Regulation of protein synthesis in vesicular stomatitis virus-infected mouse L-929 cells by decreased protein synthesis initiation factor 2 activity

Computational multigene interactions in virus growth and infection spread

Viruses persist in nature owing to their extreme genetic heterogeneity and large population sizes, which enable them to evade host immune defenses, escape antiviral drugs, and adapt to new hosts. The persistence of viruses is challenging to study because mutations affect multiple virus genes, interactions among genes in their impacts on virus growth are seldom known, and measures of viral fitness are yet to be standardized. To address these challenges, we employed a data-driven computational model of cell infection by a virus. The infection model accounted for the kinetics of viral gene expression, functional gene-gene interactions, genome replication, and allocation of host cellular resources to produce progeny of vesicular stomatitis virus, a prototype RNA virus. We used this model to computationally probe how interactions among genes carrying up to eleven deleterious mutations affect different measures of virus fitness: single-cycle growth yields and multicycle rates of infection spread. Individual mutations were implemented by perturbing biophysical parameters associated with individual gene functions of the wild-type model. Our analysis revealed synergistic epistasis among deleterious mutations in their effects on virus yield; so adverse effects of single deleterious mutations were amplified by interaction. For the same mutations, multicycle infection spread indicated weak or negligible epistasis, where single mutations act alone in their effects on infection spread. These results were robust to simulation in high- and low-host resource environments. Our work highlights how different types and magnitudes of epistasis can arise for genetically identical virus variants, depending on the fitness measure. More broadly, gene-gene interactions can differently affect how viruses grow and spread.

Characterizing Cellular Responses During Oncolytic Maraba Virus Infection

The rising demand for powerful oncolytic virotherapy agents has led to the identification of Maraba virus, one of the most potent oncolytic viruses from Rhabdoviridae family which displays high selectivity for killing malignant cells and low cytotoxicity in normal cells. Although the virus is readied to be used for clinical trials, the interactions between the virus and the host cells is still unclear. Using a newly developed interferon-sensitive mutant Maraba virus (MG1), we have identified two key regulators of global translation (4E-BP1 and eIF2α) as being involved in the regulation of protein synthesis in the infected cells. Despite the translational arrest upon viral stress, we showed an up-regulation of anti-apoptotic Bcl-xL protein that provides a survival benefit for the host cell, yet facilitates effective viral propagation. Given the fact that eIF5B canonically regulates 60S ribosome subunit end joining and is able to replace the role of eIF2 in delivering initiator tRNA to the 40S ribosome subunit upon the phosphorylation of eIF2α we have tested whether eIF5B mediates the translation of target mRNAs during MG1 infection. Our results show that the inhibition of eIF5B significantly down-regulates the level of Bcl-xL steady-state mRNA, thus indirectly attenuates viral propagation.

An inhibitor of eIF2 activity in the sRNA pool of eukaryotic cells

Eukaryotic protein synthesis is a multi-step and highly controlled process that includes an early initiation complex containing eukaryotic initiation factor 2 (eIF2), GTP, and methionine-charged initiator methionyl-tRNA (met-tRNAi). During studies to reconstruct formation of the ternary complex containing these molecules, we detected a potent inhibitor in low molecular mass RNA (sRNA) preparations of eukaryotic tRNA. The ternary complex inhibitor (TCI) was retained in the total sRNA pool after met-tRNAi was charged by aminoacyl tRNA synthetase, co-eluted with sRNA by size exclusion chromatography, but resolved from met-tRNAi by ion exchange chromatography. The adverse effect of TCI was not overcome by high GTP or magnesium omission and was independent of GTP regeneration. Rather, TCI suppressed the rate of ternary complex formation, and disrupted protein synthesis and the accumulation of heavy polymeric ribosomes in reticulocyte lysates in vitro. Lastly, a component or components in ribosome depleted cell lysate significantly reversed TCI activity. Since assembly of the met-tRNAi/eIF2/GTP ternary complex is integral to protein synthesis, awareness of TCI is important to avoid confusion in studies of translation initiation. A clear definition of TCI may also allow a better appreciation of physiologic or pathologic situations, factors, and events that control protein synthesis in vivo.

New mRNAs are preferentially translated during vesicular stomatitis virus infection

During vesicular stomatitis virus (VSV) infection, host protein synthesis is inhibited, while synthesis of viral proteins increases. VSV infection causes inhibition of host transcription and RNA transport. Therefore, most host mRNAs in the cytoplasm of infected cells were synthesized before infection. However, viral mRNAs are synthesized throughout infection and are newer than preexisting host mRNAs. To determine if the timing of appearance of mRNAs in the cytoplasm affected their translation during VSV infection, we transfected reporter mRNAs into cells at various times relative to the time of infection and measured their rate of translation in mock- and VSV-infected cells. We found that translation of mRNAs transfected during infection was not inhibited but that translation of mRNAs transfected prior to infection was inhibited during VSV infection. Based on these data, we conclude that the timing of viral mRNA appearance in the cytoplasm is responsible, at least in part, for the preferential translation of VSV mRNAs. A time course measuring translation efficiencies of viral and host mRNAs showed that the translation efficiencies of viral mRNAs increased between 4 and 8 h postinfection, while translation efficiencies of host mRNAs decreased. The increased translation efficiency of viral mRNAs occurred in cells infected with an M protein mutant virus that is defective in host shutoff, demonstrating that the enhanced translation of viral mRNA is genetically separable from inhibition of translation of host mRNA.

Model-based design of growth-attenuated viruses

Live-virus vaccines activate both humoral and cell-mediated immunity, require only a single boosting, and generally provide longer immune protection than killed or subunit vaccines. However, growth of live-virus vaccines must be attenuated to minimize their potential pathogenic effects, and mechanisms of attenuation by conventional serial-transfer viral adaptation are not well-understood. New methods of attenuation based on rational engineering of viral genomes may offer a potentially greater control if one can link defined genetic modifications to changes in virus growth. To begin to establish such links between genotype and growth phenotype, we developed a computer model for the intracellular growth of vesicular stomatitis virus (VSV), a well-studied, nonsegmented, negative-stranded RNA virus. Our model incorporated established regulatory mechanisms of VSV while integrating key wild-type infection steps: hijacking of host resources, transcription, translation, and replication, followed by assembly and release of progeny VSV particles. Generalization of the wild-type model to allow for genome rearrangements matched the experimentally observed attenuation ranking for recombinant VSV strains that altered the genome position of their nucleocapsid gene. Finally, our simulations captured previously reported experimental results showing how altering the positions of other VSV genes has the potential to attenuate the VSV growth while overexpressing the immunogenic VSV surface glycoprotein. Such models will facilitate the engineering of new live-virus vaccines by linking genomic manipulations to controlled changes in virus gene-expression and growth.

Inhibition of host and viral translation during vesicular stomatitis virus infection. eIF2 is responsible for the inhibition of viral but not host translation

In cells that allow replication of vesicular stomatitis virus (VSV), there are two phases of translation inhibition: an early block of host translation and a later inhibition of viral translation. We investigated the phosphorylation of the alpha subunit of the eIF2 complex during these two phases of viral infection. In VSV-infected cells, the accumulation of phosphorylated (inactivated) eIF2alpha did not begin until well after host protein synthesis was inhibited, suggesting that it only plays a role in blocking viral translation later after infection. Consistent with this, cells expressing an unphosphorylatable eIF2alpha showed prolonged viral protein synthesis without an effect on host protein synthesis inhibition. Induction of eIF2alpha phosphorylation at early times of viral infection by treatment with thapsigargin showed that virus and host translation are similarly inhibited, demonstrating that viral and host messages are similarly sensitive to eIF2alpha phosphorylation. A recombinant virus that expresses a mutant matrix protein and is defective in the inhibition of host and virus protein synthesis showed an altered phosphorylation of eIF2alpha, demonstrating an involvement of viral protein function in inducing this antiviral response. This analysis of eIF2alpha phosphorylation, coupled with earlier findings that the eIF4F complex is modified earlier during VSV infection, supports a temporal/kinetic model of translation control, where at times soon after infection, changes in the eIF4F complex result in the inhibition of host protein synthesis; at later times, inactivation of the eIF2 complex blocks VSV protein synthesis.

Vesicular stomatitis virus infection alters the eIF4F translation initiation complex and causes dephosphorylation of the eIF4E binding protein 4E-BP1

Vesicular stomatitis virus (VSV) modulates protein synthesis in infected cells in a way that allows the translation of its own 5'-capped mRNA but inhibits the translation of host mRNA. Previous data have shown that inactivation of eIF2alpha is important for VSV-induced inhibition of host protein synthesis. We tested whether there is a role for eIF4F in this inhibition. The multisubunit eIF4F complex is involved in the regulation of protein synthesis via phosphorylation of cap-binding protein eIF4E, a subunit of eIF4F. Translation of host mRNA is significantly reduced under conditions in which eIF4E is dephosphorylated. To determine whether VSV infection alters the eIF4F complex, we analyzed eIF4E phosphorylation and the association of eIF4E with other translation initiation factors, such as eIF4G and the translation inhibitor 4E-BP1. VSV infection of HeLa cells resulted in the dephosphorylation of eIF4E at serine 209 between 3 and 6 h postinfection. This time course corresponded well to that of the inhibition of host protein synthesis induced by VSV infection. Cells infected with a VSV mutant that is delayed in the ability to inhibit host protein synthesis were also delayed in dephosphorylation of eIF4E. In addition to decreasing eIF4E phosphorylation, VSV infection also resulted in the dephosphorylation and activation of eIF4E-binding protein 4E-BP1 between 3 and 6 h postinfection. Analysis of cap-binding complexes showed that VSV infection reduced the association of eIF4E with the eIF4G scaffolding subunit at the same time as its association with 4E-BP1 increased and that these time courses correlated with the dephosphorylation of eIF4E. These changes in the eIF4F complex occurred over the same time period as the onset of viral protein synthesis, suggesting that activation of 4E-BP1 does not inhibit translation of viral mRNAs. In support of this idea, VSV protein synthesis was not affected by the presence of rapamycin, a drug that blocks 4E-BP1 phosphorylation. These data show that VSV infection results in modifications of the eIF4F complex that are correlated with the inhibition of host protein synthesis and that translation of VSV mRNAs occurs despite lowered concentrations of the active cap-binding eIF4F complex. This is the first noted modification of both eIF4E and 4E-BP1 phosphorylation levels among viruses that produce capped mRNA for protein translation.

Cytopathogenesis and inhibition of host gene expression by RNA viruses

Many viruses interfere with host cell function in ways that are harmful or pathological. This often results in changes in cell morphology referred to as cytopathic effects. However, pathogenesis of virus infections also involves inhibition of host cell gene expression. Thus the term "cytopathogenesis," or pathogenesis at the cellular level, is meant to be broader than the term "cytopathic effects" and includes other cellular changes that contribute to viral pathogenesis in addition to those changes that are visible at the microscopic level. The goal of this review is to place recent work on the inhibition of host gene expression by RNA viruses in the context of the pathogenesis of virus infections. Three different RNA virus families, picornaviruses, influenza viruses, and rhabdoviruses, are used to illustrate common principles involved in cytopathogenesis. These examples were chosen because viral gene products responsible for inhibiting host gene expression have been identified, as have some of the molecular targets of the host. The argument is made that the role of the virus-induced inhibition of host gene expression is to inhibit the host antiviral response, such as the response to double-stranded RNA. Viral cytopathogenesis is presented as a balance between the host antiviral response and the ability of viruses to inhibit that response through the overall inhibition of host gene expression. This balance is a major determinant of viral tissue tropism in infections of intact animals.

Translational control of viral gene expression in eukaryotes

As obligate intracellular parasites, viruses rely exclusively on the translational machinery of the host cell for the synthesis of viral proteins. This relationship has imposed numerous challenges on both the infecting virus and the host cell. Importantly, viruses must compete with the endogenous transcripts of the host cell for the translation of viral mRNA. Eukaryotic viruses have thus evolved diverse mechanisms to ensure translational efficiency of viral mRNA above and beyond that of cellular mRNA. Mechanisms that facilitate the efficient and selective translation of viral mRNA may be inherent in the structure of the viral nucleic acid itself and can involve the recruitment and/or modification of specific host factors. These processes serve to redirect the translation apparatus to favor viral transcripts, and they often come at the expense of the host cell. Accordingly, eukaryotic cells have developed antiviral countermeasures to target the translational machinery and disrupt protein synthesis during the course of virus infection. Not to be outdone, many viruses have answered these countermeasures with their own mechanisms to disrupt cellular antiviral pathways, thereby ensuring the uncompromised translation of virion proteins. Here we review the varied and complex translational programs employed by eukaryotic viruses. We discuss how these translational strategies have been incorporated into the virus life cycle and examine how such programming contributes to the pathogenesis of the host cell.

Molecular mechanisms of virus-mediated cytopathology

Lytic virus infections of animal cells usually lead to a variety of morphological and biochemical lesions that include inhibition of cellular macromolecular syntheses. These cytopathic effects vary in intensity for different virus-cell combinations and probably involve several overlapping mechanisms. Inhibition may be mediated by components of parental virions or require viral gene expression. In many infected cell systems the initiation of host protein synthesis is selectively blocked. This shut-off phenomenon can result from changes in membrane permeability that alter the intracellular ionic environment in favour of viral expression, successful competition of viral mRNAs for limited translational components, or a decrease in the level of cell mRNAs by inhibition of synthesis or nucleocytoplasmic transport. However, the early onset and rapidity of virus-induced inhibition, sometimes under non-permissive conditions, implies more direct mechanisms of translational inactivation. These include enhanced degradation of cellular mRNAs or specific modification of the translation apparatus in infected cells. A dramatic example of the latter occurs in poliovirus-infected HeLa cells in which intact, functional cellular mRNA persists but host protein synthesis is almost completely inhibited. The virus-induced defect is apparently related to inactivation of a protein factor that binds to the 5' end of m7G-capped mRNAs and is required for translation of host (capped) mRNAs but not for the expression of poliovirus RNA, which is not capped. This process and other possible molecular mechanisms of virus-mediated cytopathology are discussed.

Mutations in the hydrophobic domain of poliovirus protein 3AB abrogate its permeabilizing activity

Poliovirus protein 3AB contains a predicted amphipathic helix that could lead to pore formation in membranes. We have introduced various mutations in the hydrophobic domain of the protein and the membrane-modifying properties of the resulting mutants have been analyzed. Expression of wild type 3AB protein in E. coli increases the influx and efflux of different molecules such as nucleosides, lactose analogues and antibiotics. Thus, 3AB expression makes E. coli cells two orders of magnitude more sensitive to hygromycin B, a non-permeant inhibitor of translation, and causes a 15-20-fold enhancement in the efflux of uridine. Changes in membrane permeability take place under conditions where no cellular lysis is detected and when other molecules such as beta-galactosidase or polyribonucleotides are kept inside the cell. These membrane modifications can be blocked to different extents by amino acid substitutions in the membrane-spanning region of the protein. These results suggest that poliovirus protein 3AB could possess an intrinsic ability to form pores in natural membranes, thus allowing the flux of small hydrophylic molecules through them.

Effect of vesicular stomatitis virus matrix protein on host-directed translation in vivo

Vesicular stomatitis virus infection causes a rapid and potent inhibition of both host transcription and translation. Recently, the viral matrix (M) protein was shown to inhibit host-directed transcription in vivo in the absence of any other viral component (B. L. Black and D. S. Lyles, J. Virol. 66:4058-4064, 1992). The goal of this study was to determine the effect of M protein on host-directed translation. In vitro-transcribed mRNAs encoding M protein and chloramphenicol acetyltransferase (CAT) were cotransfected into BHK cells to determine the effect of M protein expression on translation of CAT mRNA. The results presented here show that M protein did not inhibit host-directed translation of CAT mRNA. On the contrary, this study gave the unexpected result that M protein actually stimulated host-directed translation under the same conditions in which it potently inhibited host-directed transcription. Under these conditions, the combined effect on host gene expression was a greater-than-20-fold inhibition. Furthermore, the enhancement of host translation mediated by M protein was genetically correlated with M protein's ability to inhibit host transcription. Thus, the results of this study establish that M protein does not inhibit host protein synthesis under the same conditions in which it potently inhibits host transcription and suggest that the inhibition of transcription and that of translation by vesicular stomatitis virus require separate viral gene products.

5' sequence of vesicular stomatitis virus N-gene confers selective translation of mRNA

The infection of cells by vesicular stomatitis virus results in the rapid inhibition of host-cell protein synthesis, but not of viral protein synthesis. To determine if this translational selectivity might be conferred by the viral mRNA, we constructed a plasmid (pUCLN beta-4) containing the 5' end of the viral nucleocapsid (N)-gene, including the ribosome binding site, fused in frame with the gene encoding beta-galactosidase, and compared it to a control plasmid (pMC1924) containing the cellular rabbit beta-globin gene 5' end fused with the beta-galactosidase encoding gene. Both plasmids contained identical promoter and 3' nontranslated regions and expressed similar levels of beta-galactosidase in the indicator cell line 293. In cells transfected with either plasmid, viral infection resulted in a approximately 70% decrease in protein synthesis by five hours. The level of beta-galactosidase from cells transfected with pMC1924 also decreased concomitantly with the decrease in total protein synthesis. However, the level of beta-galactosidase from cells transfected with pUCLN beta-4 was not affected by viral infection. Our data suggest that sequences in the 5' end of the viral mRNA allow for the selective translation of the viral message in the presence of an inhibited translational machinery.

Protein synthesis in vaccinia virus-infected cells. I. Effect of hypertonic shock recovery

Human Hep-2 cells were submitted to hypertonic shock (210 mM NaCl) to block host protein synthesis before infection with vaccinia virus. With the start of infection, the medium isotonicity (116 mM NaCl) was restored, and the effect of viral infection on the recovery of host polyribosomes and protein synthesis was studied. Although host translation blockage was released together with infection, vaccinia virus did not affect immediately host protein synthesis. During the first hour of recovery, infected cells could perfectly rebuild the polyribosome profile and recuperate the rate of protein synthesis. Also, during recovery, formation of the initiation complex for protein synthesis was not affected by viral infection. In this period, viral mRNA and proteins were detected by slot blot and SDS-polyacrylamide gel electrophoresis. The inhibitory effect of vaccinia virus on host translation was observed after the second hour of infection. These findings suggest that vaccinia virus-mediated shutoff occurs in a later period during infection, in parallel with viral mRNA accumulation in the polyribosomes and after the on-set of viral DNA replication.

Lack of direct correlation between p220 cleavage and the shut-off of host translation after poliovirus infection

Poliovirus induces a drastic inhibition of host protein synthesis soon after infection of susceptible cells. The correlation between this inhibition and the cleavage of p220, a polypeptide that forms part of protein synthesis initiation factor elF-4F, has been examined in detail. Measurements of protein synthesis at half-hourly intervals after infection with poliovirus show the lack of direct correlation between p220 cleavage and the blockade of cellular translation. Moreover, the use of inhibitors of poliovirus RNA synthesis helped to dissociate those two events more clearly. Thus, in the presence of guanidine or Ro 09-0179 when little shut-off was induced by poliovirus extensive proteolytic degradation of p220 took place. When HeLa cells infected with poliovirus are placed at 28 degrees the inhibition of host protein synthesis is prevented and cellular translation continues for at least 8 hr, albeit at a reduced level compared to cells incubated at 37 degrees. At 28 degrees, cleavage of p220 is observed and about 80% of p220 is degraded after 6 hr of incubation at that temperature. Strikingly, when cells in which more than 50% of p220 is cleaved are shifted to 37 degrees, cellular translation recuperates to 100%, in spite of the fact that no detectable p220 is present. Furthermore, if poliovirus-infected cells are incubated for 2 hr at 37 degrees to permit the cleavage of p220 and then are shifted to 28 degrees in the presence of guanidine, cellular proteins are synthesized at the same level as uninfected HeLa cells incubated at 28 degrees. These results show that translation of cellular mRNAs takes place in cells containing a cleaved p220 and indicate that this cleavage is not directly responsible for the shut-off of host translation induced by poliovirus.

The vaccinia virus K3L gene product potentiates translation by inhibiting double-stranded-RNA-activated protein kinase and phosphorylation of the alpha subunit of eukaryotic initiation factor 2

Interferon resistance of vaccinia virus is mediated by specific inhibition of phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF-2 alpha) by the double-stranded-RNA-activated (DAI) protein kinase. Vaccinia virus encodes a homolog of eIF-2 alpha, K3L, the deletion of which renders the virus sensitive to interferon treatment. We have studied the mechanism by which this protein product elicits interferon resistance in a transient DNA transfection system designed to evaluate regulators of eIF-2 alpha phosphorylation. In this system, translation of a reporter gene mRNA is inefficient because of eIF-2 phosphorylation mediated by the DAI protein kinase. Cotransfection of the K3L gene enhances translation of the reporter mRNA in this system. The K3L protein inhibits eIF-2 alpha phosphorylation and DAI kinase activation, apparently without being phosphorylated itself. Inhibition of protein synthesis, elicited by expression of a mutant Ser-51----Asp eIF-2 alpha designed to mimic a phosphorylated serine, is not relieved by the presence of K3L, suggesting that K3L cannot bypass a block imposed by eIF-2 alpha phosphorylation. The results suggest that K3L acts as a decoy of eIF-2 alpha to inhibit DAI kinase autophosphorylation and activation. Another vaccinia virus gene product, K1L, which is required for growth of vaccinia virus on human cells, does not enhance translation in this assay.

Protein synthesis initiation factor modifications during viral infections: implications for translational control

Infection of tissue culture cells with certain viruses results in the shutoff of host cell protein synthesis. We have examined virally infected cell lysates using two-dimensional gel electrophoresis and immunoblotting to ascertain whether initiation factor protein modifications are correlated with translational repression. Moderate increases in eukaryotic initiation factor (eIF)-2 alpha phosphorylation are detected in reovirus- and adenovirus-infected cells, as reported previously (Samuel et al., 1984; O'Malley et al., 1989). Neither vesicular stomatitis virus, vaccinia virus, frog virus III, rhinovirus, nor encephalomyocarditis virus caused significantly increased 2 alpha phosphorylation. There were no reproducible, significant changes in eIF-4A, eIF-4B, or eIF-2 beta in cells infected by any of these viruses. The cleavage of eIF-4F subunit p220, such as has been previously demonstrated to occur in poliovirus (Etchison et al., 1982) and rhinovirus (Etchison and Fout, 1985), was not detected in any of the other virus infections analyzed.

Frog virus 3-induced translational shut-off: activation of an eIF-2 kinase in virus-infected cells

Infection of susceptible fathead minnow or Friend erythroleukemia cells with either infectious or heat-inactivated frog virus 3 led to the rapid inhibition of cellular protein synthesis. As seen in other cells, translational shut-off was accompanied by the dissociation of polysomes, but not the degradation of irreversible inactivation of cellular mRNAs. In addition, lysates from cells infected with heat-inactivated FV3 showed a reduced capacity to synthesize protein and to form 43S pre-initiation complexes in vitro. These results indicate that the in vitro systems accurately reflected in vivo events, and suggest that translational shut-off occurred prior to the union of the 40S ribosomal subunit and the [eIF-2.GTP.Met tRNAi] ternary complex. To determine the basis for the translational block, lysates from mock- and FV3-infected cells were assayed in vitro for their ability to phosphorylate the alpha subunit of eIF-2. In contrast to lysates from mock-infected cells, lysates from cells infected with heat-inactivated or infectious FV3 readily phosphorylated the alpha subunit of eIF-2. Since phosphorylation of the alpha subunit of eIF-2 inhibits its catalytic utilization during polypeptide chain initiation, these findings suggest that translational shut-off mediated by FV3 may be due to activation of a kinase that selectively phosphorylates this key initiation factor.

Inhibitory effects of vesicular stomatitis virus on cellular and influenza viral RNA metabolism and protein synthesis

Infection with vesicular stomatitis virus (VSV) results in the rapid inhibition of cellular macromolecular synthesis, including transcription, translation, and maturation of the U1 and U2 snRNPs. Unlike infection with VSV, influenza virus infection did not result in the inhibition of either the processing of U1 and U2 snRNAs or the assembly of the RNPs. Although influenza virus relies on the cellular splicing apparatus to generate viral mRNAs, the maturation of snRNPs was inhibited during double infections with VSV. However, this inhibition of snRNP maturation had no effect on the splicing of a cellular pre mRNA in extracts prepared from VSV-infected HeLa cells. Thus, the effects of VSV on the processing and assembly of snRNPs appear to involve virus-specific functions and unique cellular targets. Coinfection with VSV and influenza virus resulted in the dramatic inhibition of influenza virus transcription; polyadenylated mRNAs corresponding to the influenza virus NP and NS1 proteins could not be detected by Northern blot analysis. However, reduced levels of the influenza virus replicative templates were still synthesized during double infection. Coinfection with VSV also resulted in the inhibition of influenza viral mRNA translation, even when superinfection with VSV was delayed until 3 or 6 hr after influenza virus infection. VSV required at least 2 hr to affect the inhibition of translation; this corresponded to the time after VSV infection when inhibition of cellular protein synthesis was evident. These results demonstrate that, in contrast to adenovirus, the VSV-mediated inhibition of cellular macromolecular synthesis may be effective against influenza virus.

Influenza virus regulates protein synthesis during infection by repressing autophosphorylation and activity of the cellular 68,000-Mr protein kinase

We investigated the mechanisms by which influenza virus prevents shutoff of protein synthesis by a cellular protein kinase normally activated during infection. Earlier work has shown that influenza virus superinfection of cells previously infected by the adenovirus VAI RNA-negative mutant dl331 resulted in selective translation of influenza virus mRNAs and suppression of the elevated protein kinase levels normally found in cells infected by the mutant alone (M. G. Katze, B. M. Detjen, B. Safer, and R. M. Krug, Mol. Cell. Biol. 6:1741-1750, 1986). We elucidated the mechanisms of this kinase repression and can now report that influenza virus encodes a gene product which functions to directly block the autophosphorylation and activity of the interferon-induced, double-stranded-RNA-activated protein kinase, P68. Suppressed P68 activity was found not only in doubly infected cells but also in cells infected by influenza virus alone. Moreover, the decrease in P68 activity correlated with a decrease in the endogenous levels of phosphorylation of the alpha subunit of the eucaryotic initiation factor eIF-2, the natural substrate of the protein kinase. Suppression of P68 activity occurred as early as 2 h after influenza virus infection and required viral gene expression beyond the level of primary mRNA transcription to take place. We confirmed our in vivo observations with in vitro mixing experiments which showed that the influenza virus inhibitor can act in trans to block P68 activity. Combined repression of P68 function and eIF-2 alpha phosphorylation during influenza virus infection is essential for continued catalytic recycling of eIF-2 and efficient mRNA translation.

Vaccinia virus stimulates the growth of vesicular stomatitis virus at the level of protein synthesis in mouse L cells

Coinfection with vaccinia virus increases the growth of vesicular stomatitis virus (VSV) in mouse L cells by 10- to 20-fold. Although vaccinia has no significant effect on RNA synthesis by VSV, VSV protein synthesis is dramatically stimulated by double infection. The enhancement of VSV growth is correlated with the ability of vaccinia to inhibit the VSV-mediated damage to the host translational machinery. Coinfection with vaccinia fails to stimulate the growth of a VSV mutant which is deficient in its ability to shut off protein synthesis during infection.

Inhibition of adenovirus DNA replication by vesicular stomatitis virus leader RNA

Vesicular stomatitis virus (VSV) leader RNA and a synthetic oligodeoxynucleotide of the same sequence were found to inhibit the replication of adenovirus DNA in vitro. In contrast, the small RNA transcribed by the VSV defective interfering particle DI-011 did not prevent adenovirus DNA replication. The inhibition produced by leader RNA was at the level of preterminal protein (pTP)-dCMP complex formation, the initiation step of adenovirus DNA replication. Initiation requires the adenovirus pTP-adenovirus DNA polymerase complex (pTP-Adpol), the adenovirus DNA-binding protein, and nuclear factor I. Specific replication in the presence of leader RNA was restored when the concentration of adenovirus-infected or uninfected nuclear extract was increased or by the addition of purified pTP-Adpol or HeLa cell DNA polymerase alpha-primase to inhibited replication reactions. Furthermore, the activities of both purified DNA polymerases could be inhibited by the leader sequence. These results suggest that VSV leader RNA is the viral agent responsible for inhibition of adenovirus and possibly cellular DNA replication during VSV infection.

Dominance of temperature-sensitive phenotypes. II. Vesicular stomatitis virus mutants from a persistent infection interfere with shut-off of host protein synthesis by wild-type virus

The dominance of a mutant (VSV-PI) isolated from a long-term persistent infection over wild-type vesicular stomatitis virus (wt-VSV) is reported. This dominance has some important differences from and similarities to the dominance of conventional ts mutants studied previously (J. S. Youngner, D. W. Frielle, and P. Whitaker-Dowling, 1986, Virology 155, 225-235). Unlike the ts mutants representing complementation groups I and IV, coinfection with VSV-PI does not reduce the yield of infectious wt-VSV at either the permissive (37 degrees) or nonpermissive (39.5 degrees) temperatures. However, in double infections with wt-VSV and VSV-PI at 37 degrees, viral RNA synthesis patterns were converted to those of the RNA synthesis phenotype of VSV-PI: reduced mRNA transcription and enhanced replication of genomic RNA. In addition, VSV-PI which shuts off host protein synthesis very inefficiently was able to interfere in double infections with the ability of wt-VSV to rapidly shut off host protein synthesis. This finding suggests that the mutant virus is not just missing the factor(s) responsible for the inhibition of host protein synthesis but has a dominant activity which works in trans to interfere with the shut-off function of wt-VSV. Ultraviolet irradiation of VSV-PI was used to determine the target size of the interference function. The calculated value for the uv target size is equal to that of the viral genome. This suggests that either viral replication or the expression of the last gene on the viral genome (encoding the L protein) is required for interference by VSV-PI with the shut-off of host cell protein synthesis by wt-VSV.

Translational control by influenza virus: suppression of the kinase that phosphorylates the alpha subunit of initiation factor eIF-2 and selective translation of influenza viral mRNAs

Selective translation of influenza viral mRNAs occurs after influenza virus superinfection of cells infected with the VAI RNA-negative adenovirus mutant dl331 (M. G. Katze, Y.-T. Chen, and R. M. Krug, Cell 37:483-490, 1984). Cell extracts from these doubly infected cells catalyze the initiation of essentially only influenza viral protein synthesis, reproducing the in vivo situation. This selective translation is correlated with a 5- to 10-fold suppression of the dl331-induced kinase that phosphorylates the alpha subunit of eucaryotic initiation factor eIF-2. This strongly suggests that influenza virus encodes a gene product that, analogous to the adenoviral VAI RNA, prevents the shutdown of overall protein synthesis caused by an eIF-2 alpha kinase turned on by viral infection. Adenoviral mRNA translation was restored to the extract from the doubly infected cells by the addition of the guanine nucleotide exchange factor eIF-2B, which is responsible for the normal recycling of eIF-2 during protein synthesis. This indicates that the residual kinase in the doubly infected cells leads to a limitation in functional (nonsequestered) eIF-2B and hence functional (GTP-containing) eIF-2 and that under these conditions influenza viral mRNAs are selectively translated over adenoviral mRNAs. Addition of double-stranded RNA to the extracts from these cells restored the eIF-2 alpha kinase to a level approaching that seen in extracts from cells infected with dl331 alone and caused the inhibition of influenza viral mRNA translation. This suggests that the putative influenza viral gene product acts against the double-stranded RNA activation of the kinase and indicates that influenza viral mRNA translation is also linked to the level of functional eIF-2. Our results thus indicate that a limitation in functional eIF-2 which causes a nonspecific reduction in the rate of initiation of protein synthesis results in the preferential translation of the better mRNAs (influenza viral mRNAs) at the expense of the poorer mRNAs (adenoviral mRNAs).

Translational control by influenza virus: suppression of the kinase that phosphorylates the alpha subunit of initiation factor eIF-2 and selective translation of influenza viral mRNAs

Selective translation of influenza viral mRNAs occurs after influenza virus superinfection of cells infected with the VAI RNA-negative adenovirus mutant dl331 (M. G. Katze, Y.-T. Chen, and R. M. Krug, Cell 37:483-490, 1984). Cell extracts from these doubly infected cells catalyze the initiation of essentially only influenza viral protein synthesis, reproducing the in vivo situation. This selective translation is correlated with a 5- to 10-fold suppression of the dl331-induced kinase that phosphorylates the alpha subunit of eucaryotic initiation factor eIF-2. This strongly suggests that influenza virus encodes a gene product that, analogous to the adenoviral VAI RNA, prevents the shutdown of overall protein synthesis caused by an eIF-2 alpha kinase turned on by viral infection. Adenoviral mRNA translation was restored to the extract from the doubly infected cells by the addition of the guanine nucleotide exchange factor eIF-2B, which is responsible for the normal recycling of eIF-2 during protein synthesis. This indicates that the residual kinase in the doubly infected cells leads to a limitation in functional (nonsequestered) eIF-2B and hence functional (GTP-containing) eIF-2 and that under these conditions influenza viral mRNAs are selectively translated over adenoviral mRNAs. Addition of double-stranded RNA to the extracts from these cells restored the eIF-2 alpha kinase to a level approaching that seen in extracts from cells infected with dl331 alone and caused the inhibition of influenza viral mRNA translation. This suggests that the putative influenza viral gene product acts against the double-stranded RNA activation of the kinase and indicates that influenza viral mRNA translation is also linked to the level of functional eIF-2. Our results thus indicate that a limitation in functional eIF-2 which causes a nonspecific reduction in the rate of initiation of protein synthesis results in the preferential translation of the better mRNAs (influenza viral mRNAs) at the expense of the poorer mRNAs (adenoviral mRNAs).

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.

Effect of viral infection on host protein synthesis and mRNA association with the cytoplasmic cytoskeletal structure

We studied the association of several eucaryotic viral and cellular mRNAs with cytoskeletal fractions derived from normal and virus-infected cells. We found that all mRNAs appear to associate with the cytoskeletal structure during protein synthesis, irrespective of their 5' and 3' terminal structures: e.g., poliovirus that lacks a 5' cap structure or reovirus and histone mRNAs that lack a 3' poly A tail associated with the cytoskeletal framework to the same extent as capped, polyadenylated actin mRNA. Cellular (actin) and viral (vesicular stomatitis virus and reovirus) mRNAs were released from the cytoskeletal framework and their translation was inhibited when cells were infected with poliovirus. In contrast, actin mRNA was not released from the cytoskeleton during vesicular stomatitis virus infection although actin synthesis was inhibited. In addition, several other conditions under which protein synthesis is inhibited did not result in the release of mRNAs from the cytoskeletal framework. We conclude that the association of mRNA with the cytoskeletal framework is required but is not sufficient for protein synthesis in eucaryotes. Furthermore, the shut-off of host protein synthesis during poliovirus infection and not vesicular stomatitis virus infection occurs by a unique mechanism that leads to the release of host mRNAs from the cytoskeleton.

Inhibition of protein synthesis in vesicular stomatitis virus infected Chinese hamster ovary cells: role of virus mRNA-ribonucleoprotein particle

Although host protein synthesis is preferentially inhibited, there is a steady decline in the ability of Chinese hamster ovary (CHO) cells infected with vesicular stomatitis virus (VSV) to synthesize both host and viral proteins. We previously reported finding an mRNA-ribonucleoprotein particle (mRNP) that contained all five VSV mRNAs and viral N protein exclusively. This particle apparently regulates translation by sequestering a majority of the VSV mRNA made late in infection and thus rendering it unavailable for protein synthesis. In the present investigation the mRNP was also shown to inhibit in vitro protein synthesis in rabbit reticulocyte and wheat germ lysates programmed with mRNA isolated from VSV-infected cells. The synthesis of eIF-2 X GTP X Met-tRNA (ternary) complex, the first step in initiation of protein synthesis, was markedly inhibited by the mRNP. The inhibition was partially reversed by addition of purified eIF-2 to the inhibited lysate or ternary complex formation reaction. These results indicate a dual role of the mRNP in regulating protein synthesis during infection. Nucleocapsid also inhibited in vitro protein synthesis, although this inhibition was not reversed by eIF-2. Nucleocapsid did not inhibit ternary complex formation in vitro. Consequently, nucleocapsid may also regulate in vivo protein synthesis, but by a mechanism different from the mRNP.

Rapid inhibition of pinocytosis in baby hamster kidney (BHK-21) cells following infection with vesicular stomatitis virus

Infection of baby hamster kidney cells with vesicular stomatitis virus (VSV) caused a reduced rate of pinocytosis (as judged by the uptake of horseradish peroxidase) after 1 h, and maximum inhibition (60-80%) was observed at 4-6 h. This inhibition occurred 2-3 h before release of virus or changes in cell morphology. Analytical cell fractionation of homogenates of VSV-infected cells indicated that the horseradish peroxidase taken up by pinocytosis was transferred to lysosomes. The inhibition of pinocytosis required viral gene expression: little or no inhibition was detected in cells infected with UV-irradiated virus, wild-type virus in the presence of cycloheximide, or a temperature-sensitive mutant which failed to synthesize viral proteins. When cells were infected with temperature-sensitive viruses with mutations in the five VSV genes, an inhibition of pinocytosis was observed only when the viral transmembrane glycoprotein was present on the surface of the cells.

The leader RNA of vesicular stomatitis virus is bound by a cellular protein reactive with anti-La lupus antibodies

The leader RNA transcript of vesicular stomatitis virus (VSV) has been immunoprecipitated from infected BHK cell extracts by anti-La specific sera from patients with systemic lupus erythematosus (SLE). This association was specific as lupus anti-sera with other specificities failed to precipitate leader RNA. The amount of leader RNA associated with the La antigen peaked 4 hr post infection and then declined. Leader RNA complexed with viral nucleocapsid proteins increased at a slower rate but eventually predominated 6 hr post infection. By 16 hr all of the leader RNA was associated with nucleocapsid proteins. Although a significant portion of the leader RNA was present in isolated nuclei 4 hr post infection, all of the leader RNA outside the nucleus was bound to La protein. Leader RNA is the first non-RNA polymerase III product found to associate with the La protein. The proposed function of the leader-La complex in VSV transcription and replication and in viral cytopathology is discussed.

Protein synthesis in cells infected with Semliki Forest virus is not controlled by intracellular cation changes

Treatment of BHK cells with 1 microM nigericin results in a 55% decrease in K+ and a 3.3-fold increase in intracellular Na+; protein synthesis under these conditions is depressed by 35%. In BHK cells infected with Semliki Forest virus (SFV), protein synthesis is depressed by 76% 6.5 h after infection; intracellular K+ is unchanged, and intracellular Na+ is increased 1.8-fold at this time. These results suggest that the increase in intracellular Na+ in SFV-infected BHK cells does not adequately account for the decrease in protein synthesis, and makes it likely that an increased Na+ concentration is a consequence, not a cause, of alterations in protein synthesis in virally-infected cells. No evidence was obtained for the purported [Alonso, M. A. and Carrasco, L. (1980) Eur. J. Biochem. 109, 535-540; (1981) Eur. J. Biochem. 118, 289-294; (1981) FEBS Lett. 127, 112-114] ability of 1 microM nigericin to permeabilize' cells.

Inhibition of translation in lysates of mouse L cells infected with vesicular stomatitis virus: presence of a defective ribosome-associated factor

Lysates of L cells infected for 4 h with vesicular stomatitis virus were inhibited in their in vitro translational activity to about the same extent as protein synthesis was inhibited in vivo in infected L cells. Inhibition of translation occurred at the level of the ribosome as determined by reciprocal cross-reconstitution studies with polyribosomes and postribosomal supernatant fractions isolated from virus-infected and mock-infected cells. Inhibition of protein synthesis in reconstituted lysates of virus-infected cells was found to be at the level of initiation of translation as evidenced by reduction in incorporation into acid-precipitable proteins of formylatable [35S]methionine. Ribosomes from virus-infected and mock-infected cells were exposed to 0.5 M KCl and fractionated by centrifugation into salt-washed polyribosomes and supernatant fractions containing ribosome-associated proteins; reciprocal reconstitution of translational activity by a mixing of salt-washed polyribosomes and ribosome-associated proteins revealed that the defect in initiation of translation was in the ribosome-associated proteins released by salt wash from the infected-cell ribosomes. Differential ammonium sulfate precipitation of the supernatant ribosome-associated proteins from virus-infected and mock-infected cells indicated by reciprocal reconstitution studies that the defective ribosomal initiation factor(s) was (were) present primarily in the 0-40% ammonium sulfate fraction that is considered to contain primarily eIF-3 and eIF-4B. These results are similar to those found in earlier studies of defective initiation factors responsible for impaired protein synthesis in cells infected with plus-strand viruses quite different from the rhabdovirus studied in these experiments.

Protein synthesis in lysates of Aedes albopictus cells infected with vesicular stomatitis virus

Aedes albopictus cells (clone LT-C7) showed a marked cytopathic effect and inhibition of protein synthesis (both host and viral) after infection with vesicular stomatitis virus (VSV), but only if (i) cultures were incubated at 34 degrees C rather than 28 degrees C and (ii) serum was present in the medium (S. Gillies and V. Stollar, Mol. Cell. Biol. 2:66-75, 1982). To learn more about how protein synthesis is shut off in VSV-infected A. albopictus cells, we have compared cell-free protein synthesis in extracts prepared from VSV-infected cells and control cells. Extracts prepared 6 h after infection from VSV-infected cells maintained at 34 degrees C in the presence of serum reflected what was observed with intact cells in at least two respects: (i) they showed a markedly diminished capacity to carry out protein synthesis (whether directed by endogenous or exogenously added mRNA), and (ii) there was decreased phosphorylation in vitro by [gamma-32P]ATP of a specific ribosomal protein (Gillies and Stollar, Mol. Cell. Biol. 2:66-75, 1982). In addition, and consistent with a block at the level of initiation, the formation of 80S initiation complexes, as measured by binding of VSV 12 to 18S mRNA, was reduced in the inactive extracts. Addition of an S-100 fraction from uninfected cells to the inactive extract reversed each of the aforementioned changes; i.e., it restored protein synthetic activity, it stimulated the formation of 80S initiation complexes, and it increased phosphorylation of the specific ribosomal protein referred to above. The active component in the S-100 fraction was heat labile and non-dialyzable and, upon ammonium sulfate fractionation of the S-100 fraction, was found in the 40 to 70% saturation fraction. Our findings suggest that VSV infection of A. albopictus cells inhibits protein synthesis by inactivating a macromolecular component, probably a protein, in the S-100 fraction which may be involved in the initiation of protein synthesis. More specifically, we suggest that this component is involved in the joining of the ribosomal subunits to form 80S initiation complexes.

Two transcription products of the vesicular stomatitis virus genome may control L-cell protein synthesis

When mouse L-cells are infected with vesicular stomatitis virus, there is a decrease in the rate of protein synthesis ranging from 20 to 85% of that in mock-infected cells. Vesicular stomatitis virus, irradiated with increasing doses of UV light, eventually loses this capacity to inhibit protein synthesis. The UV inactivation curve was biphasic, suggesting that transcription of two regions of the viral genome is necessary for the virus to become inactivated in this capacity. The first transcription product corresponded to about 373 nucleotides, and the second corresponded to about 42 nucleotides. Inhibition of transcription of the larger product by irradiating the virus with low doses of UV light left a residual inhibition of protein synthesis consisting of approximately 60 to 65% of the total inhibition. This residual inhibition could be obviated by irradiating the virus with a UV dose of greater than 20,000 ergs/mm(2) and was thus considered to represent the effect of the smaller transcription product. In the R1 mutant of C. P. Stanners et al. (Cell 11:273-281, 1977), inhibition of transcription of the larger product sufficed to restore protein synthesis to the mock-infected level, suggesting that the smaller transcription product is nonfunctional with respect to protein synthesis inhibition. It thus appears that the inhibition of protein synthesis by wild-type vesicular stomatitis virus involved at least two separate viral transcription products, and the inhibition by the R1 mutant involved only one. Extracts from cells infected with virus irradiated with low doses of UV light showed a protein synthesis capacity quite similar to that of their in vivo counterparts, indicating that these extracts closely reflect the in vivo effects of virus infection.

Translational control of vesicular stomatitis virus protein synthesis: isolation of an mRNA-sequestering particle

An mRNA-ribonucleoprotein particle (mRNP) was found in vesicular stomatitis virus (VSV)-infected Chinese hamster ovary cells. The particle was present 3 and 4.5 h after infection but was barely discernible at 2 h. The mRNP (buoyant density, 1.56 g/cm3), which cosedimented with viral nucleocapsid in a sucrose density gradient at approximately 120 to 160S, was separable from nucleocapsid (buoyant density, 1.31 g/cm3) by CsCl density gradient centrifugation. It contained all five VSV mRNAs and, almost exclusively, viral N protein. Some host mRNA and host protein was also present in the particle. The intact mRNP was incapable of stimulating protein synthesis in an in vitro protein-synthesizing system, although the VSV mRNA isolated from the particle by phenol extraction was functional in vitro. In contrast, intact polysomes stimulated cell-free protein synthesis to the same extent as purified polysomal mRNA. By 4.5 h after infection, 97% of the functional mRNA in vivo was associated with the mRNP, and only 3% was on polysomes. The amount of polysomal mRNA at 4.5 h after infection was only 31% of that found at 2 h after infection; this was reflected by the 76% decrease observed in the rate of in vivo protein synthesis at 4.5 h relative to that found at 2 h. Thus, it appears that the mRNP serves as an organelle which sequesters the large excess of VSV mRNA that is normally made during secondary transcription.

Protein synthesis in lysates of Aedes albopictus cells infected with vesicular stomatitis virus

Aedes albopictus cells (clone LT-C7) showed a marked cytopathic effect and inhibition of protein synthesis (both host and viral) after infection with vesicular stomatitis virus (VSV), but only if (i) cultures were incubated at 34 degrees C rather than 28 degrees C and (ii) serum was present in the medium (S. Gillies and V. Stollar, Mol. Cell. Biol. 2:66-75, 1982). To learn more about how protein synthesis is shut off in VSV-infected A. albopictus cells, we have compared cell-free protein synthesis in extracts prepared from VSV-infected cells and control cells. Extracts prepared 6 h after infection from VSV-infected cells maintained at 34 degrees C in the presence of serum reflected what was observed with intact cells in at least two respects: (i) they showed a markedly diminished capacity to carry out protein synthesis (whether directed by endogenous or exogenously added mRNA), and (ii) there was decreased phosphorylation in vitro by [gamma-32P]ATP of a specific ribosomal protein (Gillies and Stollar, Mol. Cell. Biol. 2:66-75, 1982). In addition, and consistent with a block at the level of initiation, the formation of 80S initiation complexes, as measured by binding of VSV 12 to 18S mRNA, was reduced in the inactive extracts. Addition of an S-100 fraction from uninfected cells to the inactive extract reversed each of the aforementioned changes; i.e., it restored protein synthetic activity, it stimulated the formation of 80S initiation complexes, and it increased phosphorylation of the specific ribosomal protein referred to above. The active component in the S-100 fraction was heat labile and non-dialyzable and, upon ammonium sulfate fractionation of the S-100 fraction, was found in the 40 to 70% saturation fraction. Our findings suggest that VSV infection of A. albopictus cells inhibits protein synthesis by inactivating a macromolecular component, probably a protein, in the S-100 fraction which may be involved in the initiation of protein synthesis. More specifically, we suggest that this component is involved in the joining of the ribosomal subunits to form 80S initiation complexes.