Vesicular stomatitis virus mRNA and inhibition of translation of cellular mRNA--is there a P function in vesicular stomatitis virus?

Global analysis of polysome-associated mRNA in vesicular stomatitis virus infected cells

Infection of mammalian cells with vesicular stomatitis virus (VSV) results in the inhibition of cellular translation while viral translation proceeds efficiently. VSV RNA synthesis occurs entirely within the cytoplasm, where during transcription the viral polymerase produces 5 mRNAs that are structurally indistinct to cellular mRNAs with respect to their 5′ cap-structure and 3′-polyadenylate tail. Using the global approach of massively parallel sequencing of total cytoplasmic, monosome- and polysome-associated mRNA, we interrogate the impact of VSV infection of HeLa cells on translation. Analysis of sequence reads in the different fractions shows >60% of total cytoplasmic and polysome-associated reads map to the 5 viral genes by 6 hours post-infection, a time point at which robust host cell translational shut-off is observed. Consistent with an overwhelming abundance of viral mRNA in the polysome fraction, the reads mapping to cellular genes were reduced. The cellular mRNAs that remain most polysome-associated following infection had longer half-lives, were typically larger, and were more AU rich, features that are shared with the viral mRNAs. Several of those mRNAs encode proteins known to positively affect viral replication, and using chemical inhibition and siRNA depletion we confirm that the host chaperone heat shock protein 90 (hsp90) and eukaryotic translation initiation factor 3A (eIF3A)—encoded by 2 such mRNAs—support viral replication. Correspondingly, regulated in development and DNA damage 1 (Redd1) encoded by a host mRNA with reduced polysome association inhibits viral infection. These data underscore the importance of viral mRNA abundance in the shut-off of host translation in VSV infected cells and link the differential translatability of some cellular mRNAs with pro- or antiviral function.

Viruses co-opt the host translational machinery and frequently suppress host cell protein synthesis. Many positive-strand RNA viruses manipulate initiation factors while bypassing their need for viral protein production using internal ribosome entry sites. Negative-strand RNA viruses and DNA viruses produce mRNAs that contain host-like 5′ cap-structures and 3′ polyadenylate tails. Those similarities necessitate a different mechanism for controlling viral versus host protein synthesis. We infected cells with vesicular stomatitis virus and sequenced polysome-associated mRNAs at 2 and 6 hours post-infection providing 2 snapshots of how infection alters translation. We present evidence that the 5 viral mRNAs outcompete cellular mRNAs for ribosomes and demonstrate that individual host mRNAs vary in the extent to which their polysome association is altered by infection. Host mRNAs that are more abundant, have longer half-lives, greater than average length, and a similar AU content to the viral mRNAs were more likely to be enriched among polysome-associated cellular mRNAs. Several of the enriched mRNAs encode proteins that promote viral replication, whereas mRNAs that exhibit the largest decrease in polysome association include those that encode antiviral functions.

The vesicular stomatitis virus matrix protein inhibits NF-κB activation in mouse L929 cells

A previous study found that NF-κB activation is delayed in L929 cells infected with wild-type (wt) strains of VSV, while activation occurred earlier in cells infected with mutant strain T1026R1 (R1) that encodes a mutation in the cytotoxic matrix (M) protein. The integrity of the other R1 proteins is unknown; therefore our goal was to identify the viral component responsible for preventing NF-κB activation in L929 cells. We found that the M protein inhibits viral-mediated activation of NF-κB in the context of viral infection and when expressed alone via transfection, and that the M51R mutation in M abrogates this function. Addition of an IκB kinase (IKK) inhibitor blocked NF-κB activation and interferon-β mRNA expression in cells infected with viruses encoding the M51R mutation in M. These results indicate that the VSV M protein inhibits activation of NF-κB by targeting an event upstream of IKK in the canonical pathway.

Amino acid residues within conserved domain VI of the vesicular stomatitis virus large polymerase protein essential for mRNA cap methyltransferase activity

During mRNA synthesis, the polymerase of vesicular stomatitis virus (VSV) copies the genomic RNA to produce five capped and polyadenylated mRNAs with the 5'-terminal structure 7mGpppA(m)pApCpApGpNpNpApUpCp. The 5' mRNA processing events are poorly understood but presumably require triphosphatase, guanylyltransferase, [guanine-N-7]- and [ribose-2'-O]-methyltransferase (MTase) activities. Consistent with a role in mRNA methylation, conserved domain VI of the 241-kDa large (L) polymerase protein shares sequence homology with a bacterial [ribose-2'-O]-MTase, FtsJ/RrmJ. In this report, we generated six L gene mutations to test this homology. Individual substitutions to the predicted MTase active-site residues K1651, D1762, K1795, and E1833 yielded viruses with pinpoint plaque morphologies and 10- to 1,000-fold replication defects in single-step growth assays. Consistent with these defects, viral RNA and protein synthesis was diminished. In contrast, alteration of residue G1674 predicted to bind the methyl donor S-adenosylmethionine did not significantly perturb viral growth and gene expression. Analysis of the mRNA cap structure revealed that alterations to the predicted active site residues decreased [guanine-N-7]- and [ribose-2'-O]-MTase activity below the limit of detection of our assay. In contrast, the alanine substitution at G1674 had no apparent consequence. These data show that the predicted MTase active-site residues K1651, D1762, K1795, and E1833 within domain VI of the VSV L protein are essential for mRNA cap methylation. A model of mRNA processing consistent with these data is presented.

PKR-dependent and -independent mechanisms are involved in translational shutoff during Sindbis virus infection

The replication of Sindbis virus (SIN) profoundly affects the metabolism of infected vertebrate cells. One of the main events during SIN infection is the strong inhibition of translation of cellular mRNAs. In this study, we used a combination of approaches, including the study of SIN replication in PKR(-/-) mouse embryo fibroblasts or in the presence of an excess of catalytically inactive PKR. We show that the PKR-dependent inhibition of translation is not the only and most likely not the major pathway mediating translational shutoff during SIN infection. The PKR-independent mechanism strongly affects the translation of cellular templates, whereas translation of SIN subgenomic RNA is resistant to inhibition, and this leads to a benefit for viral replication. Our findings suggest that both PKR-dependent and non-PKR-dependent mechanisms of SIN-induced translational shutoff can be manipulated by using SIN replicons expressing mutated SIN nsP2 or kinase-defective PKR. Specifically, we show that expression of heterologous genes from SIN-based and most likely other alphavirus-based replicons can be increased by downregulating both the PKR-dependent and PKR-independent translational shutoffs.

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.

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.

Vesicular stomatitis virus P function depends on cellular growth cycle

The P function of vesicular stomatitis virus (VSV) is defined as the viral function which results in a reduced rate of total protein synthesis (viral plus cellular) arising from a nonspecific reduction in the efficiency of the translational machinery in infected cells. The existence of P function has been challenged by Lodish and Porter who were unable to detect it in L-strain mouse cells infected with wild-type VSV (HR) or, as expected, with the P- mutant, T1026-R1. Although other groups have subsequently confirmed the existence of P function and the difference between HR and T1026-R1, we have sought an explanation for the difference between Lodish and Porter's results and those of other laboratories. We show that the VSV P function depends on the phase of the growth cycle of infected L-cell cultures. In very early exponential phase, as used by Lodish and Porter, HR has very little demonstrable P function; as the growth cycle proceeds toward stationary phase, P function becomes more and more manifest. Under the same conditions, T1026-R1 shows no P function throughout the growth cycle. Furthermore we show that the VSV M protein mutant tsG31 has a P++ phenotype reducing total protein synthesis below that seen with wild-type HR. P function can be observed in cells infected with tsG31, even early in the exponential phase of the cellular growth cycle.

The isolation of interferon-inducing mutants of vesicular stomatitis virus with altered viral P function for the inhibition of total protein synthesis

We have previously reported that T1026, a temperature-sensitive (ts) noncytocidal mutant of VSV, and its ts revertant, T1026-R1, are nonconditional mutants in the VSV function "P" for the inhibition of total protein synthesis (viral plus cellular) in infected cells (C. P. Stanners, A. M. Francoeur, and T. Lam, 1977, Cell 11, 273-281; C. P. Stanners, S. Kennedy, and L. Poliquin, 1987, Virology 160, 255-258). We have also shown that P- mutants such as these are superior interferon inducers relative to their parental P+ wild-type virus, HR, and that P- mutants may be distinguished from P+ virus using the plaque interferon production of PIF assay. (A. M. Francoeur, T. Lam, and C. P. Stanners, 1980, Virology 105, 526-536). In order to carry the analysis of VSV P function further, a number of independent mutants in the VSV P function are required. We show here that the PIF assay may be used to isolate spontaneously occurring interferon-inducing mutants (PIF+ mutants) from wild-type VSV (PIF- virus) populations. About one-half of the PIF+ mutants isolated with the PIF assay were found to have alterations in the VSV P function. As well as mutants that were defective for the inhibition of total protein synthesis, the assay yielded a new class of VSV P function mutants which appear to inhibit protein synthesis more severely than does P+ virus. The majority of newly isolated PIF+ mutants was also found to be temperature sensitive for growth. The ts phenotype, however, could be reverted for most PIF+ mutants with little effect on the PIF or P phenotype. These findings show that interferon induction and P function are related functions of VSV; this fact has allowed the isolation of a repertoire of mutants with widely varying P function.

Dominance of the CPE(+) phenotype in hybrid Aedes albopictus cells infected with Sindbis virus

The effect of Sindbis virus (SV) infection was analyzed in hybrid Aedes albopictus cells formed by fusing ouabain-resistant CPE(+) cells to CPE(-) alpha-amanitin resistant cells. Although the 24-h yields of virus from the parental CPE(+) and CPE(-) clones were similar, the rates of viral RNA synthesis and virus release at early times post-infection were higher in the CPE(+) cells. In all eight hybrid clones studied, the CPE(+) phenotype was dominant. In addition, the kinetics of viral RNA synthesis and virus release in the hybrids closely resembled what was observed in the CPE(+) parent clone. These data indicate that both in the parental and in the hybrid cells, expression of SV-induced CPE is associated with a high level of viral RNA synthesis and a rapid production of virus during the early period after infection. It is suggested that CPE(+) cells contain some factor or activity which is lacking or less abundant in the CPE(-) cells and that this activity is important in the regulation of viral RNA synthesis.

Interaction of mRNA with proteins in vesicular stomatitis virus-infected cells

The interaction of mRNA with proteins in vesicular stomatitis virus (VSV)-infected cells was studied by photochemical cross-linking in intact cells. The major [35S]methionine-labeled proteins which became cross-linked by UV light to mRNA in uninfected and in VSV-infected HeLa cells were similar and had apparent mobilities in sodium dodecyl sulfate-polyacrylamide gel electrophoresis corresponding to 135, 93, 72, 68, 53, 50, 43, and 36 kilodaltons. The proteins which were cross-linked in vivo specifically to the five mRNAs of VSV were labeled through radioactive nucleotides incorporated only into VSV mRNAs under conditions (5 micrograms of actinomycin D per ml) in which only VSV mRNAs are labeled. The same major mRNP proteins that became cross-linked to host mRNAs also became cross-linked to VSV mRNAs, although several quantitative differences were detected. Photochemical cross-linking and immunoblotting of cross-linked mRNPs with VSV antiserum demonstrated that in addition to host proteins VSV mRNAs also became cross-linked to the VSV-encoded N protein. The poly(A) segment of both host and VSV mRNAs was associated in vivo selectively with the 72-kilodalton polypeptide. The major proteins of mRNA-ribonucleoprotein complexes are therefore ubiquitous and common to different mRNAs. Furthermore, since the major messenger ribonucleoproteins interact also with VSV mRNAs even though these mRNAs are transcribed in the cytoplasm, it appears that nuclear transcription and nucleocytoplasmic transport are not necessary for mRNA to interact with these proteins.

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.

Translation of vaccinia virus and cellular mRNA in cell-free systems prepared from uninfected and vaccinia virus infected L929 cells

Cell-free translation systems were prepared from uninfected and vaccinia infected (3 and 5 hours post-infection) L929 cells. The systems were made mRNA dependent in order to translate exogenous mRNA mixtures. The overall rate of protein synthesis was similar in the three translation systems. However, one-dimensional electrophoresis showed that the systems differed in terms of the translation efficiency for individual mRNAs. This could be demonstrated with each of the following mRNA mixtures: early vaccinia mRNA synthesized by vaccinia cores in vitro, mRNA isolated from polysomes of vaccinia infected HeLa cells ("late" vaccinia mRNA) and cytoplasmic ascites mRNA. When the above mentioned groups of mRNAs were allowed to compete for translation in the cell-free systems and their products were analyzed on one-dimensional gels, the following order of translational efficiency was observed: the most prominent species of vaccinia early mRNA (other species could not be judged) were translated better than some late vaccinia mRNA species which in turn were slightly more efficiently translated than cellular mRNAs.

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.

Vaccinia virus induces cellular mRNA degradation

The infection of mouse L cells with vaccinia virus induced a rapid inhibition of cellular polypeptide synthesis and a diversion of protein synthesis to the exclusive production of viral polypeptides. This shutoff of cell-specific protein synthesis was achieved by a novel mechanism by which the virus induced the rapid degradation of cellular mRNAs. Concurrent with the degradation of cellular mRNA, the virus proceeds in the orderly temporal expression of its own genetic information. The effect of vaccinia virus infection upon two abundant L-cell mRNAs was assessed by using the highly conserved cDNA sequences that encode chicken beta-actin and rat alpha-tubulin. Hybridization analyses demonstrated that throughout infection there is a rapid and progressive degradation of both of these mRNAs. In fact, after 3 h of infection they are reduced to less than 50% of their concentration in uninfected L cells, and between 8 to 10 h they are almost entirely degraded. This observation explains in part the mechanism by which vaccinia virus inhibits host cell protein synthesis.

Reversible block in intracellular transport and budding of mutant vesicular stomatitis virus glycoproteins

The blocks in intracellular maturation of glycoprotein (G protein) synthesized by two temperature-sensitive vesicular stomatite's virus (VSV) mutants are reversible. Our earlier work demonstrated that at 40 degrees, the nonpermissive temperature, mutant ts L513(V) G protein accumulates in the rough endoplasmic reticulum. Here we show that when the temperature is lowered the high-mannose oligosaccharides on a significant fraction of ts L513(V) G protein, synthesized at 40 degrees, will be modified to the complex type. Moreover, when the temperature is lowered ts L513(V) G protein accumulated at 40 degrees will mature to the cell surface, as evidenced by its accessibility to extracellular trinitrobenzene sulfonate, and into virions. G protein synthesized at 40 degrees in ts L511(V)-infected cells undergoes most of the processing events characteristic of the Golgi complex. Although we reported previously that no ts L511(V) G protein reaches the plasma membrane at 40 degrees, we now find, using more sensitive techniques, that an appreciable fraction does reach the cell surface. ts L511(V) G protein is lost from the cells but is not incorporated into virions. However, an appreciable fraction of the ts L511(V) G protein which accumulates in cells at 40 degrees will mature into virions when the temperature is lowered. These results exclude irreversible denaturation of mutant G proteins as a cause of the block in intracellular maturation and virus budding.

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.

Inhibition by vesicular stomatitis virus of herpes simplex virus-directed protein synthesis

Infection of mammalian cells with either herpes simplex virus (HSV) or vesicular stomatitis virus (VSV) results in a marked inhibition of host protein synthesis. These viruses employ different mechanisms to turn off the host. In previous studies we showed that following infection with HSV, cellular mRNA was degraded and host polyribosomes were dissociated (Nishioka and Silverstein, Proc. Nat. Acad. Sci. USA 74, 2370-2374, 1977; Nishioka and Silverstein, J. Virol. 25, 422-426, 1978a). Degradation required synthesis of an HSV-specified polypeptide whereas dissociation appeared to be mediated by a heat-labile virion associated function (Nishioka and Silverstein, J. Virol. 27, 619-627, 1978b). In contrast, when cells are infected with VSV, host mRNAs are not degraded and polyribosome profiles are not drastically altered (Nishioka and Silverstein, 1978a). We have exploited the properties of these two viruses by infecting cells either simultaneously or sequentially in an effort to test our previous hypotheses. Analyses of the distribution of polyribosomes, stability of mRNA, synthesis of mRNA, and patterns of protein synthesis in coinfected cells permit us to conclude that dissociation of polyribosomes in cells infected with HSV results from expression of a virion associated function, degradation of cellular mRNA requires expression of the HSV genome, and VSV is dominant in doubly infected cells because it inhibits de novo transcription of the HSV genome.

Effect of intracellular vesicular stomatitis virus mRNA concentration on the inhibition of host cell protein synthesis

Inhibition of host cellular protein synthesis by vesicular stomatitis virus (VSV) has been suggested to be primarily the result of competition for ribosomes between cellular and viral mRNAs (H. F. Lodish and M. Porter, J. Virol., 36:719-733, 1980; Lodish and Porter, J. Virol. 38:504-517, 1981). This hypothesis was investigated by regulating the extent of VSV mRNA synthesis through the use of defective interfering particles. Although intracellular VSV mRNA concentrations decreased by as much as a factor of 14 at high multiplicities of infection of defective interfering particles, the inhibition of host cell protein synthesis by VSV decreased by a maximum of only 10%. The data also indicated that under these conditions the protein-synthesizing capacity of the cells was not exhausted. We concluded that competition for cellular ribosomes could not have been the major factor in the inhibition of host cell protein synthesis by VSV. This conclusion was further supported by inhibition data obtained with VSV mutants. The ts G22 mutant, defective in replication but not in primary transcription, inhibited host protein synthesis at the nonpermissive temperature (39 degrees C) to the same extent as did wild-type virus, even though it generated only 30 to 50% of the amount of viral mRNA as did wild-type virus. Conversely, in infections with the R1 mutant, which did not inhibit host cell protein synthesis, the amount of total and polysome-bound viral mRNA was indistinguishable from that obtained in infections by wild-type virus.

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.

Inhibition of host translation in encephalomyocarditis virus-infected L cells: a novel mechanism

Encephalomyocarditis virus induced a rapid shutoff of host translation in mouse L cells shortly after infection and before viral proteins were made in detectable amounts. This kinetic pattern is similar to that seen in poliovirus-infected HeLa cells. However, the mechanisms of host shutoff are different in these two cases, for no reduction in the ability of lysates from encephalomyocarditis virus-infected L cells to translate capped mRNAs was observed. Instead, a change in the subcellular distribution of one or more initiation factors was seen. In particular, cap recognition activity in the high-speed supernatant fraction (S200) prepared from cell lysates increased threefold as a result of virus infection. The significance of this observation in terms of possible shutoff mechanisms is discussed. Inasmuch as the rapid host shutoff is not induced in at least four other cell types by encephalomyocarditis virus infection, it may be concluded that host shutoff mechanisms not only vary within the picornavirus group, but also depend upon the particular cell type employed.

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

Infection of mouse L-cell spinner cultures by vesicular stomatitis virus (VSV) effected the selective translation of viral mRNA by 4h after viral adsorption. Cell-free systems prepared from mock- and VSV-infected cells reflected this phenomenon; protein synthesis was reduced in the virus-infected cell lysate by approximately 75% compared with the mock-infected (control) lysate. This effect appeared to be specific to protein synthesis initiation since (i) methionine incorporation into protein from an exogenous preparation of initiator methionyl-tRNA gave completely analogous results and (ii) the addition of a ribosomal salt wash (containing protein synthesis initiation factors) stimulated protein synthesis by the infected cell lysate but had no effect on protein synthesis by the control. Micrococcal nuclease-treated (initiation-dependent) VSV-infected cell lysates were not able to translate L-cell mRNA unless they were supplemented with a ribosomal salt wash; a salt wash from ribosomes from uninfected cells effected a quicker recovery than a salt wash from ribosomes from infected cells. When salt wash preparations from ribosomes from uninfected and infected cells were tested for initiation factor 2 (eIF-2)-dependent ternary complex capacity with added GTP and initiator methionyl-tRNA, we found that the two preparations contained equivalent levels of eIF-2. However, initiation complex formation by the factor from virus-infected cells proceeded at a reduced initial rate compared with the control. When the lysates were supplemented with a partially purified eIF-2 preparation, recovery of activity by the infected cell lysate was observed. Mechanisms by which downward regulation of eIF-2 activity might direct the selective translation of viral mRNA in VSV-infected cells are proposed.