Specific proteins synthesized during the viral lytic cycle in vaccinia virus-infected HeLa cells: analysis by high-resolution, two-dimensional gel electrophoresis

Translation of mRNAs from vesicular stomatitis virus and vaccinia virus is differentially blocked in cells with depletion of eIF4GI and/or eIF4GII

Cytolytic viruses abrogate host protein synthesis to maximize the translation of their own mRNAs. In this study, we analyzed the eukaryotic initiation factor (eIF) 4G requirement for translation of vesicular stomatitis virus (VSV) and vaccinia virus (VV) mRNAs in HeLa cells using two different strategies: eIF4G depletion by small interfering RNAs or cleavage of eIF4G by expression of poliovirus 2A protease. Depletion of eIF4GI or eIF4GII moderately inhibits cellular protein synthesis, whereas silencing of both factors has only a slightly higher effect. Under these conditions, the extent of VSV protein synthesis is similar to that of nondepleted control cells, whereas VV expression is substantially reduced. Similar results were obtained when eIF4E was depleted. On the other hand, eIF4G cleavage by poliovirus 2A protease strongly inhibits translation of VV protein expression, whereas translation directed by VSV mRNAs is not abrogated, even though VSV mRNAs are capped. Therefore, the requirement for eIF4F activity is different for VV and VSV, suggesting that the molecular mechanism by which their mRNAs initiate their translation is also different. Consistent with these findings, eIF4GI does not colocalize with ribosomes in VSV-infected cells, while eIF2alpha locates at perinuclear sites coincident with ribosomes.

Identification of the major membrane and core proteins of vaccinia virus by two-dimensional electrophoresis

Vaccinia virus assembly has been well studied at the ultrastructural level, but little is known about the molecular events that occur during that process. Towards this goal, we have identified the major membrane and core proteins of the intracellular mature virus (IMV). Pure IMV preparations were subjected to Nonidet P-40 (NP-40) and dithiothreitol (DTT) treatment to separate the core proteins from the membrane proteins. These proteins were subsequently separated by two-dimensional (2D) gel electrophoresis, and the major polypeptide spots, as detected by silver staining and 35S labeling, were identified by either matrix-assisted laser desorption/ionization mass spectrometry, N-terminal amino acid sequencing, or immunoprecipitation with defined antibodies. Sixteen major spots that partitioned into the NP-40-DTT-soluble fraction were identified; 11 of these were previously described virally encoded proteins and 5 were cellular proteins, mostly of mitochondrial origin. The core fraction revealed four major spots of previously described core proteins, two of which were also detected in the membrane fraction. Subsequently, the NP-40-DTT-soluble and -insoluble fractions from purified virus preparations, separated by 2D gels, were compared with postnuclear supernatants of infected cells that had been metabolically labeled at late times (6 to 8 h) postinfection. This relatively short labeling period as well as the apparent shutoff of host protein synthesis allowed the selective detection in such postnuclear supernatants of virus-encoded proteins. These postnuclear supernatants were subsequently treated with Triton X-114 or with sodium carbonate to distinguish the membrane proteins from the soluble proteins. We have identified the major late membrane and nonmembrane proteins of the IMV as they occur in the virus as well as in infected cells. This 2D gel map should provide an important reference for future molecular studies of vaccinia virus morphogenesis.

Effects of poliovirus 2A(pro) on vaccinia virus gene expression

The effects of transient expression of poliovirus 2A(pro) on p220 cleavage in COS cells have been analyzed. When 2A(pro) was cloned in plasmid pTM1 and transiently expressed in COS cells, efficient cleavage of p220 occurred after infection of these cells with a recombinant vaccinia virus bearing phage T7 RNA polymerase. High numbers of COS cells were transfected with pTM1-2A, as judged by p220 cleavage, thereby allowing an analysis of the effects of poliovirus 2A(pro) on vaccinia virus gene expression. A 40-50% cleavage of p220 by transfected poliovirus 2A(pro) was observed ten hours post infection and cleavage was almost complete (80-90%) 20-25 hours post infection with vaccinia virus. Profound inhibition of vaccinia virus protein synthesis was detectable ten hours post infection and was maximal 20-25 hours post infection. This inhibition resulted from neither a blockade of transcription of vaccinia virus nor a lack of translatability of the mRNAs present in cells that synthesize poliovirus 2A(pro). Addition of ara-C inhibited the replication of vaccinia virus and allowed the continued synthesis of cellular proteins. Under these conditions, 2A(pro) is expressed and blocks cellular translation. Finally, p220 cleavage by 2A(pro) did not inhibit the translation of a mRNA encoding poliovirus protein 2C, as directed by the 5' leader sequences of encephalomiocarditis virus. Therefore, these findings show a correlation between p220 cleavage and inhibition of translation from newly made mRNAs. Our results are discussed in the light of present knowledge of p220 function, and new approaches are considered that might provide further insights into the function(s) of initiation factor eIF-4F.

Protein mutations revealed by two-dimensional electrophoresis

High-resolution two-dimensional electrophoresis (2DE) can resolve many hundreds of proteins present in complex mixtures depending on the method of detection. These proteins can be characterised qualitatively, with respect to their electrophoretic mobilities (i.e. charge and apparent molecular mass) and quantitatively, using densitometry, to determine their amounts. There has been a widespread application of 2DE in the analysis and characterisation of protein mutations for a range of organisms. This review presents examples of the use of 2DE to study naturally occurring protein mutations and polymorphisms as well as the characterisation of induced protein mutations in prokaryotes and eukaryotes. Examples are presented to illustrate the use of 2DE to detect mutations affecting the electrophoretic mobility and biosynthesis of individual proteins as well as mutations leading to global alterations in cellular protein synthesis. The advantages and disadvantages of 2DE in the detection of protein mutations are discussed.

The vaccinia virus K2L gene encodes a serine protease inhibitor which inhibits cell-cell fusion

In certain circumstances, cells infected with vaccinia virus (VV) undergo fusion, but this does not occur in tissue cultures infected with wild-type VV. The VV genome includes three genes (B24R, B13R, and K2L) encoding polypeptides that are structurally related to members of the plasma serine proteases inhibitor (SPI) superfamily. In this study, we demonstrate by deleting these genes singly or in combination that the K2L gene encoding SPI-3, but not the B24R or B13R genes encoding SPI-1 and SPI-2, inhibits cell-cell fusion in VV-infected cells. A VV-encoded hemagglutinin (HA) has previously been demonstrated to inhibit cell-cell fusion, but fusion-promoting VVs with K2L gene deletions had normal expression and cellular location of the VV HA. As both HA and SPI-3 independently inhibit cell-cell fusion in VV-infected cells, there must be at least two fusion-promoting mechanisms encoded by VV. These may play different roles in virus-cell fusion and in cell-cell fusion after VV infection.

Misfolding and aggregation of newly synthesized proteins in the endoplasmic reticulum

As a part of our studies on the folding of glycoproteins in the ER, we analyzed the fate of viral glycoproteins that have misfolded either spontaneously or through inhibition of N-linked glycosylation. Newly synthesized Semliki Forest virus spike glycoproteins E1 and p62 and influenza hemagglutinin were studied in infected and transfected tissue culture cells. Misfolded proteins aggregated in less than 1 min after release from polysomes and aberrant interchain disulfide bonds were formed immediately. When more than one protein was misfolded, mixed aggregates were generated. This indicated that the formation of complexes was nonspecific, random, and not restricted to products from single polysomes. The size of the aggregates varied from small oligomers to complexes of several million daltons. BiP was associated noncovalently with the aggregates and with some of the nonaggregated products. We conclude that aggregation reflects the poor solubility of incompletely folded polypeptide chains.

Isolation and characterization of mutants of vaccinia virus with a modified 94-kDa inclusion protein

We have characterized one of the most highly expressed genes of vaccinia virus, WR strain, in the wild type and in several spontaneous mutants isolated from persistently infected cells. This gene encodes the 94-kDa inclusion protein, which is the vaccinia virus counterpart of the 160-kDa A-type inclusion (ATI) protein of cowpox virus. The homology index between both genes is greater than 95%. A deletion of two consecutive adenylate residues is responsible for a frameshift mutation and premature translational termination in the vaccinia virus gene. In addition, several point mutations and small deletions occur in the 94K gene. The deduced protein contains 725 amino acids, and 4 of the 10 repeated motifs present in the carboxyl terminus of the cowpox virus 160-kDa protein are conserved. In several mutants independently isolated from untreated and interferon-treated persistently infected cells, the gene encodes a 40-kDa protein. In mutant 87-4, this truncated protein is due to the insertion of a cytidilate residue that produces a frameshift mutation and premature translational termination. The deduced protein contains 366 amino acids and has lost all the repetitions. Transcriptional analysis has shown that the steady-state levels of mRNAs in cells infected with the mutants or wild-type vaccinia virus are similar. However, the accumulation of this protein in cells infected with the mutants is reduced indicating some instability. In addition the mutated protein is not recognized by polyclonal antisera. Existence of tandemly repeated sequences at the carboxyl terminus of this family of inclusion proteins correlates with their antigenicity. These results indicate a high degree of mutability of the ATI gene and products, which apparently has no consequence on replication in vitro, but could have relevance to control of the infection by immune responses in animal hosts.

Hemadsorption and fusion inhibition activities of hemagglutinin analyzed by vaccinia virus mutants

Vaccinia virus IHD-J strain induces hemagglutinin (HA) on the surface membrane of infected cells and does not elicit cell-cell fusion (F-). We isolated 21 independent hemadsorption-negative (HAD-) mutant viruses from IHD-J and five HAD+ revertants from one of these mutants. Of the 21 mutants, 19 that synthesized either no or little HA at the cell surface caused cell-cell fusion (F+), whereas none of the five revertants that synthesized HA at the cell surface induced cell-cell fusion. Furthermore, anti-HA monoclonal antibody B2D10 induced extensive polykaryocytosis of IHD-J-infected cells and suppressed the ability of the IHD-J-infected cell extract to inhibit the polykaryocytosis induced by IHD-W. The other 2 of the 21 HAD- mutants, B1 and A2, which induced HAs at the cell surface, showed F- and F+ phenotype, respectively. The HA molecule of mutant B1 had a single amino acid substitution of Lys for Glu-121 in its extracellular domain, whereas that of mutant A2 had a single substitution mutation of Tyr for Cys-103. We conclude that the vaccinia HA is a fusion inhibition protein, that the active sites for the two activities reside separately in its extracellular domain, and that cysteine-103 is important in forming the proper tertiary structure of the protein to exert both activities.

Identification of the point mutations in two vaccinia virus nucleoside triphosphate phosphohydrolase I temperature-sensitive mutants and role of this DNA-dependent ATPase enzyme in virus gene expression

The biological function of the nucleoside triphosphate phosphohydrolase I (NTPase I) enzyme of vaccinia virus is not yet known. In this investigation we have identified the genetic lesion of two temperature-sensitive mutants of vaccinia virus, ts50 and ts36, as single point mutations contained within the 5'615 nucleotides of the NTPase I gene (ts50, G to A at position 131; ts36, C to T at position 556). The point mutations result in amino acid substitutions of Gly to Glu-44 (ts50) and Pro to Ser-186 (ts36). In monkey BSC-40 cells, ts50 and ts36 behave phenotypically like wild-type virus with respect to replication and synthesis of viral DNA but are defective in late polypeptide synthesis. However, these two ts mutants displayed a drastically different phenotype in virus-infected human HeLa cells at the restrictive temperature; viral DNA replication did not occur and late polypeptide synthesis was absent. Moreover, if the early block was overcome by a temperature shift-up, then HeLa cells infected with the ts mutants displayed a profile characteristic of defective late viral polypeptide synthesis. Our results reveal that vaccinia NTPase I enzyme functions early and late in the viral replication cycle and that the phenotype of these ts mutants is dependent upon the cell type.

Characterization of vaccinia virus early promoters and evaluation of their informational content

We have reported the isolation of cis-acting regulatory DNA sequences promoting expression of the herpes virus thymidine kinase gene in vaccinia virus recombinants. In this work we show that each of the inserts from recombinants VpT25, 28, 36 and 56 contains a vaccinia virus early promoter. The position of each of the early RNA start sites in the nucleotide sequence of these four vaccinia virus inserts was precisely mapped by an S1 nuclease mapping procedure. Among the four recombinants analysed only VpT56-infected cells also contained a substantial amount of a transcript with the same 5' end at late period. The insert present in VpT25 contained a new late RNA start site 50 nucleotides upstream from that of the early RNA. The four inserts were mapped on the vaccinia virus genome. We also localized the 5' end of the mRNA of a vaccinia virus host-range gene, whose DNA nucleotide sequence has recently been established. The 45 nucleotides preceding the RNA start site from most of 19 known vaccinia virus early promoters were found to be A + T-rich (at least 80%) and contained shorter A-rich (at least 60%) regions, beginning approximately 25 nucleotides upstream from the RNA start site. The information content, as expressed by the parameter Rsequence, of early vaccinia virus promoters revealed ten bits of information in the sequence of 28 nucleotides upstream from the early RNA start sites. Most of the information needed to locate an early promoter is contained within the nucleotide sequence upstream from an RNA start site. A consensus sequence consists of two blocks: the sequence AA(A/T)N(T/A)N(A/G)AAAANAANA starting at position -27 and the sequence (T/A)(C/T)N(A/T)T(A/G) starting at position -5. It was concluded that vaccinia virus early promoters may be characterized by an A + T-rich region of approximately 45 nucleotides preceding the RNA start site and include a specific 3'-terminal sequence of 28 nucleotides containing at least ten bits of information. A procedure for localizing putative early RNA start sites in nucleotide sequences is proposed.

Structural and functional studies of a 39,000-Mr immunodominant protein of vaccinia virus

Little is known about the nature of poxvirus proteins involved in the host immune response. Screening a lambda gt11 expression library of genomic rabbit poxvirus DNA with hyperimmune rabbit anti-vaccinia virus serum and selection of monospecific antibodies identified a highly antigenic viral protein of about 39,000 molecular weight (39K protein). The same-size protein of vaccinia virus was also identified with a monoclonal antibody (MAb B6) obtained from hybridomas generated after fusion of hyperimmunized mouse spleen cells with mouse myeloma cells. Structural analysis revealed that the 39K protein is an acidic polypeptide, that it can exist in two molecular forms because of intramolecular disulfide linkages, and that it is part of the virus core. This protein shares antigenic determinants with a cytoplasmic component(s) from uninfected cells. Functional studies revealed that the 39K protein is synthesized at late times postinfection and appears to be required for virus assembly. This protein is highly conserved in members of the Orthopoxvirus group, but in cowpox virus, a 41K virion protein was specifically recognized by antibodies that reacted against the vaccinia virus 39K protein. Significantly, during long-term passages of Friend erythroleukemia cells persistently infected with vaccinia virus, some virus mutants were found to increase or decrease by about 2 kilodaltons the size of the 39K protein. Mapping analysis localized sequences encoding the 39K protein in a rifampin-sensitive gene cluster between the two major core-associated viral polypeptides, 4a and 4b. The fact that the 39K core protein of vaccinia virus elicits strong humoral immune response, induces antibodies that react against a host component(s), and is subjected to genetic variability suggests that this protein has important biological functions.

Proteins synthesized in African swine fever virus-infected cells analyzed by two-dimensional gel electrophoresis

At least 74 acidic and 37 basic proteins are synthesized in African swine fever virus (ASFV)-infected monkey cells not detected in uninfected cells analyzed by two-dimensional gel electrophoresis. Essentially all the proteins synthesized early during infection are also observed at late times. The use of inhibitors such as cycloheximide and phosphonoacetate has led to the identification of 34 immediate early and 13 delayed early polypeptides. Therefore 64 proteins were classified as late polypeptides. Several ASFV-induced proteins are phosphorylated as proteins a1, a4, a20, a41, a48, a49, a51, a52, a55, a58, a67, b2, b12, b28, and b32.

A 14,000-Mr envelope protein of vaccinia virus is involved in cell fusion and forms covalently linked trimers

A monoclonal antibody, MAbC3, that reacts with a 14,000-molecular-weight envelope protein (14K protein) of vaccinia virus completely inhibited virus-induced cell fusion during infection. Immunoblot and immunofluorescence studies revealed that the 14K protein was synthesized at about 6 to 7 h postinfection and transported from the cytoplasm to the cell surface. Synthesis and transport of the 14K protein during infection occurred in the presence of rifampin, an inhibitor of virus maturation. One- and two-dimensional gel electrophoretic analyses demonstrated that the 14K protein forms largely trimers (42K) that are covalently linked by disulfide bonds. The facts that MAbC3 prevents virus uncoating and blocks virus-induced cell fusion but does not prevent virus attachment to cells and the 14K envelope protein forms trimers all suggest that this protein plays major role in virus penetration.