Synthesis and processing of the major envelope glycoprotein of murine cytomegalovirus

Mutagenesis of varicella-zoster virus glycoprotein B: putative fusion loop residues are essential for viral replication, and the furin cleavage motif contributes to pathogenesis in skin tissue in vivo

Glycoprotein B (gB), the most conserved protein in the family Herpesviridae, is essential for the fusion of viral and cellular membranes. Information about varicella-zoster virus (VZV) gB is limited, but homology modeling showed that the structure of VZV gB was similar to that of herpes simplex virus (HSV) gB, including the putative fusion loops. In contrast to HSV gB, VZV gB had a furin recognition motif ([R]-X-[KR]-R-|-X, where | indicates the position at which the polypeptide is cleaved) at residues 491 to 494, thought to be required for gB cleavage into two polypeptides. To investigate their contribution, the putative primary fusion loop or the furin recognition motif was mutated in expression constructs and in the context of the VZV genome. Substitutions in the primary loop, W180G and Y185G, plus the deletion mutation Delta491RSRR494 and point mutation 491GSGG494 in the furin recognition motif did not affect gB expression or cellular localization in transfected cells. Infectious VZV was recovered from parental Oka (pOka)-bacterial artificial chromosomes that had either the Delta491RSRR494 or 491GSGG494 mutation but not the point mutations W180G and Y185G, demonstrating that residues in the primary loop of gB were essential but gB cleavage was not required for VZV replication in vitro. Virion morphology, protein localization, plaque size, and replication were unaffected for the pOka-gBDelta491RSRR494 or pOka-gB491GSGG494 virus compared to pOka in vitro. However, deletion of the furin recognition motif caused attenuation of VZV replication in human skin xenografts in vivo. This is the first evidence that cleavage of a herpesvirus fusion protein contributes to viral pathogenesis in vivo, as seen for fusion proteins in other virus families.

Cleavage of Epstein-Barr virus glycoprotein B is required for full function in cell-cell fusion with both epithelial and B cells

Glycoprotein B (gB) homologues within the herpesvirus family display high sequence conservation, and a number of gB homologues contain a cleavage motif R-X-K/R-R recognized by the cellular protease furin. Epstein-Barr virus (EBV) gB contains this motif and cleaved gB is found in EBV virions. To determine the functional significance of this cleavage motif in EBV gB, a deletion mutant (gB Deltafurin) was created lacking the motif. This cleavage mutant was expressed well in cell culture but was not cleaved. Experiments examining gB Deltafurin in a cell-fusion assay revealed that fusion was reduced by 52 % in epithelial and 28 % in B cells when compared with wild-type EBV gB. This decrease in cell-cell fusion is similar to that observed with multiple alphaherpesvirus gB cleavage mutants and supports a conserved function for cleaved gB.

The identification and characterization of fusogenic domains in herpes virus glycoprotein B molecules

The molecular mechanism of entry of herpes viruses requires a multicomponent fusion system. Virus entry and cell-cell fusion of Herpes simplex virus (HSV) requires four glycoproteins: gD, gB and gH/gL. The role of gB remained elusive until recently, when the crystal structure of HSV-1 gB became available. Glycoprotein B homologues represent the most highly conserved group of herpes virus glycoproteins; however, despite the high degree of sequence and structural conservation, differences in post-translational processing are observed for different members of this virus family. Whereas gB of HSV is not proteolytically processed after oligomerization, most other gB homologues are cleaved by a cellular protease into subunits that remain linked through disulfide bonds. Proteolytic cleavage is common for activation of many other viral fusion proteins, so it remains difficult to envisage a common role for different herpes virus gB structures in the fusion mechanism. We selected bovine herpes virus type 1 (BoHV-1) and herpes simplex virus type 1 (HSV-1) as representative viruses expressing cleaved and uncleaved gBs, and have screened their amino acid sequences for regions of highly interfacial hydrophobicity. Synthetic peptides corresponding to such regions were tested for their ability to induce the fusion of large unilamellar vesicles and to inhibit herpes virus infection. These results underline that several regions of the gB protein are involved in the mechanism of membrane interaction.

Murine cytomegalovirus m142 and m143 are both required to block protein kinase R-mediated shutdown of protein synthesis

Cytomegaloviruses carry the US22 family of genes, which have common sequence motifs but diverse functions. Only two of the 12 US22 family genes of murine cytomegalovirus (MCMV) are essential for virus replication, but their functions have remained unknown. In the present study, we deleted the essential US22 family genes, m142 and m143, from the MCMV genome and propagated the mutant viruses on complementing cells. The m142 and the m143 deletion mutants were both unable to replicate in noncomplementing cells at low and high multiplicities of infection. In cells infected with the deletion mutants, viral immediate-early and early proteins were expressed, but viral DNA replication and synthesis of the late-gene product glycoprotein B were inhibited, even though mRNAs of late genes were present. Global protein synthesis was impaired in these cells, which correlated with phosphorylation of the double-stranded RNA-dependent protein kinase R (PKR) and its target protein, the eukaryotic translation initiation factor 2alpha, suggesting that m142 and m143 are necessary to block the PKR-mediated shutdown of protein synthesis. Replication of the m142 and m143 knockout mutants was partially restored by expression of the human cytomegalovirus TRS1 gene, a known double-stranded-RNA-binding protein that inhibits PKR activation. These results indicate that m142 and m143 are both required for inhibition of the PKR-mediated host antiviral response.

The murine cytomegalovirus M73.5 gene, a member of a 3' co-terminal alternatively spliced gene family, encodes the gp24 virion glycoprotein

We have identified a novel family of five 3' co-terminal transcripts in murine cytomegalovirus (MCMV) arranged in a tail-to-tail orientation with respect to the MCMV glycoprotein H (gH) gene M75. They share the same exon 2 sequence but possess different exon 1 sequences. Two of these spliced transcripts (M73) encode the MCMV homolog of glycoprotein N (gN) entirely within exon 1. Two other transcripts designated M73.5 encode a previously described virion glycoprotein gp24 that shares its first 20 amino acids with gN, but which has another 64 amino acids encoded within exon 2. The fifth transcript, designated m60, has an 80-bp exon 1 near the MCMV oriLyt region 10.8 kb upstream of exon 2. Both MCMV M73.5 and m60 encode type II glycoproteins expressed at the cell surface. Their shared exon 2 coding sequences likely represent the highly conserved region of an as yet unidentified betaherpesvirus-specific glycoprotein species.

Proteolytic processing of human cytomegalovirus glycoprotein B is dispensable for viral growth in culture

Glycoprotein B (gB) of human cytomegalovirus (HCMV), which is considered essential for the viral life cycle, is proteolytically processed during maturation. Since gB homologues of several other herpesviruses remain uncleaved, the relevance of this property of HCMV gB for viral infectivity is unclear. Here we report on the construction of a viral mutant in which the recognition site of gB for the cellular endoprotease furin was destroyed. Because mutagenesis of essential proteins may result in a lethal phenotype, a replication-deficient HCMV gB-null genome encoding enhanced green fluorescent protein was constructed, and complementation by mutant gBs was initially evaluated in transient-cotransfection assays. Cotransfection of plasmids expressing authentic gB or gB with a mutated cleavage site (gB-DeltaFur) led to the formation of green fluorescent miniplaques which were considered to result from one cycle of phenotypic complementation of the gB-null genome. To verify these results, two recombinant HCMV genomes were constructed: HCMV-BAC-DeltaMhdI, with a deletion of hydrophobic domain 1 of gB that appeared to be essential for viral growth in the cotransfection experiments, and HCMV-BACDeltaFur, in which the gB cleavage site was mutated by amino acid substitution. Consistent with the results of the cotransfection assays, only the DeltaFur mutant replicated in human fibroblasts, showing growth kinetics comparable to that of wild-type virus. gB in mutant-infected cells was uncleaved, whereas glycosylation and transport to the cell surface were not impaired. Extracellular mutant virus contained exclusively uncleaved gB, indicating that proteolytic processing of gB is dispensable for viral replication in cell culture.

Monoclonal antibodies specific for human tumor-associated antigen 90K/Mac-2 binding protein: tools to examine protein conformation and function

As part of our effort to identify glycoproteins that contribute to colon cancer progression, we have previously described a family of structurally related glycoproteins expressing beta1-6 branched asparagine(Asn)-linked oligosaccharides defined by monoclonal antibody (MAb 1H9), which are differentially expressed, processed, and glycosylated by human colon carcinoma cell lines (Laferté and Loh [1992]; Biochem J; 283:193-201). MAb 1H9 immunoprecipitates three glycoproteins having apparent sizes of 92-100, 66-70, and 25 kDa, the size heterogeneity attributable to cell-type specific glycosylation differences. We report on the basis of partial protein and cDNA sequence information, that the 100-kDa glycoprotein detected by MAb 1H9 is identical to the 90-kDa glycoprotein variably known as tumor-associated antigen 90K (TAA90K), Mac-2 binding protein, and cyclophilin C-associated protein. Using a PCR-based cloning strategy, the complete cDNA encoding TAA90K was cloned into the eukaryotic expression vector pCDNA-3 (pCD-TAA90K(wt)) and the protein expressed in COS-1 cells. A [(35)S]methionine-labeled 60-kDa polypeptide, processed to an endoglycosidase H-sensitive 74-kDa glycoprotein in the presence of dog pancreas microsomes, was detected in a coupled transcription/translation in vitro reaction. The in vitro-translated 60-kDa polypeptide and N-glycanase-treated TAA90K (60-kDa species) immunoprecipitated from HT29 cells were shown to be structurally identical by limited proteolytic peptide mapping. Using a new panel of 11 TAA90K-specific monoclonal antibodies, including five specific for human TAA90K and six cross-reactive with a 90-kDa species expressed by COS-1 cells, we have detected conformational differences between recombinant wild-type TAA90K, in vitro-synthesized TAA90K, and mutant forms of TAA90K containing point mutations at residues 189, 223, and 259. Furthermore, we have shown that these mutant forms of TAA90K, as well as a truncated form of TAA90K containing amino acid residues 1-383, are defective in secretion. These studies demonstrate the potential usefulness of TAA90K-specific monoclonal antibodies for examining the structure and function of TAA90K, and highlight the contribution of specific amino acid residues to its normal processing and secretion.

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

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

Identification of the murine cytomegalovirus glycoprotein B gene and its expression by recombinant vaccinia virus

The gene encoding glycoprotein B (gB) of murine cytomegalovirus (MCMV) strain Smith was identified, sequenced, and expressed by recombinant vaccinia virus. The gB gene was found adjacent to the polymerase gene, as it is in the genome of human cytomegalovirus (HCMV). The open reading frame consists of 2,784 nucleotides capable of encoding a protein of 928 amino acids. Comparison with gB homologs of other herpesviruses revealed a high degree of homology. The similarity between the MCMV gB and the HCMV gB is most prominent, since 45% of the amino acids are identical. In addition, all cysteine residues are at homologous positions, indicating a similar tertiary structure of the two proteins. In contrast to HCMV, the MCMV gB mRNA is a true late transcript. A recombinant vaccinia virus expressing the MCMV gB gene has been constructed (Vac-gB). Antibodies raised against the Vac-gB recombinant precipitated proteins of 130, 105, and 52 kDa from MCMV-infected cells. The identity of the MCMV gB with the major envelope glycoprotein of MCMV described by Loh et al. was shown (L. C. Loh, N. Balachandran, and L. F. Qualtiere, Virology 166:206-216, 1988). Immunization of mice with the Vac-gB recombinant gave rise to neutralizing antibodies.

Characterization of a family of structurally related glycoproteins expressing beta 1-6-branched asparagine-linked oligosaccharides in human colon carcinoma cells

Previous studies have established that metastatic tumour cells express high levels of beta 1-6-branched Asn-linked oligosaccharides which can be detected with the lectin leucoagglutinin (L-PHA) [Dennis, Laferté, Waghorne, Breitman & Kerbel (1987) Science 236, 582-585]. In order to identify L-PHA-binding glycoproteins which may play a role specifically in colon cancer, we have prepared monoclonal antibodies (MAbs) to the moderately well-differentiated human colon carcinoma cell line HT29. In this paper we present the initial characterization of a family of structurally related L-PHA-binding glycoproteins detected by MAb 1H9 which are differentially expressed and processed by HT29 cells and by two other human colon carcinoma cell lines, SW480 and SW620. In contrast to HT29, the SW480 and SW620 cell lines were established from a poorly differentiated grade III/IV primary tumour and one of its lymph node metastases respectively. MAb 1H9 detects in HT29 cells a conformational determinant present on three L-PHA-binding glycoproteins of 100, 70 and 25kDa, as well as a 74 kDa glycoprotein with high-mannose-type Asn-linked oligosaccharides. Pulse-chase experiments and peptide mapping analyses revealed that the 74 kDa and 100 kDa species are related by carbohydrate processing and are probably derived from a common 76 kDa precursor. On the other hand, the 70 kDa glycoprotein is synthesized from an endoglycosidase H-sensitive precursor of 56 kDa which is structurally related to, but distinct from, the aforementioned 76 kDa precursor. In addition, the 100 kDa species is secreted into the culture medium, whereas the 70 kDa glycoprotein is retained intracellularly. SW480 and SW620 cells showed qualitative and quantitative differences from HT29 cells, including increased secretion of a smaller L-PHA-binding glycoprotein of 92 kDa into the culture medium, as well as apparent differences in glycosylation of the intracellular 66 kDa glycoprotein. These results suggested that the expression, glycosylation and subcellular localization of this family of L-PHA-binding glycoproteins may correlate with the differentiation status of colon cancer cells and/or reflect biochemical changes. characteristic of more progressive metastatic tumours.

Characterization of a membrane-associated phosphoprotein of murine cytomegalovirus (pp50) and its immunological cross-reactivity with a human cytomegalovirus protein

We have identified an abundant 50K phosphoprotein (pp50) in MCMV-infected 3T3-L1 cells and shown by immunofluorescence microscopy and surface-iodination experiments that pp50 is localized to the plasma membrane of the infected cell. Furthermore, the kinetics of its synthesis suggests that it belongs to the early-late class of herpesvirus proteins. Using monoclonal antibodies specific for pp50 to screen a lambda ZAP II expression library constructed from poly(A)+ mRNA of MCMV-infected cells, we have isolated a cDNA clone that synthesizes a truncated form of pp50 as a beta-galactosidase fusion protein. This allowed us to localize the partial pp50 transcript to a region between map coordinates 0.228 and 0.260 of the MCMV genome (Smith strain, Vancouver). Finally, we demonstrated that the MAb 5H10.21A recognizes an antigenic determinant that is conserved between pp50 and a 50K human cytomegalovirus (HCMV) nonstructural protein.