Restriction of vesicular stomatitis virus in a nonpermissive rabbit cell line is at the level of protein synthesis

Growth restriction of negative-strand-RNA viruses in a rat 3Y1 cell line

A rat fibroblast cell line, 3Y1 is nonpermissive for infection by several negative-strand-RNA viruses including influenza virus A, Sendai virus, Newcastle disease virus and vesicular stomatitis virus (VSV), but not refractory to that of a positive-strand-RNA virus, Sindbis virus. To elucidate the mechanism of the restricted viral growth, we compared the replication pattern of VSV in 3Y1 cells and in baby hamster kidney cells, which are fully permissive for those viruses. The results indicated that the restriction was imposed predominantly on the transcription of the viral RNA. The subsequent steps such as protein synthesis and nucleocapsid assembly appeared to occur normally, although these levels remained low due to the restricted transcription. Virus attachment onto, and penetration into, 3Y1 cells were also not restricted.

Stress-induced increase of hexose transport as a novel index of cytopathic effects in virus-infected cells: role of the L protein in the action of vesicular stomatitis virus

The VSV-specific increase in hexose transport by BHK cells has been measured by assay of the [3H]dGlc/[14C]AIB uptake ratio. The effect was abolished by uv-irradiation of the virus, indicating that viral gene expression is required. Cells infected with the T1026 R1 mutant of VSV, which causes only slight cytopathic changes, exhibited only a slight increase in hexose uptake. Cells infected with temperature-sensitive (ts) mutants of VSV that are defective in the function of the viral N, NS, G, or M proteins at the restrictive temperature (39.5 degrees) exhibited increased [3H]dGLC/[14C]AIB uptake ratios typical of wild-type virus at either restrictive (39.5 degrees) or permissive temperature (34 degrees). Cells infected with a mutant defective in the function of the viral L protein exhibited an increased [3H]dGlc/[14C]AIB uptake ratio at permissive temperature (34 degrees) only; at restrictive temperature (39.5 degrees) the uptake ratio was essentially the same as that of mock-infected cells. Temperature-shift experiments indicated that the effect on hexose transport persisted for at least 6 hr in cells which no longer expressed function L protein, and that when expression of L was restricted to the first 2 hr of infection, an almost complete stimulation of hexose transport was observed 4 hr later. These results indicate that expression of the L gene is a necessary factor for inducing an increased hexose uptake in VSV-infected BHK cells. They also suggest that the action of the L protein on hexose transport is indirect, and is presumably mediated by other cellular constituents. The studies support the concept that an increased dGlc uptake may be a useful index of the cytopathic consequences of virus infection.

Regulation of cardiac myosin synthesis: studies of RNA content in cultured heart cells

Contraction regulates the myosin content and the rate of myosin synthesis in cultured neonatal rat heart cells. To further explore the mechanism for this regulation we examined various parameters of RNA content and RNA synthesis in contacting versus noncontracting myocytes. While contraction stimulated myosin heavy chain (MHC) synthesis by 72% compared to that of KCl-arrested cells, simultaneous analyses of polysome profiles were no different under the two culture conditions. Incorporation of [3H]uridine monophosphate into cellular RNA revealed no change in the rate of total RNA or ribosomal subunits synthesis. In vitro translation of cellular RNA yielded similar incorporation of [35S]methionine into trichloroacetic acid precipitable protein. Specific transcription of the MHC gene was examined by dot-blot analysis and was unaltered by contraction. Northern blot analysis of the MHC sequences detected by a cDNA probe revealed an mRNA sequence corresponding to a molecular weight of approximately 30 S. These data suggest that RNA synthesis and RNA content are unaltered by contraction in cultured heart cells and therefore the changes in myosin synthesis may be mediated at a post-transcriptional control level.

Dominance of temperature-sensitive phenotypes. I. Studies of the mechanism of inhibition of the growth of wild-type vesicular stomatitis virus

It has been reported previously that temperature-sensitive (ts) mutants of vesicular stomatitis virus (VSV) with an RNA- phenotype interfere with the growth of wild-type (wt)-VSV at both the permissive (34 degrees) and nonpermissive (39.5 degrees) temperatures (J. S. Youngner and D. O. Quagliana (1976) J. Virol. 19, 102-107). Investigation of the mechanism of this interference has revealed the following information. In double infection with RNA- ts mutants and wt-VSV, the cumulative synthesis of viral RNA is inhibited. By varying the relative multiplicities of the two viruses, it was observed that the level of RNA synthesis reflects the level of interference with wt-VSV growth. Although viral RNA synthesis was severely compromised in double infections, this inhibition was not at the level of primary transcription or the translation of primary transcripts. Rather, secondary transcription and genome RNA replication were drastically reduced. Sequential infection with wt-VSV and the ts mutants revealed that there is an early point in the replication cycle of wt-VSV (1 to 2 hr) after which the ts mutants can no longer interfere with the growth of wt-VSV. Ultraviolet irradiation of ts G 41, a mutant belonging to complementation group IV, was used to determine the target size of the interference function. The calculated value for the target size was very close to the target size of the N gene. Additional experiments showed that RNA+ ts mutants representing complementation groups III and V also were able to interfere with the growth of wt-VSV.

Vaccinia-mediated rescue of encephalomyocarditis virus from the inhibitory effects of interferon

Coinfection of mouse L cells with vaccinia virus rescues encephalomyocarditis virus (EMC) from the inhibitory effect of interferon (IFN). The vaccinia-mediated rescue of EMC growth increases the yield of EMC as much as 1000-fold and is optimum when vaccinia is used at a multiplicity of infection of 1. This rescue correlates with a vaccinia-dependent stimulation of EMC gene expression. Evidence is presented to indicate that the rescue by vaccinia does not involve a block of the 2'-5'A synthetase pathway. However, the vaccinia rescue function is correlated with a vaccinia-mediated inhibition of the IFN-induced protein kinase.

Regulation of protein synthesis in virus-infected animal cells

This chapter summarizes the structural features that govern the translation of viral mRNAs: where the synthesis of a protein starts and ends, how many proteins can be produced from one mRNA, and how efficiently. It focuses on the interplay between viral and cellular mRNAs and the translational machinery. That interplay, together with the intrinsic structure of viral mRNAs, determines the patterns of translation in infected cells. It also points out some possibilities for translational regulation that can only be glimpsed at present, but are likely to come into focus in the future. The mechanism of selecting the initiation site for protein synthesis appears to follow a single formula. The translational machinery displays a certain flexibility that is exploited more frequently by viral than by cellular mRNAs. Although some of the parameters that determine efficiency have been identified, how efficiently a given mRNA will be translated cannot be predicted by summing the known parameters.

Evidence for the presence of an inhibitor on ribosomes in mouse L cells infected with mengovirus

After infection of mouse L cells with mengovirus, there is a rapid inhibition of protein synthesis, a concurrent disaggregation of polysomes, and an accumulation of 80S ribosomes. These 80S ribosomes could not be chased back into polysomes under an elongation block. The infected-cell 80S-ribosome fraction contained twice as much initiator methionyl-tRNA and mRNA as the analogous fraction from uninfected cells. Since the proportion of 80S ribosomes that were resistant to pronase digestion also increased after infection, these data suggest that the accumulated 80S ribosomes may be in the form of initiation complexes. The specific protein synthetic activity of polysomal ribosomes also decreased with time of infection. However, the transit times in mock-infected and infected cells remained the same. Cell-free translation systems from infected cells reflected the decreased protein synthetic activity of intact cells. The addition of reticulocyte initiation factors to such systems failed to relieve the inhibition. Fractionation of the infected-cell lysate revealed that the ribosomes were the predominant target affected. Washing the infected-cell ribosomes with 0.5 M KCI restored their translational activity. In turn, the salt wash from infected-cell ribosomes inhibited translation in lysates from mock-infected cells. The inhibitor in the ribosomal salt wash was temperature sensitive and micrococcal nuclease resistant. A model is proposed wherein virus infection activates (or induces the synthesis of) an inhibitor that binds to ribosomes and stops translation after the formation of the 80S-ribosome initiation complex but before elongation. The presence of such an inhibitor on ribosomes could prevent them from being remobilized into polysomes in the presence of an inhibitor of polypeptide elongation.

Characterization of the infections of permissive and nonpermissive cells by host range mutants of vesicular stomatitis virus defective in RNA methylation

Two host range mutants of VSV, hr 1 and hr 8, which, unlike the wild-type virus, have a mRNA methylation defect and direct the in vitro synthesis of full-length capped but unmethylated viral mRNAs have been described previously (S.M. Horikami and S.A. Moyer, 1982, Proc. Natl, Acad. Sci. USA 79, 7694-7698). It is shown that the in vivo nonpermissive infection of HEp-2 cells by either of these two mutants is characterized by the reduced synthesis of full-length mRNAs at levels characteristic of primary transcription and the total lack of synthesis of genome-length RNA. The VSV mRNAs synthesized by either mutant in HEp-2 cells are not translated either in vivo or in vitro in mRNA-dependent rabbit reticulocyte lysates. Subsequent isolation and analysis of the mRNAs from infected HEp-2 cells has shown that the 5' termini of the messages contain a cap structure which is guanylylated, but unmethylated (GpppA), a finding that might account for the lack of translatability. Hence these mutants are unable to properly methylate mRNAs whether they are synthesized in vitro or in vivo within nonpermissively infected cells. It is also shown that unlike hr 1, the undermethylation of mRNA synthesized by hr 8 is partially reversible by the addition of high levels of AdoMet in vitro. It is interesting to note, therefore, that permissive baby hamster kidney (BHK) cells have a 10-fold higher level of endogenous AdoMet than the nonpermissive HEp-2 cells. Unlike singly infected cells, the coinfection of HEp-2 cells with either hr mutant and a poxvirus yields a permissive infection for these two host range mutants. Analysis of the VSV mRNAs produced in vivo under the conditions of rescue reveals the presence of fully methylated caps (7mGppp(m)Am), suggesting that poxvirus may rescue the mutants by converting the VSV mRNAs to a translationally active form due to methylation by the cytoplasmic poxvirus mRNA methyltransferase enzymes. Both mutants are, however, able to grow normally in permissive BHK cells. An analysis of the translationally active mRNAs from infected permissive cells shows the presence primarily of a 5'-monomethylated cap, 7mGpppA. Finally, we have examined the nonpermissive infections of two other host range mutants of VSV (hr 5 and hr 7). Unlike mutants hr 1 and hr 8 described above, these two mutants synthesize mRNA in HEp-2 cells which is translated both in vivo and in vitro.

Characterization of a specific kinase inhibitory factor produced by vaccinia virus which inhibits the interferon-induced protein kinase

When mouse L cells are infected by vaccinia virus, a specific kinase inhibitory factor is produced which inhibits the interferon-induced, double-stranded RNA-dependent protein kinase (P. Whitaker-Dowling and J. S. Youngner (1983) Virology 131, 128-136). This inhibitory factor appears early in vaccinia infection (90 min) and its production requires protein synthesis. It inhibits the phosphorylation of the alpha subunit of protein synthesis initiation factor eIF-2 and it is active in mixed extracts of IFN-treated cells and vaccinia-infected cells. The vaccinia-mediated inhibition of the IFN-induced protein kinase is not due to a specific phosphatase or a specific protease and can be reversed by the addition of excess double-stranded RNA. Evidence is presented which suggests that the specific kinase inhibitory factor interacts in a stoichiometric manner with the double-stranded RNA which is required for the activation of the interferon-induced protein kinase.

Vaccinia rescue of VSV from interferon-induced resistance: reversal of translation block and inhibition of protein kinase activity

Coinfection with vaccinia virus rescues vesicular stomatitis virus (VSV) from the inhibitory effects of interferon (IFN) in mouse L cells. While vaccinia infection does not significantly affect VSV RNA synthesis, coinfection with vaccinia dramatically increases VSV protein synthesis in IFN-treated cells. Evidence is provided that vaccinia inhibits the activity of the IFN-induced dsRNA-dependent protein kinase.

Superinfection exclusion by vesicular stomatitis virus

The infection of baby hamster kidney (BHK21) cells by the Indiana strain of vesicular stomatitis virus (VSV) causes a rapid loss of the ability of the cells to be superinfected by VSV virions or defective-interfering particles. This exclusion phenomenon is at the level of virus penetration and requires viral gene expression and a functional VSV transmembrane glycoprotein G. Infection with the New Jersey serotype of VSV also inhibits the uptake of the Indiana serotype. However, infection of BHK21 cells with either encephalomyocarditis, Newcastle disease, or influenza A viruses does not inhibit superinfection by VSV.