 (91K). GVP 6 and GVP 11a, as well as GVP 6 and GVP 16, showed at least partial sequence homology, since they shared several antigenic determinants and peptides. In addition, GVP 6, GVP 11a, and GVP 16 were derived from one primary precursor. These results, as well as the ratio of their apparent molecular weights, indicated that the GVP 6/11a/16 complex consists of two forms: one in which GVP 6 (130K) is uncleaved and the other one in which GVP 6 is cleaved and composed of GVP 11a (74K) and GVP 16 (55K), linked by disulfide bridges. An antigenically distinct precursor to each of the four BHV-1 glycoproteins or glycoprotein complexes was identified by monoclonal antibodies. These precursors, pGVP 6 (117K), pGVP 11a (62K), pGVP 7 (100K), pGVP 9 (69K), and pGVP 11b (63K) were sensitive to endo-beta-N-acetylglucosaminidase H treatment, indicating that they represent the partially glycosylated high-mannose-type intermediate forms generated by cotranslational glycosylation of the primary, unglycosylated precursors to GVP 6/11a/16, GVP 7, GVP 3/9, and GVP 11b, which were identified as having apparent molecular weights of 105,000, 90,000, 61,000, and 58,000, respectively. A new nomenclature for the BHV-1 glycoproteins, based on roman numerals, is proposed.

Polypeptide specificity of the antibody response after primary and recurrent infection with bovine herpesvirus 1

The polypeptide specificities and defense mechanisms of the humoral immune response to bovine herpesvirus 1 were analyzed. Sequential serum samples taken from cows which were experimentally infected with bovine herpesvirus 1 were tested for their reactivity with individual bovine herpesvirus 1 polypeptides by immunoprecipitation, immunoblotting, and enzyme-linked immunosorbent assay. All bovine immune sera reacted with each of the three major bovine herpesvirus 1 glycoproteins, GVP 6/11a/16, GVP 3/9, and GVP 11b, during primary as well as recurrent infection. Among these glycoproteins, GVP 6/11a/16 induced the earliest and most consistent immune response. The levels of antibody to GVP 3/9 and GVP 11b varied among the animals, and they were slightly lower than the level of antibody to GVP 6/11a/16. Antibodies to several nonglycosylated polypeptides and two additional glycoproteins were also detected with the immunoblot assay. However, these antibodies were usually apparent only during recurrent infection, whereas they were undetectable or low during primary infection. The antibodies in the sera from all animals mediated virus neutralization and destruction of virus-infected cells by two immune mechanisms, e.g., antibody- and complement-mediated lysis and antibody-dependent cell-mediated cytotoxicity.

Antigenic and immunogenic characteristics of bovine herpesvirus type-1 glycoproteins GVP 3/9 and GVP 6/11a/16, purified by immunoadsorbent chromatography

Glycoproteins GVP 3/9 and GVP 6/11a/16, two of the major glycosylated proteins specified by bovine herpesvirus type-1 (BHV-1), were purified on immunoadsorbents consisting of the appropriate monoclonal antibodies linked to Affigel-10. Each glycoprotein, whether purified from virus-infected cells or from virus, retained antigenic activity and induced high titers of monospecific antibodies in rabbits. These antibodies could neutralize virus and mediate complement-dependent lysis of virus-infected cells. Denatured glycoproteins GVP 3, GVP 6, GVP 11a, and GVP 16, which were purified by immunoadsorbent chromatography, followed by preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis, also retained antigenicity and immunogenicity, though to a lesser extent than the native glycoproteins. Antibodies induced by GVP 9, GVP 6, and GVP 11a could also neutralize and mediate immune lysis. Even though GVP 16 induced high levels of antibody, these antibodies could not neutralize virus or participate in antibody and complement-mediated cytolysis. These results may suggest that the orientation of GVP 6/11a/16 in the membrane is such that GVP 11a is better exposed on the virion envelope and the cell surface than GVP 16. Cross-reactivity between monospecific antibodies against GVP 3 and GVP 9, as well as GVP 6, GVP 11a, and GVP 16 supported the previously proposed hypothesis that GVP 3 (180K) is a dimer of GVP 9 (91K) and that GVP 6 exists in two forms: one being a 130K polypeptide and the other composed of GVP 11a (74K) and GVP 16 (55K) linked by disulfide bonds. These data suggest that, thus far, either GVP 6/11a/16 or GVP 3/9 may be a potential candidate for a subunit vaccine against BHV-1 infection.