Posttranslational modification of vaccinia virus proteins

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.

Genetic analysis of the vaccinia virus I6 telomere-binding protein uncovers a key role in genome encapsidation

The linear, double-stranded DNA genome of vaccinia virus contains covalently closed hairpin termini. These hairpin termini comprise a terminal loop and an A+T-rich duplex stem that has 12 extrahelical bases. DeMasi et al. have shown previously that proteins present in infected cells and in virions form distinct complexes with the telomeric hairpins and that these interactions require the extrahelical bases. The vaccinia virus I6 protein was identified as the protein showing the greatest specificity and affinity for interaction with the viral hairpins (J. DeMasi, S. Du, D. Lennon, and P. Traktman, J. Virol. 75:10090-10105, 2001). To gain insight into the role of I6 in vivo, we generated eight recombinant viruses bearing altered alleles of I6 in which clusters of charged amino acids were changed to alanine residues. One allele (temperature-sensitive I6-12 [tsI6-12]) conferred a tight ts phenotype and was used to examine the stage(s) of the viral life cycle that was affected at the nonpermissive temperature. Gene expression, DNA replication, and genome resolution proceeded normally in this mutant. However, proteolytic processing of structural proteins, which accompanies virus maturation, was incomplete. Electron microscopic studies confirmed a severe block in morphogenesis in which immature, but no mature, virions were observed. Instead, aberrant spherical virions and large crystalloids were seen. When purified, these aberrant virions were found to have normal protein content but to be devoid of viral DNA. We propose that the binding of I6 to viral telomeres directs genome encapsidation into the virus particle.

Analysis of a vaccinia virus mutant expressing a nonpalmitylated form of p37, a mediator of virion envelopment

Vaccinia virus encodes a 37-kDa palmitylated protein (p37) that is required for envelopment, translocation, and cell-to-cell spread of virions. We have analyzed the biological significance of the palmitate modification by constructing a recombinant vaccinia virus that expresses a nonpalmitylated p37 and comparing its biological activity to that of the wild-type virus. The mutant virus is inefficient at cell-to-cell spread and does not produce or release enveloped virions, although it produces normal amounts of nonenveloped virions. Furthermore, the mutant virus is not able to nucleate actin to propel itself through and out of the cell, a function requiring the indirect participation of p37. The deficiency in protein function appears to result from a lack of appropriate targeting to the membranes of the trans-Golgi network (TGN) which leaves p37 soluble in the cytoplasm. We conclude that the palmitate moiety is necessary for targeting or anchoring p37 to the TGN membrane, where, along with other vaccinia virus-encoded proteins, p37 is involved in the complex process of virion envelopment and release.

Identification and analysis of three myristylated vaccinia virus late proteins

Previous studies have shown that at least three vaccinia virus (VV) late proteins (with apparent molecular asses of 37, 35, and 25 kDa) label with myristic acid. Time course labeling of VV-infected cells with [3H]myristic acid reveals at least three additional putative myristylproteins, with apparent molecular masses of 92, 17, and 14 kDa. The 25-kDa protein has previously been identified as that encoded by the L1R open reading frame, leaving the identities of the remaining proteins to be determined. Sequence analysis led to the preliminary identification of the 37-, 35-, and 17-kDa proteins as G9R, A16L, and E7R, respectively. Using synthetic oligonucleotides and PCR techniques, each of these open reading frames was amplified by using VV DNA as a template and then cloned individually into expression vectors behind T7 promoters. These plasmid constructs were then transcribed in vitro, and the resulting mRNAs were translated in wheat germ extracts and radiolabeled with either [35S]methionine or [3H]myristic acid. Each wild-type polypeptide was labeled with [35S]methionine or [3H]myristic acid in the translation reactions, while mutants containing an alanine in place of glycine at the N terminus were labeled only with [35S]methionine, not with myristic acid. This result provided strong evidence that the open reading frames had been correctly identified and that each protein is myristylated on a glycine residue adjacent to the initiating methionine. Subcellular fractionations of VV-infected cells suggested that A16L and E7R are soluble, in contrast to L1R, which is a membrane-associated protein.

Isolation and analysis of vaccinia virus previrions

Vaccinia virus (VV) virion morphogenesis is a complex sequence of events that occurs late in viral infection that is essential for the production of mature progeny. Electron microscopy studies have identified multiple morphogenic forms of virus particles, apparently assembled in a sequence from immature to mature particles that correlates with distinct physical changes. This assembly process is, however, rather poorly understood at the molecular level. To better characterize the multiple forms of VV previrions, sucrose log gradient fractionation of VV-infected cells was used to separate radiolabeled immature and mature forms of the virus. Depending on time postinfection that the infected cells were harvested, four distinct peaks of acid-precipitable counts could be detected that displayed different rates of sedimentation. Using pulse-chase analysis procedures, the labeled peaks were shown to have precursor-product relationships as slower sedimenting entities chased to faster sedimenting ones with time. These peaks were referred to as A, B, C, and V particles, with A being the initial precursor form found near the top of the gradient and V being the fastest sedimenting product. As the previrions mature, they migrated faster in the gradient and became infectious and resistant to treatment with DNase I. The core protein composition of the A particles was predominantly uncleaved precursors, with only small amounts of the mature core proteins 4a, 4b, 25K, and 23K evident. However, as the sedimentation rate of the particles increased, proteolytic maturation proceeded such that C particles were composed almost exclusively of mature core proteins. Together these results indicate that several distinct and separable forms of VV previrions exist, that VV core protein precursors are associated with the previrions prior to cleavage, and that maturation of the core proteins is coordinately linked to the conversion from noninfectious previrions to infectious viral particles.

Further characterization of an adenosine-containing modification of vaccinia virus proteins

Three vaccinia virus (VV) core proteins which become labeled when virus is grown in the presence of radiolabeled adenosine or orthophosphate were identified as the major viral core proteins 4A, 4B, and 25K on the basis of comigration with [35S]methionine-labeled viral proteins and immunoprecipitation with monospecific polyclonal antisera. Boronate affinity chromatography and HPLC analysis suggested that a cis-diol-containing adenosine compound is present on this set of viral proteins. The replication of VV in tissue culture cells was prevented by the ADP-ribosylation inhibitors nicotinamide (NIC), 3-aminobenzamide (3-AB), and meta-iodobenzylguanidine (MIBG). None of these compounds significantly affected viral DNA synthesis at lower drug concentrations, although at higher concentrations of the three drugs a reduction in viral DNA synthesis was evident. Total VV protein synthesis also decreased at higher inhibitor levels, and the proteolytic processing of the major virion core proteins was greatly diminished as well. The three inhibitors also affected labeling of viral core proteins and cellular histone proteins by [8-14C]adenosine. In addition, mature, infectious virus particles were not formed in the presence of either 60 mM NIC or 3-AB, or 0.6 mM MIBG. These results provide evidence that the major VV core proteins are subject to modification by an adenosine compound, and suggest the possibility that this modification might represent ADP-ribosylation.

Evidence for multiple species of vaccinia virus-encoded palmitylated proteins

When cells were infected with vaccinia virus in the presence of [3H]palmitic acid, radiolabel was incorporated into six viral proteins with apparent molecular weights of 92, 41, 37, 26, 17, and 14 kDa, all of which are expressed at late times during the infection cycle. The [3H]palmitate-labeled fatty acid moieties from the modified proteins were isolated, converted to p-nitrophenacyl derivatives, and subjected to reverse phase HPLC analysis which confirmed the identity of the fatty acid group as palmitic acid. Furthermore, the radiolabeled palmitate-protein bonds were sensitive to treatment with neutral hydroxylamine, suggesting that association of the fatty acid moieties with these proteins occurs via a thioester linkage. Previous studies by other investigators have identified the 37-kDa protein as the major antigen present in the outer membrane of extracellular enveloped virions, and demonstrated that the protein is modified by palmitic acid but is not glycosylated (G. Hiller and K. Weber J. Virol. (1985) 55, 651-659). Growth of vaccinia virus in the presence of tunicamycin indicated that the 41- and 26-kDa palmitylated proteins were also subject to modification by glycosylation, whereas like the 37-kDa protein, the 92-, 17-, and 14-kDa species did not appear to be glycosylated. Subcellular fractionation studies provided evidence that all of the viral palmitylated proteins were membrane-associated. Extraction of purified vaccinia virus with NP-40 and DTT demonstrated that the palmitylated proteins were associated with one of the viral membranes rather than the core of the virion. Viewing these results together with the previous reports of myristylated VV proteins (Franke et al. J. Virol. (1989) 63, 4285-4291), suggests that acylation of VV proteins represents a major modification pathway utilized by VV proteins during the assembly of progeny virions.

The vaccinia virus B1R gene product is a serine/threonine protein kinase

The nucleotide sequence of the vaccinia virus open reading frame B1 predicts a polypeptide with significant sequence similarity to the catalytic domain of known protein kinases. To determine whether the B1R polypeptide is a protein kinase, we have expressed it in bacteria as a fusion with glutathione S-transferase. Affinity-purified preparations of the fusion protein were found to undergo autophosphorylation and also phosphorylated the exogenous substrates casein and histone H1. Mutation of lysine 41 to glutamine within the conserved kinase catalytic domain II abrogated protein kinase activity on all three protein substrates, supporting the notion that the protein kinase activity is inherent to the B1R polypeptide. Casein and histone H1 were phosphorylated on serine and threonine residues. The B1R fusion protein was phosphorylated on a threonine residue(s) by an apparently intramolecular mechanism. The autophosphorylation reaction resulted in phosphorylation of the glutathione S-transferase portion of the fusion and not the protein kinase domain. The protein kinase activity of B1R was specific for ATP as the phosphate donor; GTP was not utilized to a detectable extent. Immunoblotting experiments with anti-B1R antiserum showed that the protein kinase is located in the virion particle. Chromatography of virion extracts resulted in separation of the B1R protein kinase from the bulk of the total protein kinase activity, indicating that multiple protein kinases are present in the virion particle and that B1R is distinct from the previously described vaccinia virus-associated protein kinase.

Identification and characterization of vaccinia virus genes encoding proteins that are highly antigenic in animals and are immunodominant in vaccinated humans

Vaccinia virus (VV) is a potent immunogen, but the nature of VV proteins involved in the activation of the immune response of the host is not yet known. By screening a lambda gt11 expression library of rabbitpox virus DNA with serum from humans vaccinated against smallpox or with serum from VV-immunized animals, we identified several VV genes that encode highly antigenic viral proteins with molecular masses of 62, 39, 32, 25, 21, and 14 kDa. It was found that VV proteins of 62, 39, 25, and 21 kDa are part of the virus core, while proteins of 32 and 14 kDa are part of the virus envelope. All of these proteins were synthesized at late times postinfection. Proteins of 62 and 25 kDa were produced by cleavage of larger precursors of 95 kDa (p4a) and 28 kDa, respectively. The 21-kDa protein was the result of a cleavage of p4a, presumably at amino acid Gly-697. DNA sequence analysis, in comparison with the known nucleotide sequence of VV, provided identification of the corresponding open reading frames. Expression of the viral genes in Escherichia coli was used to monitor which of the viral antigens elicit immunodominant responses and the location of antigenic domains. Three viral antigens of 62, 39, and 32 kDa exhibited immunodominant characteristics. The most antigenic sites of 62 and 39 kDa were identified at the N terminus (amino acids 132 to 295) and C terminus (last 103 amino acids), respectively. Immunization of mice with the 62-, 39-, or 14-kDa antigenic proteins conferred different degrees of protection from VV challenge. Proteins of 32 and 14 kDa induced cellular proliferative responses in VV-infected mice. Our findings demonstrate the nature of VV proteins involved in the activation of host immune responses after vaccination, provide identification of the viral gene locus, and define structural and immunological properties of these antigenic VV proteins.

The multistep proteolytic maturation pathway utilized by vaccinia virus P4a protein: a degenerate conserved cleavage motif within core proteins

The most abundant vaccinia virus (VV) core protein found within the virion is protein 4a, which represents approximately 14% of the particle's dry weight. The 4a protein is synthesized as a 102.5-kDa precursor, which is proteolytically processed to a 62-kDa product concomitant with virion assembly. To identify the pathway by which P4a is converted into 4a, immunological reagents which are specific for subregions of the P4a precursor were developed and used in concert with peptide mapping and protein sequencing procedures. The results obtained suggest that the 891 amino acid P4a precursor is cleaved at two locations, between residues 614 and 615 and 697 and 698. Both the large amino-terminal 4a protein (residues 1-614) and the carboxy-terminal-derived 23-kDa protein (residues 698-891) become major virion constituents. The location and fate of the small internal peptide (residues 615-697) is not known. Interestingly, an analysis of the predicted amino acid sequences at the sites of cleavage within the P4a precursor indicated the presence of an Ala-Gly decreases Thr motif flanking the 697-698 site and an Ala-Gly decreases Ser motif flanking the 614-615 site. Since both of these signals are quite similar to the Ala-Gly decreases Ala signal previously identified as the cleavage point within the VV P4b and P25K core protein precursors (VanSlyke et al., 1991.J. Gen. Virol. 72, 411-416), this suggests that processing of all three core protein precursors may be coordinately linked and/or catalyzed by the same proteinase during viral assembly.

Vaccinia virus: a tool for research and vaccine development

Vaccinia virus is no longer needed for smallpox immunization, but now serves as a useful vector for expressing genes within the cytoplasm of eukaryotic cells. As a research tool, recombinant vaccinia viruses are used to synthesize biologically active proteins and analyze structure-function relations, determine the targets of humoral- and cell-mediated immunity, and investigate the immune responses needed for protection against specific infectious diseases. When more data on safety and efficacy are available, recombinant vaccinia and related poxviruses may be candidates for live vaccines and for cancer immunotherapy.