Glycoprotein synthesis in cells infected with vaccinia virus. II. A glycoprotein component of the virion

Vaccinia virus proteolysis--a review

It is well known that viruses, as obligate intracellular parasites, must use their hosts' metabolic machinery in order to replicate their genomes and form infectious progeny virions. What is less well known are the details of how viruses make sure that once all the necessary proteins are made, that they assume the correct configuration at the proper time in order to catalyse the efficient assembly of infectious virions. One of the methods employed by viruses to regulate this process is the proteolytic cleavage of viral proteins. Over the past several decades, studies in numerous laboratories have demonstrated that morphogenic proteolysis plays a major and essential role during the assembly and maturation of infectious poxvirus virions. In this review we describe the history of vaccinia virus proteolysis as a prototypic viral system.

The formation and function of extracellular enveloped vaccinia virus

Vaccinia virus produces four different types of virion from each infected cell called intracellular mature virus (IMV), intracellular enveloped virus (IEV), cell-associated enveloped virus (CEV) and extracellular enveloped virus (EEV). These virions have different abundance, structure, location and roles in the virus life-cycle. Here, the formation and function of these virions are considered with emphasis on the EEV form and its precursors, IEV and CEV. IMV is the most abundant form of virus and is retained in cells until lysis; it is a robust, stable virion and is well suited to transmit infection between hosts. IEV is formed by wrapping of IMV with intracellular membranes, and is an intermediate between IMV and CEV/EEV that enables efficient virus dissemination to the cell surface on microtubules. CEV induces the formation of actin tails that drive CEV particles away from the cell and is important for cell-to-cell spread. Lastly, EEV mediates the long-range dissemination of virus in cell culture and, probably, in vivo. Seven virus-encoded proteins have been identified that are components of IEV, and five of them are present in CEV or EEV. The roles of these proteins in virus morphogenesis and dissemination, and as targets for neutralizing antibody are reviewed. The production of several different virus particles in the VV replication cycle represents a coordinated strategy to exploit cell biology to promote virus spread and to aid virus evasion of antibody and complement.

A vaccinia virus core protein, p39, is membrane associated

We describe herein the characterization of p39, the product of the A4L gene of vaccinia virus. By immunolabelling of thawed cryosections from infected HeLa cells, we show that this protein is initially located in the central region, or viroplasm, of the viral factories, as well as in the immature virions, with very small amounts of labelling observed on the surrounding membranes. The localization of p39 changes dramatically during the transition of the immature virion to the intracellular mature virus (IMV), coincident with the appearance of the core structure in the center of the IMV, with p39 located between this core and the surrounding membranes. Complementary biochemical data, such as partitioning into the Triton X-114 detergent phase and stripping of the viral membranes with Nonidet P-40 and dithiothreitol, suggest that p39 is associated with the innermost of the two membranes surrounding the core. Sodium carbonate treatment also indicates that p39 is associated with membranes, even at the early stages of viral assembly. However, following in vitro translation of p39 in the presence of microsomal membranes, we failed to detect any association of the independently expressed protein with membranes. We also failed to detect any posttranslational acylation of p39 with myristate or palmitate, suggesting that p39 does not achieve its membrane association through lipid anchors. Therefore, p39 is most likely membrane associated through an interaction with an integral membrane protein(s) present in the innermost of the two membranes surrounding the IMV. These data, together with our recent data showing that p39 colocalizes with the spike-like protrusions on the IMV core (N. Roos, M. Cyrklaff, S. Cudmore, R. Blasco, J. Krijnse-Locker, and G. Griffiths, EMBO J. 15:2343-2355, 1996), suggest that p39 may form part of this spike and that it possibly functions as a matrix-like linker protein between the core and the innermost of the two membranes surrounding the IMV.

The role of a 21-kDa viral membrane protein in the assembly of vaccinia virus from the intermediate compartment

We have recently provided morphological evidence that a key event in the assembly of vaccinia virus is the formation of a novel cisternal domain of the intermediate compartment (IC) between the endoplasmic reticulum and the Golgi complex (Sodeik, B., Doms, R. W., Ericsson, M., Hiller, G., Machamer, C. E., van't Hof, W., van Meer, G., Moss, B., and Griffiths, G. (1993) J. Cell Biol. 121, 521-541). This tightly apposed cisternal domain incompletely surrounds the spherical immature virus that matures into the first of the two distinct infectious forms of vaccinia, the intracellular mature virus (IMV). In this study we describe the characterization of an abundant membrane protein of the IMV, the gene product of A17L, a 21-kDa protein that has recently been shown to be essential for the formation of the viral membranes (Rodriguez, D., Esteban, M., and Rodriguez, J. R. (1995) J. Virol. 69, 4640-4648). Upon translation in vitro, p21 associated with rough microsomal membranes in a co-translational manner. Using NH2- and COOH-terminal specific antibodies, we show that both in vitro as well as in vivo, p21 adopts a topology where the NH2 and COOH termini are cytoplasmically orientated. Immunocytochemical experiments demonstrated that p21 is a component of the inner of the two cisternal membranes of the immature virus as well as of membranes of the IC, identified using antibodies against Rab1. Taken together, these data provide the first molecular evidence in support of our assembly model; they show that an essential membrane protein of the IMV inserts into the rough endoplasmic reticulum, but gets efficiently targeted to the IC and membranes of the viral factory.

A prominent antigenic surface polypeptide involved in the biogenesis and function of the vaccinia virus envelope

Polypeptides of the vaccinia virus envelope exposed on the surface were identified by means of sulfo-N-hydroxysuccinimidobiotin as a surface tag. Among surface expressed polypeptides is the 35-kDa antigen, previously designated Ag35. Both monoclonal (mAb) and monospecific affinity pure antibodies directed against Ag35 neutralized vaccinia infectiousness, indicating that this prominent surface antigen has a function during early virus-host cell interactions. The binding of several monoclonal antibodies to various regions of Ag35 was tested by reacting CNBr fragments, derived from the polypeptide, employing Western blotting. All mAbs tested reacted with the same region of Ag35. Estimation of the molecular weights (MW), based on migration of the CNBr peptides in sodium dodecyl sulfate-polyacrylamide gel electrophoresis, revealed that those partial digestion products which contained a proline-rich 99 amino acid limit digest fragment were present at a position approximately 12.5 kDa larger than that predicted from the DNA sequence. By contrast, partial and limit digest products lacking the proline-rich fragment migrated to the MW position expected from the length of the DNA sequence. This observation demonstrates that departure from a predicted 22.3 kDa to an anomalous MW of Ag35 is conferred by the proline-rich peptide. The surface location of Ag35 was confirmed by immune electron microscopy. In a competition test the binding specificity of mAb and affinity-purified antibodies at the surface of virions could be demonstrated. Evidence for an association of Ag35 with the virus envelope at various stages during biogenesis of vaccinia was obtained by immune electron microscopy of whole mounts and thin sections. Presence of Ag35 as an early component of immature and mature virions, probably residing in the bilayer membrane structure was detected. A distinction can, therefore, be made between Ag35 and several other vaccinia envelope polypeptides which are synthesized as late functions and added during late stages of envelope assembly.

Transcription mapping of glycoprotein I (gpI) and gpIV of varicella-zoster virus and immunological analysis of the gpI produced in cells infected with the recombinant vaccinia virus

In order to determine the transcripts of gpI and gpIV of varicella-zoster virus (VZV), RNA was isolated from human embryonic fibroblast cells infected with VZV and Northern blot analysis was carried out using cloned DNA probes of unique short region including gpI and gpIV genes. The analysis of RNA revealed two discrete transcripts of 3.6 and 2.15 kilobases (kb) and three transcripts of 3.6, 2.9, and 1.6 kb which hybridized to DNA probes covering the gpI and gpIV region, respectively. Next, mRNAs were hybrid-selected, translated in vitro and the polypeptide products were immunoprecipitated with antibodies against these glycoproteins. The polypeptides with a molecular weight of 70,000 (70K) and 37K which were in vitro translational products of mRNA hybrid-selected with the DNA clone covering gpI and gpIV were detected using antibodies against gpI and gpIV, respectively. The result showed that the 70K polypeptide is presumably the translational product of 2.15 kb mRNA and the 37K polypeptide is that of 1.6 kb mRNA. DNA fragment encoding gpI or gpIV was inserted into vaccinia virus DNA and the recombinant viruses, mO74 (gpI) and mO39 (gpIV), were used for immunological analysis. In consequence, the gpI derived from cells infected with mO74 showed antigenic characteristics similar to those of gpI from VZV-infected cells as determined from the immunoprecipitation pattern, although the molecular weight of each polypeptide was different, and antibody produced in rabbits infected with recombinant virus had a high neutralizing activity, when the reaction was performed with complement. This suggested that gpI plays an important role for protection and recovery from VZV infection.

Glycosylated components induced in African swine fever (ASF) virus-infected Vero cells

African swine fever (ASF) virus production was inhibited more than 100 fold by 5 mM glucosamine, 2 mM 2-deoxyglucose and 3 microM tunicamycin. ASF virus induced in Vero cells the synthesis of 19 glycosylated components of molecular weights ranging from 9K to 220K, the major ones being those of 9K, 13K, 14K, 74K and 220K. At least five of the induced glycosylated components, of molecular weights 13K, 33K, 34K, 38K and 220K, were probably virus-coded glycoproteins, as suggested by a comparative analysis of the time course of synthesis and the antigenicity of these components in extracts from [35S]methionine or [14C]glucosamine-labeled infected cells. The non-protein glycosylated components present in extracellular ASF virus particles had a cellular origin.

Glycosylated components of African swine fever virus particles

Extracellular African swine fever (ASF) virus particles were specifically agglutinated by several lectins, suggesting the presence of surface glycosylated component(s) containing at least glucose, mannose, or both; galactose, N-acetylgalactosamine, or both; N-acetylneuraminic acid and N-acetylglucosamine, but not fucose. When virions were purified from infected Vero cells labeled with [14C]glucosamine, [14C]galactose and analyzed by polyacrylamide gel electrophoresis, no major structural glycoproteins were detected. However, several species of glycolipids were found when virions were extracted with organic solvents and analyzed by thin layer chromatography. These, plus two minor glycosylated structural components, of apparent mol wt 230K and 95K, could account for the agglutination of ASF virions with concanavalin A.

Golgi-derived membranes that contain an acylated viral polypeptide are used for vaccinia virus envelopment

A 37,000-dalton polypeptide (p37K) present on purified extracellular vaccinia virus but absent from intracellular virus particles of classical morphology (G. Hiller et al., J. Virol. 39:903-913, 1981; L. G. Payne, J. Virol. 27:28-37, 1978) was further characterized. The polypeptide was only expressed in infected cells after onset of viral DNA replication. Phase partition experiments showed that it is relatively hydrophobic. Although p37K apparently is not a glycoprotein, in vivo radioisotope labeling detected tightly associated palmitic acid. Antibodies to p37K were used to monitor its distribution within infected cells at the light and electron microscopic levels. After synthesis p37K first accumulated in the Golgi region due to a tight membrane association. During progressing infection p37K-carrying membranes were used to form double-walled envelopes around brick-shaped vaccinia particles. Within these specialized vesicles vaccinia particles were moved through the cytoplasm toward the cell's surface, presumably along cellular routes for certain secretory products. Finally, single enveloped viruses were released into the extracellular space by an exocytotic process.

Effect of glycosylation inhibitors on the release of enveloped vaccinia virus

Addition of 1 to 10 mM 2-deoxy-D-glucose (2-dg) or glucosamine (gln) to the growth medium of vaccinia virus-infected cells inhibited the release of extracellular enveloped vaccinia virus (EEV) without affecting the production of intracellular naked vaccinia virus (INV) particles. In contrast, INV infectivity (particles per PFU) was decreased sevenfold by 50 mM 2-dg. Treatment with 2-dg reduced but did not eliminate glycosylation of the INV 37,000-molecular-weight glycoprotein. The kinetics of sensitivity to inhibitor addition experiments and inhibitor reversal experiments indicated that EEV release was dependent on glycosylation before 8 h postinfection. This was supported by polyacrylamide gel electrophoretic analysis of the synthesis kinetics for cell membrane-associated vaccinia glycoproteins in 2-dg-inhibited infected cells. The dependence of vaccinia protein glycosylation before 8 h postinfection for efficient EEV release was observed in spite of the fact that the period of greatest glycoprotein synthesis was 8 to 12 h postinfection. The presence of 2-dg resulted in an incompletely glycosylated 89,000-molecular-weight glycoprotein, as indicated by a reduction in the apparent glycoprotein molecular weight. The morphological event affected by the inhibitors was the acquisition by INV of a double-membrane structure from the Golgi apparatus. This morphological intermediate is necessary for release of EEV.

Phosphoprotein component of vaccinia virions

The recent discovery of a protein kinase activity in vaccinia virions led us to search for a viral protein which is phosphorylated in vivo. Vaccinia virus was radioactively labeled by infecting cells in the presence of (32)P(1). A phosphoprotein was isolated from purified delipidated virions by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The phosphoprotein appeared to be a specific viral component induced after infection. More than 60% of the phosphoprotein was associated with viral cores. The electrophoretic mobility of the protein suggested that it has a molecular weight of 11,000 to 12,000. Phosphoserine was liberated by acid hydrolysis and identified by electrophoresis with known standards. Tryptic digests of the purified phosphoprotein were analyzed by two-dimensional electrophoresis and chromatography on thin-layer cellulose plates, and a single major phosphopeptide was resolved. The high selectivity of phosphorylation suggested that the process has a specific function.