Cotranslational and posttranslational processing of viral glycoproteins

Deciphering transmission dynamics and spillover of avian influenza viruses from avian species to swine populations globally

Genome sequences of eleven avian influenza virus (AIV) subtypes have been reported in swine populations from seven countries until August 2020. To unravel the transmission dynamics and spillover events of AIVs from avian reservoirs to swine, full-length hemagglutinin (HA) sequences of AIV subtypes (n = 11) reported from various avian species and swine were retrieved from the ‘Influenza Research Database’. Phylogenetic analysis identified closely related avian and swine AIV sequences suggesting potential spillover events from multiple domestic and wild avian species, including chicken, duck, pigeon, goose, quail, and aquatic birds to swine. Furthermore, N-linked glycosylation analysis of these closely related AIV sequences supported the possibility of multiple spillover events of highly pathogenic H5N1 and low pathogenic H9N2 viruses from various avian species to swine. The principal coordinate analysis further validated these findings for H5N1 and H9N2 viruses; however, spillover events of the other nine AIV subtypes were limited. Interestingly, the presence of potential mammalian adaptation markers, particularly in some of the swine H5N1, H7N9, and H9N2 viruses, suggested that these viruses may have already adapted in swine. The occurrence and circulation of these AIVs in swine, especially the H5N1 and H9N2 viruses with numerous spillover events from the avian reservoirs to swine, pose a significant threat in terms of their reassortment with endemic swine viruses or circulating human influenza viruses within the swine which may facilitate the emergence of a novel influenza virus strain with pandemic potential.

Neutralization of budded Autographa californica NPV by a monoclonal antibody: identification of the target antigen

A neutralizing monoclonal antibody of the budded phenotype of Autographa californica nuclear polyhedrons virus did not react with the occluded form of the virus as determined by neutralization, ELISA, and indirect immunoperoxidase staining. The antibody did react with the surface of infected cells in the prepolyhedra stage of cytopathic effect, the period of active virus budding. Immunoelectron microscopy indicated the antigen(s) reactive with the neutralizing antibody was present all around the viral envelope, but was more highly concentrated at the end with peplomers. Four proteins were immunoprecipitated from solubilized, radiolabeled budded virus preparations with the monoclonal antibody. The major protein had a molecular weight of approximately 64,000, while the other three were approximately 127,000, 59,000, and 49,000. All four proteins could be labeled with N-acetyl-d-[1-³H]glucosamine. This glycosylation reaction could be inhibited by tunicamycin.

Regulation of ganglioside biosynthesis in the nervous system

Ganglioside biosynthesis is strictly regulated by the activities of glycosyltransferases and is necessarily controlled at the levels of gene transcription and posttranslational modification. Cells can switch between expressing simple and complex gangliosides or between different series within these two groups during brain development. The sequential biosynthesis of gangliosides in parallel enzymatic pathways, however, requires fine-tuned subcellular sequestration and orchestration of glycosyltransferases. A popular model predicts that this regulation is achieved by the vectorial organization of ganglioside biosynthesis: sequential biosynthetic steps occur with the traffic of ganglioside intermediates through subsequent subcellular compartments. Here, we review current models for the subcellular distribution of glycosyltransferases and discuss results that suggest a critical role of N-glycosylation for the processing, transport, and complex formation of these enzymes. In this context, we attempt to illustrate the regulation of ganglioside biosynthesis as well as the biological significance of N-glycosylation as a posttranslational regulatory mechanism. We also review the results of analyses of the 5' regulatory sequences of several glycosyltransferases in ganglioside biosynthesis and provide insights into how their synthesis can be regulated at the level of transcription.

Trypsinogen hL expressed in the human lung is a new member of the trypsinogen family

Molecular cloning of cDNA encoding a new member of the trypsinogen family, named trypsinogen hL, was carried out by PCR using human lung cDNAs as templates. The primary structure of trypsinogen hL was found to be a prepro-protein and a catalytic triad, 64His, 108Asp and 201Ser. It was also found that trypsinogen hL is specifically expressed in the human lung, the expression level being 30-times higher than those in other tissues tested. A phylogenic tree analysis showed that trypsinogen hL is a new member of the trypsinogen family, a family of serine protease family proteins.

Targeted delivery of human neurofibromin and c-Raf-1 mutants to the cytoplasmic membrane by use of the influenza virus hemagglutinin

Mutants of human neurofibromin and c-Raf-1 genes were fused to the 3' end of the hemagglutinin (HA) gene of influenza A virus by oligonucleotide-directed polymerase chain reaction (PCR). The two resulting chimeric genes, HA (1-534)/NF1 (1441-1518) and HA (1-534)/Raf-1 (51-132) which we designated HN and HR, respectively, were cloned in a vaccinia virus expression vector (pTMI) under the control of a T7 RNA polymerase promoter. The clones were expressed in a monkey cell line (CV-1) and the resulting chimeric proteins analysed. We found that expression levels of the chimeric proteins were similar to that of wild-type HA protein. Comparative endoglycosidase treatment revealed that the expressed chimeric proteins HN and HR were processed as wild-type HA, and FACS-analysis showed that both chimeric expression products localised in the cell membrane as the wild-type control. HN and HR expressing cells showed similar fusogenic activity as CV-1 cells transfected with wild-type HA indicating the correct topology of the fusion inducing portion (HA) of these chimera in the membrane. These findings show that the influenza virus hemagglutinin (HA) is a suitable vehicle to target foreign proteins with therapeutical potential into the cell membrane. In this respect HN and HR could potentially be used to block the abnormal signals generated by particular proteins in the cell membrane that lead to cell transformation.

Intra-nuclear localization of two envelope proteins, gB and gD, of herpes simplex virus

The envelopes of herpes simplex virus (HSV) particles are acquired from the inner nuclear membrane (INM) of the infected cell and virus-coded glycoproteins are present in the envelope of mature virions. Our ultrastructural study examined the process of virus envelopment and the targeting of two major viral glycoproteins, gB and gD, to the INM in HSV-infected human embryonic fibroblasts. It was shown that envelopment and transport of virus particles from the nucleus is facilitated by the formation of a dynamic tubulo-reticulum arising from the INM. Capsids were assembled in the nucleus and collected within INM tubules which protruded into the perinuclear space and thence into the cisternae of the endoplasmic reticulum (ER). Envelopment occurred by constriction and fusion of the tubular channel walls, releasing enveloped virions into the ER. Transport to the cell surface took place in membrane-bound compartments and probably followed the normal secretory pathway through the Golgi apparatus. Immunogold probes, tagged with specific monoclonal antibodies, were used to localize gB and gD during the process of virus maturation. Cytoplasmic membranes were not labelled, but probes bound inside the nucleus, mainly at sites of virus assembly. Labelling occurred on the nucleoplasmic side of the INM which surrounded capsids in the process of envelopment, but not on the outside of that membrane, although characteristic gB glycoprotein spikes were labelled on the envelopes of extracellular virus particles and on virions in trans-Golgi transport vesicles just prior to their release from the infected cell. gB was not detected on the surface of enveloped virions in the perinuclear space, or the cisternae of the ER or cis-Golgi, which suggests that the specific epitope was masked during that stage of intracellular processing. gD probes bound to virion envelopes and also to the tegument region of some particles found in both perinuclear and extracellular sites. We postulate the precursor core proteins for both gB and gD are transported first to the nucleus, and then, together with maturing capsids, are targeted to the INM, and later inserted into viral envelopes at the site of budding. Post-translational glycosylation of envelope proteins could occur as virus particles exit the nucleus and travel through the ER and Golgi compartments.

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.

Differential transport and processing of variant mouse mammary tumor virus glycoproteins

The transport and proteolytic processing of two individual gene isolates of the mouse mammary tumor virus (MMTV) glycoprotein were compared in transfected rat HTC hepatoma cells. Plasmids were constructed such that the MMTV glycoprotein genes were constitutively expressed from the promoter within the Rous Sarcoma Virus 5' Long Terminal Repeat in the absence of other MMTV proteins. An isolate of the GR strain MMTV glycoprotein was efficiently transported and processed resulting in the localization of MMTV glycoproteins at the cell surface and in the extracellular environment. Moreover, the kinetics of acquisition of endoglycosidase H resistant oligosaccharide side chains and the rate of endoproteolytic cleavage of the glycosylated polyprotein expressed in transfected cells were virtually identical to that observed in viral-infected rat hepatoma cells. In contrast, a natural variant of the C3H strain MMTV glycoprotein expressed in transfected cells was retained in an intracellular compartment by a heavy chain binding protein (BiP)-independent pathway in an endoglycosidase H sensitive and uncleaved form. This MMTV glycoprotein isolate was retained early in the exocytic pathway and displayed a half-life of approximately 45 min in transfected cells. Only a minor fraction of the expressed C3H variant glycoprotein was detected at the cell surface but was not externalized. Our results suggest that the variant C3H MMTV glycoprotein contains one or more mutations that preclude its efficient transport through the exocytic pathway.

Expression of the Marek's disease virus homolog of herpes simplex virus glycoprotein B in Escherichia coli and its identification as B antigen

Marek's disease (MD) is an oncogenic disease of chickens caused by MD virus (MDV). Among the major glycoproteins found in MDV-infected cells are gp100, gp60, and gp49, detected by immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis with antisera previously shown to be reactive with B antigen in immunodiffusion analysis. Following treatment with tunicamycin (TM), an inhibitor of N-linked glycosylation, the same sera were reported to detect two molecules called pr88 and pr44. However, the gene encoding B antigen was not unequivocally identified. Recently, an MDV homolog of the gene encoding herpes simplex virus glycoprotein B (gB) was identified and sequenced (L. J. N. Ross, M. Sanderson, S. D. Scott, M. M. Binns, T. Doel, and B. Milne, J. Gen. Virol. 70:1789-1804, 1989). To determine whether the MDV gB homolog gene might encode the B antigen, antisera against trpE fusion proteins of the MDV gB homolog (trpE-MDV-gB) were prepared. These antisera immunoprecipitated gp100, gp60, gp49, and a 92-kDa precursor polypeptide (pr88, now designated 92-kDa pr88, in the presence of TM) from MDV-infected cell lysates. On the basis of size comparison, trpE-MDV-gB competition and blocking assays, and the fact that gp100, gp60, gp49, and 92-kDa pr88 could be detected in MDV-infected cells with antisera specific to both MDV B antigen and the gB homolog, it was concluded that (i) the MDV gB homolog gene encodes MDV B antigen and (ii) 92-kDa pr88 is the primary precursor polypeptide. The antisera against trpE-MDV-gB also contained antibody reactive with the herpesvirus of turkey gB homolog, consistent with the known antigenic relatedness between the MDV and herpesvirus of turkey B antigens. TM inhibition data and results from pulse-chase analysis with MDV-infected cells show that MDV gB homolog processing involves cotranslational glycosylation of 92-kDa pr88 to form gp100, which is then cleaved to form gp60 and gp49, the N- and C-terminal halves, respectively, of gp100. This processing pathway is consistent with those of other gB homologs, further supporting the gene identification described above. The conclusions of this study will facilitate future research on the immunobiology of MD, especially studies on the mechanism of immunoprotection.

Adaptation of transmissible gastroenteritis virus to growth in non-permissive Vero cells

The CPK cells derived from swine kidney were infected with the attenuated TO-163 strain of transmissible gastroenteritis (TGE) virus, and fused with uninfected Vero cells in the presence of polyethylene glycol. Repeated cocultivation of the fused cells with uninfected Vero cells rendered the virus to grow in Vero cells. The Vero cell-adapted virus acquired the ability to infect and produce cytopathic effects in several other non-permissive cell lines of non-porcine origin. No major differences in viral polypeptides were shown between the Vero cell-adapted TO-163 strain and its parent strain by indirect immunofluorescence and Western blotting using monoclonal and polyclonal antibodies to TGE virus.

Differences in sensitivity in haemagglutinin inhibition assays between A/equine/H3N8 viruses isolated in eggs and MDCK cells are linked to cleavage of the haemagglutinin molecule

Two primary isolates of A/equine/H3N8 viruses were obtained in embryonated hens' eggs and in Madin-Darby canine kidney (MDCK) cells. Viruses isolated in MDCK cells were significantly more sensitive as antigens in haemagglutination inhibition (HI) tests. This sensitivity appeared to be primarily linked to the extent of cleavage of the haemagglutinin molecule.

Stimulation of tumor necrosis factor secretion by purified influenza virus neuraminidase

We showed that purified neuraminidase (NA) of influenza virus, but not hemagglutinin (HA), possessed the potential to increase in vitro and in vivo the interleukin 1 (IL 1) activity of mouse peritoneal macrophages. In this study, we report the effect of NA and HA on the secretion of tumor necrosis factor (TNF) activity by murine peritoneal macrophages. TNF being a cytokine sharing many related and overlapping biological functions with IL 1. The two glycoproteins of the strain A/USSR/90/77 (H1N1) were purified electrophoretically and were tested in vitro at doses ranging from 0.5 to 5.0 micrograms using the adherent peritoneal macrophages of C3H/HeN mice elicited with thioglycolate. The TNF activity of culture supernatants, collected 24 hr after stimulation with viral protein, was evaluated by the standard cytolytic assay using L929 and WEHI.164 cells. No increase of the TNF activity was observed at 0.5 micrograms of NA (4.8 Units (U)/ml in the L929 assay and 20.4 U/ml in the WEHI assay) but further increase of NA to 1.0 microgram had a significant effect on the TNF activity (39.7 and 88.8 U/ml, respectively). Higher concentrations of NA (2.0 and 5.0 micrograms) did not improve the TNF activity. The addition of a rabbit anti-TNF-alpha serum to the assay system reduced the lysis of L929 cells by 85%, suggesting that the observed activity was due to TNF. In parallel, the enhancement of IL 1 activity due to NA was reverified using D10.G4.1 cells instead of the C3H/HeJ thymocytes assay used previously. NA augmented the IL 1 activity up to 1.0 micrograms (25.8 U/ml). The addition of monoclonal anti-IL 1 antibodies (100 neutralizing units) to the supernatants reduced the incorporation of [3H]-thymidine by 90 to 95%, suggesting that the observed activity was due to IL 1. Comparative results of NA and HA showed that only NA stimulated the TNF and IL 1 activities of murine macrophages.

Mutations at the cleavage site of the hemagglutinin after the pathogenicity of influenza virus A/chick/Penn/83 (H5N2)

Six variants that form plaques in chick embryo cells in the absence of trypsin have been isolated from the apathogenic avian influenza virus A/chick/Pennsylvania/1/83 (H5N2). Unlike the wild-type, the plaque variants contain a hemagglutinin that is cleaved in chick embryo cells and MDCK cells. The variants differ also from the wild-type in their pathogenicity for chickens. Nucleotide sequence and oligosaccharide analysis of the hemagglutinin have revealed that, unlike natural isolates with increased pathogenicity (Y. Kawaoka et al., 1984, Virology 139, 303-316; Y. Kawaoka and R. G. Webster, 1985, Virology 146, 130-137), the variants obtained in vitro have retained an oligosaccharide at asparagine 11 that is believed to interfere with the cleavage site of the wild-type. However, all variants showed mutations in the hemagglutinin resulting in an increased number of basic groups at the cleavage site. These observations demonstrate that masking of the cleavage site by an oligosaccharide is overcome by an enhancement of the basic charge at the cleavage site.

Sequence of the coding regions from the 3.0 kb and 3.9 kb mRNA. Subgenomic species from a virulent isolate of transmissible gastroenteritis virus

Subgenomic mRNA from a virulent isolate of porcine transmissible gastroenteritis virus (TGEV) was used to produce cDNA clones covering the genome region from the 3' end of the peplomer gene to the start of the integral membrane protein gene. The nucleotide sequence of this area was determined using clone pTG11 and a previously reported cDNA clone pTG22. Three open reading frames (ORFs) were identified encoding putative polypeptides of relative molecular masses (Mr) 6,600, 27,600, and 9,200. The sequence encoding the Mr 9,200 polypeptide was found to be present on the "unique" 5' region of the 3.0 kb mRNA species whereas the other two ORFs mapped on the 3.9 kb mRNA species. Differences between the ORFs from this strain of TGEV and those from a previously reported avirulent strain of TGEV were compared.

Modification of membrane permeability by animal viruses

Animal viruses modify membrane permeability during lytic infection. There is a co-entry of macromolecules and virion particules during virus penetration and a drastic change in transport and membrane permeability at the late stages of the lytic cycle. Both events are of importance to understand different molecular aspects of viral infection, as virus entry into the cell and the interference of virus infection with cellular metabolism. Other methods of cell permeabilization of potential relevance to understand the mechanism of viral damage of the membrane are also discussed.

Inducible expression of herpes simplex virus type 2 glycoprotein gene gG-2 in a mammalian cell line

The gG-2 glycoprotein gene of herpes simplex virus type 2 (HSV-2) was cloned into the mammalian expression vector pMSG under the control of the inducible mouse mammary tumor virus promoter. Transfection of this cloned gG-2 construct into NIH 3T3 cells resulted in the stable expression of gG-2 upon induction with dexamethasone. In addition, the 104,000-molecular-weight (104K) and 72K gG-2 precursors as well as the 34K secreted component were generated in the transformed cells. The synthesis of gG-2 in these transformed cells appeared to follow the same cleavage-processing pathway as gG-2 synthesis during an HSV-2 infection. These results indicate that the processing of gG-2 can occur in the absence of an HSV-2 infection.

Resistance of the 64K protein of budded Autographa californica nuclear polyhedrosis virus to functional inactivation by proteolysis

The 64K surface protein of budded Autographa californica nuclear polyhedrosis virus (AcMNPV BV) is known to play a role in the functional entry of AcMNPV BV into Spodoptera frugiperda IPLB-SF-21 cells by adsorptive endocytosis. AcV1, a neutralizing monoclonal antibody, reacts with the 64K protein and in doing so prevents efficient entry. In this communication we report that treatment of AcMNPV BV with either trypsin or proteinase K cleaves the 64K protein into one major fragment of 34.6K and two minor fragments of 36K to 37.2K that are retained with the virus. All of the fragments are glycosylated. Protease treatment does not reduce viral infectivity, but it does result in the destruction of the AcV1-reactive epitope; thus AcV1 is not able to neutralize protease-treated AcMNPV BV. Polyclonal antiserum to BV is able to recognize both cleaved and uncleaved 64K and neutralize both protease-treated and untreated virus. Protease treatment does not diminish the sensitivity of AcMNPV BV to chloroquine, but it does cause the virus to become more susceptible to inactivation by 2-mercaptoethanol (2-ME) even though exposure to 2-ME does not result in dissociation of the fragments from the virus.

The integral membrane protein from a virulent isolate of transmissible gastroenteritis virus: molecular characterization, sequence and expression in Escherichia coli

Subgenomic mRNA from a virulent isolate of porcine transmissible gastroenteritis virus (TGEV) was used to produce cDNA clones. Part of a new clone and a previously reported clone were sequenced and used to construct the viral gene for integral membrane protein. A single open reading frame (ORF) encoding a polypeptide of 262 amino acids, relative molecular mass (Mr) 29,459, was identified. The positive identification of the polypeptide as the integral membrane protein was demonstrated by the production in E. coli of a chimaeric protein comprising most of the ORF encoding the Mr 29,459 polypeptide and beta-galactosidase. The chimaeric protein reacted with a specific monoclonal antibody to viral integral membrane protein and antibodies raised against the chimaeric protein immune precipitated the viral protein. Comparison with the sequence of an avirulent isolate indicates amino acid residues that may be important in pathogenicity.

Megalomycin C, a macrolide antibiotic that blocks protein glycosylation and shows antiviral activity

Megalomycin C, a natural macrolide antibiotic showed a potent antiherpetic activity. At concentrations that efficiently prevented HSV-1 multiplication, the compound had no cytotoxic or antiproliferative effects. Viral DNA and protein synthesis took place at normal levels in the presence of the antibiotic, suggesting that neither the translation of viral mRNA, nor the synthesis of viral nucleic acids was affected. The incorporation of mannose and galactosamine into viral proteins was blocked and precursor, but not mature, HSV-1 glycoproteins were detected in the presence of megalomycin C. Non-infectious HSV-1 viral particles were formed when the compound was present, but their glycoproteins were not properly glycosylated.

Monoclonal antibodies localize events in the folding, assembly, and intracellular transport of the influenza virus hemagglutinin glycoprotein

We used monoclonal antibodies that recognize monomeric and/or trimeric forms of the influenza virus hemagglutinin (HA) to study biosynthesis of this integral membrane protein in influenza virus-infected cells. We find the following: First, the globular head of the HA folds into its mature conformation in the endoplasmic reticulum prior to the assembly of HA monomers into trimers. Second, trimerization begins within 1 to 2 min following synthesis, with a half-time of approximately 5 min. Third, trimerization occurs only after the HA has been transported from the endoplasmic reticulum. Fourth, newly formed trimers are sensitive to acid-induced conformational alterations associated with viral fusion activity.

The molecular biology of influenza virus pathogenicity

It is an accepted concept that the pathogenicity of a virus is of polygenic nature. Because of their segmented genome, influenza viruses provide a suitable system to prove this concept. The studies employing virus mutants and reassortants have indicated that the pathogenicity depends on the functional integrity of each gene and on a gene constellation optimal for the infection of a given host. As a consequence, virtually every gene product of influenza virus has been reported to contribute to pathogenicity, but evidence is steadily growing that a key role has to be assigned to hemagglutinin. As the initiator of infection, hemagglutinin has a double function: (1) promotion of adsorption of the virus to the cell surface, and (2) penetration of the viral genome through a fusion process among viral and cellular membranes. Adsorption is based on the binding to neuraminic acid-containing receptors, and different virus strains display a distinct preference for specific oligosaccharides. Fusion capacity depends on proteolytic cleavage by host proteases, and variations in amino acid sequence at the cleavage site determine whether hemagglutinin is activated in a given cell. Differences in cleavability and presumably also in receptor specificity are important determinants for host tropism, spread of infection, and pathogenicity. The concept that proteolytic activation is a determinant for pathogenicity was originally derived from studies on avian influenza viruses, but there is now evidence that it may also be relevant for the disease in humans because bacterial proteases have been found to promote the development of influenza pneumonia in mammals.

Molecular aspects of the epidemiology of virus disease

With regard to molecular epidemiology, influenza A viruses belong to the best-studied virus systems. At least two large reservoirs of influenza A viruses have been built up in nature, one in humans and another one in water fowls. The latter one is very heterogenous, consisting of viruses belonging to 13 hemagglutinin (HA) and 9 neuraminidase (NA) subtypes in almost all possible combinations. The segmented structure of the influenza virus genome allows the creation of new influenza strains by reassortment. By replacement of the HA gene of human strains new pandemic viruses can be generated (antigenic shift). The particular structure of the HA enables the human influenza A-viruses to create variants which can escape the immune response of the host (antigenic drift). The nucleoprotein is responsible for keeping those two large reservoirs apart. Mixing of genes of viruses from these two reservoirs seems to happen predominantly by double infection of pigs, which apparently are tolerant for infection by either human or avian influenza viruses. The molecular mechanisms described for influenza viruses can be explained by the particular structure of their genome and their components and cannot be generalized. Each virus has developed its own strategy to multiply and to spread.

The cellular secretory pathway is not utilized for biosynthesis, modification, or intracellular transport of the simian virus 40 large tumor antigen

Unlike most proteins, which are localized within a single subcellular compartment in the eucaryotic cell, the simian virus 40 (SV40) large tumor antigen (T-ag) is associated with both the nucleus and the plasma membrane. Current knowledge of protein processing would predict a role for the secretory pathway in the biosynthesis and transport of at least a subpopulation of T-ag to account for certain of its chemical modifications and for its ability to reach the cell surface. We have examined this prediction by using in vitro translation and translocation experiments. Preliminary experiments established that translation of T-ag was detectable with as little as 0.1 microgram of the total cytoplasmic RNA from SV40-infected cells. Therefore, by using a 100-fold excess of this RNA, the sensitivity of the assays was above the limits necessary to detect the theoretical fraction of RNA equivalent to the subpopulation of plasma-membrane-associated T-ag (2 to 5% of total T-ag). In contrast to a control rotavirus glycoprotein, the electrophoretic mobility of T-ag was not changed by the addition of microsomal vesicles to the in vitro translation mixture. Furthermore, T-ag did not undergo translocation in the presence of microsomal vesicles, as evidenced by its sensitivity to trypsin treatment and its absence in the purified vesicles. Identical results were obtained with either cytoplasmic RNA from SV40-infected cells or SV40 early RNA transcribed in vitro from a recombinant plasmid containing the SP6 promoter. SV40 early mRNA in infected cells was detected in association with free, but not with membrane-bound, polyribosomes. Finally, monensin, an inhibitor of Golgi function, failed to specifically prevent either glycosylation or cell surface expression of T-ag, although it did depress overall protein synthesis in TC-7 cells. We conclude from these observations that the constituent organelles of the secretory pathway are not involved in the biosynthesis, modification, or intracellular transport of T-ag. The initial step in the pathway of T-ag biosynthesis appears to be translation on free cytoplasmic polyribosomes. With the exclusion of the secretory pathway, we suggest that T-ag glycosylation, palmitylation, and transport to the plasma membrane are accomplished by previously unrecognized cellular mechanisms.

Significance of viral glycoproteins for infectivity and pathogenicity

Disease resulting from virus infection is a complex event depending on the close interaction of viral and cellular factors. Through the application of biochemical and genetic methods, it is now possible to gain an insight into the molecular basis of these interactions. Thus, it has been shown that the glycoproteins of enveloped viruses play a central role in the initiation of infection. They are responsible not only for the adsorption of virions to cellular receptors, but are also for the entry of the genome into the cell by the fusion of viral envelopes with cellular membranes. Evidence is growing that the fusogenic glycoproteins are frequently activated by cellular proteases. The structure of the proteins at the cleavage site and the availability of a suitable protease are critical for tissue tropism, spread of the virus in the infected organism and, thus, for pathogenicity. This will be demonstrated here by the example of the haemagglutinin of influenza viruses.

Sequence analysis of the bovine coronavirus nucleocapsid and matrix protein genes

The 3' end of the 20-kb genome of the Mebus strain of bovine enteric coronavirus (BCV) was copied into cDNA and cloned into the PstI site of the pUC9 vector. Four clones from the 3' end of the genome were sequenced either completely or in part to determine the sequence of the first 2451 bases. Within this sequence were identified, in order, a 3'-noncoding region of 291 bases, the gene for a 448-amino acid nucleocapsid protein (N) having a molecular weight of 49,379, and the gene for a 230-amino acid matrix protein (M) having a molecular weight of 26,376. A third large open reading frame is contained entirely within the N gene sequence but is positioned in a different reading frame; it potentially encodes a polypeptide of 207 amino acids having a molecular weight of 23,057. A higher degree of amino acid sequence homology was found between the M proteins of BCV and MHV (87%) than between the N proteins (70%). For the M proteins of BCV and MHV, notable differences were found at the amino terminus, the most probable site of O-glycosylation, where the sequence is N-Met-Ser-Ser-Val-Thr-Thr for BCV and N-Met-Ser-Ser-Thr-Thr for MHV. BCV apparently uses two of its six potential O-glycosylation sites.

Deduced amino acid sequence and potential O-glycosylation sites for the bovine coronavirus matrix protein

The nucleotide sequence of the matrix (M) protein gene of the bovine coronavirus (BCV) was determined by sequencing cDNA clones derived from genomic RNA. The gene was found to map at the 5' side of the nucleocapsid protein gene and its sequence predicts a protein of 230 amino acids having a molecular weight of 26,376. The BCV M protein shares extensive sequence homology with the matrix protein of the mouse hepatitis coronavirus (MHV) but differs notably in the amino terminal region external to the virion envelope where BCV apparently uses at least two of its six potential O-glycosylation sites.

Glycosylation of the bovine coronavirus hemagglutinin protein

The bovine coronavirus hemagglutinin protein gp140 is composed of disulfide-linked subunits of 65 kDa. This protein was further characterized with regard to its glycosylation. The glycosylated subunits of the protein are polypeptides having a molecular mass of 42.5 kDa. Both subunits appear to be processed to the same extent by the addition of N-linked oligosaccharides. Each subunit on the mature virion has 6-7 high mannose and 3-4 complex type carbohydrate chains attached to it.

Formation of influenza virus particles lacking hemagglutinin on the viral envelope

We investigated the intracellular block in the transport of hemagglutinin (HA) and the role of HA in virus particle formation by using temperature-sensitive (ts) mutants (ts134 and ts61S) of influenza virus A/WSN/33. We found that at the nonpermissive temperature (39.5 degrees C), the exit of ts HA from the rough endoplasmic reticulum to the Golgi complex was blocked and that no additional block was apparent in either the exit from the Golgi complex or post-Golgi complex transport. When MDBK cells were infected with these mutant viruses, they produced noninfectious virus particles at 39.5 degrees C. The efficiency of particle formation at 39.5 degrees C was essentially the same for both wild-type (wt) and ts virus-infected cells. When compared with the wt virus produced at either 33 or 39.5 degrees C or the ts virus formed at 33 degrees C, these noninfectious virus particles were lighter in density and lacked spikes on the envelope. However, they contained the full complement of genomic RNA as well as all of the structural polypeptides of influenza virus with the exception of HA. In these spikeless particles, HA could not be detected at the limit of 0.2% of the HA present in wt virions. In contrast, neuraminidase appeared to be present in a twofold excess over the amount present in ts virus formed at 33 degrees C. These observations suggest that the presence of HA is not an obligatory requirement for the assembly and budding of influenza virus particles from infected cells. The implications of these results and the possible role of other viral proteins in influenza virus morphogenesis are discussed.

The properties and sequence of glycoprotein H of herpes simplex virus type 1

The map position of the coding sequence of glycoprotein H of herpes simplex virus type 1 was determined by marker transfer studies in which DNA fragments cloned from a virus resistant to neutralisation by an anti-gH monoclonal antibody were used to transfer antibody resistance to wild type virus DNA following cotransfection. The gH coding sequence was mapped to the BglII "m" fragment of HSV-1 DNA (map coordinates 0.27-0.312), confirming the map position previously determined by intertypic recombinant analysis (Buckmaster et al., 1984). The complete nucleotide sequence of the BglII "m" fragment revealed two large open reading frames in addition to the thymidine kinase gene. The open reading frame lying immediately 3' of the thymidine kinase gene has a predicted translation product with the features of a large glycoprotein. This open reading frame translates to an amino acid sequence of 90,323 mol wt with a signal peptide, a membrane anchor sequence, a large external domain containing potential N-glycosylation sites, and a charged C- terminal cytoplasmic domain. We suppose that this amino acid sequence corresponds to gH of HSV-1, and A. Davison (personal communication) has noted the existence of homologous glycoproteins predicted from the nucleotide sequences of Varicella-zoster virus and Epstein-Barr virus. The properties of monoclonal antibody LP11, directed against gH show remarkable similarities to the properties for gD antibodies. LP11 efficiently neutralizes virus infectivity, blocks cell fusion by syncytial virus strains, and inhibits the formation of plaques when added to cell monolayers after infection. These similarities in antibody activity imply functional relatedness between gH and gD of herpes simplex virus.

Temperature-sensitive mutants of fowl plague virus defective in the intracellular transport of the hemagglutinin

Nine mutants of fowl plague virus with temperature-sensitive defects in the biosynthesis of the hemagglutinin have been characterized by analyzing the processing and the intracellular location of this glycoprotein in MDCK and chick embryo cells. It was found that with all of these mutants the transport of the hemagglutinin to the cell surface was impeded at the non-permissive temperature. There were differences, however, in the site of the block. With mutants tsl, ts227, ts478 and ts658 the precursor HA was not cleaved and the oligosaccharide side chains remained sensitive to endoglucosaminidase H. When the hemagglutinin was analyzed in permeabilized cells by immunofluorescence, usually only cytoplasmic labeling was seen. Immunofluorescence of non-permeabilized cells and hemadsorption revealed that the hemagglutinin did not reach the cell surface. In contrast, the hemagglutinin of mutants ts79, ts482, ts532, ts546 and ts651 was cleaved and oligosaccharides were processed to the endoglucosaminidase H-resistant form at non-permissive temperature. In permeabilized cells, the cytoplasm and juxtanuclear regions typical for the Golgi apparatus were labeled by immunofluorescence. Except for ts482, ts532 and ts546 which were leaky, hemagglutinin could not be detected at the cell surface. These observations indicate that, with the first group of mutants, hemagglutinin transport is usually arrested already in the rough endoplasmic reticulum, whereas with the second group it is inhibited at a late stage between the Golgi apparatus and the plasma membrane.

A reassortant between influenza A viruses (H7N2) synthesizing an enzymatically inactive neuraminidase at 40 degrees which is not incorporated into infectious particles

Cells infected with a reassortant (113/Ho, H7N2) between A/fowl plague/Rostock/34 (FPV, H7N1) and A/Hong Kong/1/68 (H3N2) carrying RNA segments 1 and 6 of the Hong Kong virus and the residual genes of FPV, synthesized at 40 degrees a neuraminidase (NA) which is enzymatically not active and which is not incorporated into infectious particles. At 40 degrees NA accumulates in the rough endoplasmic reticulum. It contains mainly carbohydrate side chains of the mannose type, and fucose is only scarcely incorporated. At 33 degrees NA of the reassortant is overproduced, and at least some of it is active and is incorporated into viral particles. Under nonreducing conditions during PAGE its NA migrates to the same position as after heating with mercaptoethanol, in contrast to the Hong Kong parent virus. It is speculated that at 40 degrees the tetramerization of the NA in the rough endoplasmic reticulum does not function, and in this way its migration to the cytoplasmic membrane and its incorporation into infectious particles does not occur. Since 113/Ho is as pathogenic for the chicken (body temperature of 41 degrees) as is FPV, the question arises which role the NA plays in virus replication and spread in the infected organism.

Coronavirus IBV: partial amino terminal sequencing of spike polypeptide S2 identifies the sequence Arg-Arg-Phe-Arg-Arg at the cleavage site of the spike precursor propolypeptide of IBV strains Beaudette and M41

The spike protein of avian infectious bronchitis coronavirus comprises two glycopolypeptides S1 and S2 derived by cleavage of a proglycopolypeptide So, the nucleotide sequence of which has recently been determined for the Beaudette strain (Binns, M.M. et al., 1985, J. Gen. Virol. 66, 719-726). The order of the two glycopolypeptides within So is aminoterminus(N)-S1-S2-carboxyterminus(C). To locate the N-terminus of S2 we have performed partial amino acid sequencing on S2 from IBV-Beaudette labelled with [3H]serine and from the related strain labelled with [3H]valine, leucine and isoleucine. The residues identified and their positions relative to the N-terminus of S2 were: serine, 13; valine, 6, 12; leucine, none in the first 20 residues; isoleucine, 2, 19. These results identified the N-terminus of S2 of IBV-Beaudette as serine, 520 residues from the N-terminus of S1, excluding the signal sequence. Immediately to the N-terminal side of residue 520 So has the sequence Arg-Arg-Phe-Arg-Arg; similar basic connecting peptides are a feature of several other virus spike glycoproteins. It was deduced that for IBV-Beaudette S1 comprises 519 residues (Mr 57.0K) or 514 residues (56.2K) if the connecting peptide was to be removed by carboxypeptidase-like activity in vivo while S2 has 625 residues (69.2K). Nucleotide sequencing of the cleavage region of the So gene of IBV-M41 revealed the same connecting peptide as IBV-Beaudette and that the first 20 N-terminal residues of S2 of IBV-M41 were identical to those of the Beaudette strain. IBV-Beaudette grown in Vero cells had some uncleaved So; this was cleavable by 10 micrograms/ml of trypsin and of chymotrypsin. Partial N-terminal analysis of S1 from IBV-M41 identified leucine and valine residues at positions 2 and 9 respectively from the N-terminus. This confirms the identification, made by Binns et al. (1985), of the N-terminus of S1 and the end of the signal sequence of the IBV-Beaudette spike propolypeptide. N-terminal sequencing of [3H]leucine-labelled IBV-Beaudette membrane (M) polypeptide showed leucine residues at positions 8, 16 and 22 from the N-terminus; these results confirm the open reading frame identified by M.E.G. Boursnell et al. (1984, Virus Res. 1, 303-313) in the nucleotide sequence of M. The N-terminus of the nucleocapsid (N) polypeptide appeared to be blocked.

Deletions into an NH2-terminal hydrophobic domain result in secretion of rotavirus VP7, a resident endoplasmic reticulum membrane glycoprotein

Rotavirus, a non-enveloped reovirus, buds into the rough endoplasmic reticulum and transiently acquires a membrane. The structural glycoprotein, VP7, a 38-kD integral membrane protein of the endoplasmic reticulum (ER), presumably transfers to virus in this process. The gene for VP7 potentially encodes a protein of 326 amino acids which has two tandem hydrophobic domains at the NH2-terminal, each preceded by an in-frame ATG codon. A series of deletion mutants constructed from a full-length cDNA clone of the Simian 11 rotavirus VP7 gene were expressed in COS 7 cells. Products from wild-type, and mutants which did not affect the second hydrophobic domain of VP7, were localized by immunofluorescence to elements of the ER only. However, deletions affecting the second hydrophobic domain (mutants 42-61, 43-61, 47-61) showed immunofluorescent localization of VP7 which coincided with that of wheat germ agglutinin, indicating transport to the Golgi apparatus. Immunoprecipitable wild-type protein, or an altered protein lacking the first hydrophobic sequence, remained intracellular and endo-beta-N-acetylglucosaminidase H sensitive. In contrast, products of mutants 42-61, 43-61, and 47-61 were transported from the ER, and secreted. Glycosylation of the secreted molecules was inhibited by tunicamycin, resistant to endo-beta-N-acetylglucosaminidase H digestion and therefore of the N-linked complex type. An unglycosylated version of VP7 was also secreted. We suggest that the second hydrophobic domain contributes to a positive signal for ER location and a membrane anchor function. Secretion of the mutant glycoprotein implies that transport can be constitutive with the destination being dictated by an overriding compartmentalization signal.

Proteolytic cleavage of the E2 glycoprotein of murine coronavirus: activation of cell-fusing activity of virions by trypsin and separation of two different 90K cleavage fragments

In the murine coronavirus mouse hepatitis virus, a single glycoprotein, E2, is required both for attachment to cells and for cell fusion. Cell fusion induced by infection with mouse hepatitis virus strain A59 was inhibited by the addition of monospecific anti-E2 antibody after virus adsorption and penetration. Adsorption of concentrated coronavirions to uninfected cells did not cause cell fusion in the presence of cycloheximide. Thus, cell fusion was induced by E2 on the plasma membrane of infected 17 Cl 1 cells but not by E2 on virions grown in these cells. Trypsin treatment of virions purified from 17 Cl 1 cells quantitatively cleaved 180K E2 to 90K E2 and activated cell-fusing activity of the virions. This proteolytic cleavage yielded two different 90K species which were separable by sodium dodecyl sulfate-hydroxyapatite chromatography. One of the trypsin cleavage products, 90A, was acylated and may be associated with the lipid bilayer. The other, 90B, was not acylated and yielded different peptides than did 90A upon limited digestion with thermolysin or staphylococcal V8 protease. Thus, the cell-fusing activity of a coronavirus required proteolytic cleavage of the E2 glycoprotein, either by the addition of a protease to virions or by cellular proteases acting on E2, which was transported to the plasma membrane during virus maturation. There is a striking functional similarity between the E2 glycoprotein of coronavirus, which is a positive-strand RNA virus, and the hemagglutinin glycoprotein of negative-strand orthomyxoviruses, in that a single glycoprotein has both attachment and protease-activated cell-fusing activities.

Processing of the rough endoplasmic reticulum membrane glycoproteins of rotavirus SA11

The synthesis and oligosaccharide processing of the glycoproteins of SA11 rotavirus in infected Ma104 cells was examined. Rotavirus assembles in the rough endoplasmic reticulum (RER) and encodes two glycoproteins: VP7, a component of the outer viral capsid, and NCVP5, a nonstructural protein. A variety of evidence suggests the molecules are limited to the ER, a location consistent with the high mannose N-linked oligosaccharides modifying these proteins. VP7 and NCVP5 were shown to be integral membrane proteins. In an in vitro translation system supplemented with dog pancreas microsomes, they remained membrane associated after high salt treatment and sodium carbonate-mediated release of microsomal contents. In infected cells, the oligosaccharide processing of these molecules proceeded in a time-dependent manner. For VP7, Man8GlcNAc2 and Man6GlcNAc2 were the predominant intracellular species after a 5-min pulse with [3H]mannose and a 90 min chase, while in contrast, trimming of NCVP5 halted at Man8GlcNAc2. VP7 on mature virus was processed to Man5GlcNAc2. It is suggested that the alpha-mannosidase activities responsible for the formation of these structures reside in the ER. In the presence of the energy inhibitor carbonyl cyanide m-chlorophenylhydrazone (CCCP), processing of VP7 and the vesicular stomatitis virus G protein was blocked at Man8GlcNAc2. After a 20-min chase of [3H]mannose-labeled molecules followed by addition of CCCP, trimming of VP7 could continue while processing of G protein remained blocked. Thus, an energy-sensitive translocation step within the ER may mark the divergence of the processing pathways of these glycoproteins.

Effect of monensin on the assembly of Uukuniemi virus in the Golgi complex

The effect of the carboxylic ionophore monensin on the maturation of Uukuniemi virus, a bunyavirus, and the transport of its two membrane glycoproteins, G1 and G2, were studied in chicken embryo fibroblasts and baby hamster kidney cells. Virus maturation, which occurs in the Golgi complex (E. Kuismanen, K. Hedman, J. Saraste, and R. F. Pettersson, Mol. Cell. Biol. 2:1444-1458, 1982; E. Kuismanen, B. Bång, M. Hurme, and R. F. Pettersson, J. Virol. 51:137-146, 1984), was effectively inhibited by the drug (1 or 10 microM) as studied by electron microscopy and by assaying the release of infectious or radiolabeled virus. Immunoelectron microscopy showed that association of viral nucleocapsids with the cytoplasmic surface of glycoprotein-containing Golgi membranes, a prerequisite for virus budding, was unaffected by monensin. In the presence of the drug, the virus glycoproteins assembled into long, tubular structures extending into the lumen of Golgi-derived vacuoles, suggesting that monensin inhibited a terminal step in the assembly of the virus. Intracellular transport and expression of the virus membrane glycoproteins G1 and G2 at the cell surface were not inhibited by monensin as studied by immunocytochemical and radiolabeling techniques. Pulse-chase experiments in the presence of monensin showed that intracellular G1 acquired only partially endo-H-resistant glycans. The sialylation of G1 appearing on the cell surface in the presence of the drug was decreased, whereas sialylation of G2 apparently was inhibited to a lesser extent, as shown by external labeling of the cells with the periodate-boro[3H]hydride method. Thus, monensin exerted a differential effect on the terminal glycosylation of G1 and G2. Unlike several membrane and secretory glycoproteins, both G1 and G2 could enter a functional transport pathway in the presence of monensin and become expressed at the cell surface.

Nucleotide sequence of the G protein gene of human respiratory syncytial virus reveals an unusual type of viral membrane protein

The major surface glycoprotein (G) of human respiratory syncytial (RS) virus has an estimated mature Mr of 84,000-90,000. Among a library of cDNA clones prepared from RS virus mRNAs, we identified clones that hybridized to a message that encoded a Mr 36,000 polypeptide that was specifically immunoprecipitated with anti-G antiserum. The amino acid sequence of the G protein backbone was determined by nucleotide sequence analysis of several of the cDNA clones. It contains a combination of structural features that make it unique among the known viral glycoproteins. The G mRNA is 918 nucleotides long and contains a single major open reading frame that encodes a polypeptide having 298 amino acid residues with a Mr of 32,587, a finding consistent with the Mr 36,000 estimate for the in vitro translation product of the G mRNA. This suggests that greater than 50% of the molecular weight of the mature glycoprotein may be contributed by carbohydrate. Glycosylation of G is largely resistant to tunicamycin, an inhibitor of the attachment of N-linked oligosaccharides, suggesting that the majority of the carbohydrate residues are attached via O-glycosidic bonds. In accordance with this, serine and threonine residues, the acceptor sites for O-linked oligosaccharides, comprise 30.6% of the total amino acid composition. There are also four potential acceptor sites for N-linked oligosaccharides. The amino acid sequence lacks both an NH2-terminal hydrophobic signal sequence and a COOH-terminal hydrophobic region. Instead, a strongly hydrophobic region is located between amino acid residues 38 and 66. This region may serve as both the signal to insert the nascent polypeptide through the membrane and as the membrane anchor site.