Cellular protein synthesis shutoff by mengovirus: translation of nonviral and viral mRNA's in extracts from uninfected and infected Ehrlich ascites tumor cells

Avian reovirus nonstructural protein p17-induced G(2)/M cell cycle arrest and host cellular protein translation shutoff involve activation of p53-dependent pathways

The effects of avian reovirus (ARV) p17 protein on cell cycle progression and host cellular protein translation were studied. ARV infection and ARV p17 transfection resulted in the accumulation of infected and/or transfected cells in the G(2)/M phase of the cell cycle. The accumulation of cells in the G(2)/M phase was accompanied by upregulation and phosphorylation of the G(2)/M-phase proteins ATM, p53, p21(cip1/waf1), Cdc2, cyclin B1, Chk1, Chk2, and Cdc25C, suggesting that p17 induces a G(2)/M cell cycle arrest through activation of the ATM/p53/p21(cip1/waf1)/Cdc2/cyclin B1 and ATM/Chk1/Chk2/Cdc25C pathways. The G(2)/M cell cycle arrest resulted in increased virus replication. In the present study, we also provide evidence demonstrating that p17 protein is responsible for ARV-induced host cellular protein translation shutoff. Increased phosphorylation levels of the eukaryotic translation elongation factor 2 (eEF2) and initiation factor eIF2alpha and reduced phosphorylation levels of the eukaryotic translation initiation factors eIF4E, eIF4B, and eIF4G, as well as 4E-BP1 and Mnk-1 in p17-transfected cells, demonstrated that ARV p17 suppresses translation initiation factors and translation elongation factors to induce host cellular protein translation shutoff. Inhibition of mTOR by rapamycin resulted in a decrease in the levels of phosphorylated 4E-BP1, eIF4B, and eIF4G and an increase in the levels eEF2 but did not affect ARV replication, suggesting that ARV replication was not hindered by inhibition of cap-dependent translation. Taken together, our data indicate that ARV p17-induced G(2)/M arrest and host cellular translation shutoff resulted in increased ARV replication.

Mayaro virus infection alters glucose metabolism in cultured cells through activation of the enzyme 6-phosphofructo 1-kinase

Although it is well established that cellular transformation with tumor virus leads to changes on glucose metabolism, the effects of cell infection by non-transforming virus are far to be completely elucidated. In this study, we report the first evidence that cultured Vero cells infected with the alphavirus Mayaro show several alterations on glucose metabolism. Infected cells presented a two fold increase on glucose consumption, accompanied by an increment in lactate production. This increase in glycolytic flux was also demonstrated by a significant increase on the activity of 6-phosphofructo 1-kinase, one of the regulatory enzymes of glycolysis. Analysis of the kinetic parameters revealed that the regulation of 6-phosphofructo 1-kinase is altered in infected cells, presenting an increase in Vmax along with a decrease in Km for fructose-6-phosphate. Another fact contributing to an increase in enzyme activity was the decrease in ATP levels observed in infected cells. Additionally, the levels of fructose 2,6-bisphosphate, a potent activator of this enzyme, was significantly reduced in infected cells. These observations suggest that the increase in PFK activity may be a compensatory cellular response to the viral-induced metabolic alterations that could lead to an impairment of the glycolytic flux and energy production.

Rapamycin stimulates viral protein synthesis and augments the shutoff of host protein synthesis upon picornavirus infection

The immunosuppressant drug rapamycin blocks progression of the cell cycle at G1 in mammalian cells and yeast. We recently showed that rapamycin inhibits both in vitro and in vivo cap-dependent, but not cap-independent, translation. This inhibition is causally related to reduced phosphorylation and consequent activation of 4E-BP1, a repressor of the function of the cap-binding protein, eIF4E. Two members of the picornavirus family, encephalomyocarditis virus and poliovirus, inhibit phosphorylation of 4E-BP1. Since translation of picornavirus mRNAs is cap independent, inhibition of phosphorylation of 4E-BP1 could contribute to the shutoff of host protein synthesis. Here, we show that rapamycin augments both the shutoff of host protein synthesis and the initial rate of synthesis of viral proteins in cells infected with encephalomyocarditis virus and poliovirus.

Activation of the translational suppressor 4E-BP1 following infection with encephalomyocarditis virus and poliovirus

Infection of cells with picornaviruses, such as poliovirus and encephalomyocarditis virus (EMCV), causes a shutoff of host protein synthesis. The molecular mechanism of the shutoff has been partly elucidated for poliovirus but not for EMCV. Translation initiation in eukaryotes is facilitated by the mRNA 5' cap structure to which the multisubunit translation initiation factor eIF4F binds to promote ribosome binding. Picornaviruses use a mechanism for the translation of their RNA that is independent of the cap structure. Poliovirus infection engenders the cleavage of the eIF4G (formerly p220) component of eIF4F and renders this complex inactive for cap-dependent translation. In contrast, EMCV infection does not result in eIF4G cleavage. Here, we report that both EMCV and poliovirus activate a translational repressor, 4E-BP1, that inhibits cap-dependent translation by binding to the cap-binding subunit eIF4E. Binding of eIF4E occurs only to the underphosphorylated form of 4E-BP1, and this interaction is highly regulated in cells. We show that 4E-BP1 becomes dephosphorylated upon infection with both EMCV and poliovirus. Dephosphorylation of 4E-BP1 temporally coincides with the shutoff of protein synthesis by EMCV but lags behind the shutoff and eIF4G cleavage in poliovirus-infected cells. Dephosphorylation of 4E-BP1 by specifically inhibiting cap-dependent translation may be the major cause of the shutoff phenomenon in EMCV-infected cells.

Induction of ferritin synthesis in cells infected with Mengo virus

We have recently identified ferritin as a cellular protein particle whose synthesis is stimulated in mouse or human cells infected by the picornavirus Mengo. Immunoprecipitation of the particle from infected murine L929 cells showed a 4- and 6-fold increase in the intracellular concentrations of H and L apoferritin subunits, respectively. This differential expression altered the H/L subunit ratio from 3.0 in uninfected cells to 2.2 in Mengo virus-infected cells. The induction is not due to an increase in transcription of the apoferritin L and H genes, nor is it due to an increase in stability of the apoferritin mRNAs. At the level of translation, the iron regulatory protein (IRP) remained intact, with similar amounts being detected in uninfected and infected cells. The Mengo virus RNA genome does not compete with the iron regulatory element (IRE) for the binding of IRP, and sequence analysis confirmed that there are no IREs in the virus RNA. The IRE binding activity of IRP in infected cells decreased approximately 30% compared with uninfected cells. The decrease in binding activity could be overcome by the addition of Desferal (deferoxamine mesylate; CIBA) an intracellular iron chelator, which suggests that virus infection causes an increase in intracellular free iron. Electron paramagnetic resonance (EPR) studies have confirmed the increase in free iron in Mengo virus infected cells. The permeability of cells for iron does not change in virus infected cells, suggesting that the induction of ferritin by Mengo virus is due to a change in the form of intracellular iron from a bound to a free state.

Modification of eukaryotic initiation factor 4F during infection by influenza virus

Influenza virus infection of cells is accompanied by a striking shutoff of cellular protein synthesis, resulting in the exclusive translation of viral mRNAs. The mechanism for control of cellular protein synthesis by influenza virus is poorly understood, but several translation properties of influenza virus mRNAs which are potentially involved have been described. Influenza virus mRNAs possess the surprising ability to translate in the presence of inhibitory levels of inactive (phosphorylated) eukaryotic initiation factor 2 (eIF-2). In addition, influenza virus mRNAs were shown to be capable of translating in cells during the late phase of adenovirus infection but not in cells infected by poliovirus. Since both adenovirus and poliovirus facilitate virus-specific translation by impairing the activity of initiation factor eIF-4F (cap-binding protein complex) but through different mechanisms, we investigated the translation properties of influenza virus mRNAs in more detail. We show that influenza virus infection is associated with the significant dephosphorylation and inactivation of eIF-4E (cap-binding protein), a component of eIF-4F, and accordingly that influenza virus mRNAs possess a moderate ability to translate by using low levels of eIF-4F. We also confirm the ability of influenza virus mRNAs to translate in the presence of high levels of inactive (phosphorylated) eIF-2 but to a more limited extent than reported previously. We suggest a potential mechanism for the regulation of protein synthesis by influenza virus involving a decreased requirement for large pools of active eIF-4F and eIF-2.

The alpha subunit of eucaryotic initiation factor 2 is phosphorylated in mengovirus-infected mouse L cells

Infection of mouse L cells with mengovirus resulted in the activation of a protein kinase (PK) that selectively phosphorylated the small, 38,000-molecular-weight alpha subunit of eucaryotic initiation factor 2 (eIF-2) in vitro. The mengovirus-activated kinase was detected in vitro approximately 3 h after virus adsorption. The ratio of phosphorylated to unphosphorylated eIF-2 also increased in vivo between 3 and 7 h after adsorption. The virus-activated kinase fractionated with the ribosomal pellet and had a high affinity for DEAE-cellulose and Mono Q ion-exchange columns. Gel electrophoresis of the kinase activity eluting from the Mono Q column and silver staining of the gel revealed only one protein band with a molecular mass of 70 kilodaltons. The optimal assay conditions for the mengovirus-activated kinase paralleled those of the double-stranded RNA-activated PK (dsRNA-PK). Lysates from infected cells contained elements capable of activating partially purified dsRNA-PK. These elements were identified as double-stranded RNA by their sensitivity to double-stranded RNase. The phosphorylation of the alpha subunit of eIF-2 coincided with the synthesis of dsRNA in infected cells, suggesting that the mengovirus-activated kinase is the dsRNA-PK. The phosphorylation of the alpha subunit of eIF-2 correlated with the global inhibition of protein synthesis that occurs at late times after infection.

The adenovirus tripartite leader may eliminate the requirement for cap-binding protein complex during translation initiation

The adenovirus tripartite leader is a 200-nucleotide 5' noncoding region that is found on all late viral mRNAs. This segment is required for preferential translation of viral mRNAs at late times during infection. Most tripartite leader-containing mRNAs appear to exhibit little if any requirement for intact cap-binding protein complex, a property previously established only for uncapped poliovirus mRNAs and capped mRNAs with minimal secondary structure. The tripartite leader also permits the translation of mRNAs in poliovirus-infected cells in the apparent absence of active cap-binding protein complex and does not require any adenovirus gene products for this activity. The preferential translation of viral late mRNAs may involve this unusual property.

A kinase able to phosphorylate exogenous protein synthesis initiation factor eIF-2 alpha is present in lysates of mengovirus-infected L cells

Infection of mouse L929 cells by mengovirus resulted in the expression of a kinase activity that selectively phosphorylated the small, 38,000-molecular-weight subunit of eucaryotic initiation factor 2 and histone H2. This kinase activity was independent of host cell RNA synthesis and was located in the postribosomal supernatant (S-100 fraction) early after infection (up to 3 h). At later times after infection (5 h), kinase activity was also associated with the polysome fraction. The kinase present in the S-100 fraction bound strongly to DEAE-cellulose, its peak activity eluting at 0.5 M KCl. Kinase activity was independent of the presence of exogenous double-stranded RNA, and KCl at concentrations greater than 0.1 M inhibited eucaryotic initiation factor 2 phosphorylation. The 67,000-molecular-weight phosphoprotein activated in interferon-treated cells by double-stranded RNA was not detected by standard phosphorylation assays in lysates from mengovirus-infected cells. Labeling of this protein in vivo during 5 h of infection was also not detected. The DEAE-cellulose-purified mengovirus kinase inhibited protein synthesis in reticulocyte lysates, and the inhibition was not reversible by high concentrations of poly(I).poly(C).

Regulation of protein synthesis in virus-infected animal cells

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

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

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

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.

Role of mRNA competition in regulating translation: further characterization of mRNA discriminatory initiation factors

Host and reovirus mRNAs compete with one another for translation in infected cells. Kinetic analysis has suggested that the site of competition is a message discriminatory initiation factor which must bind to the mRNA before it can interact with the 40S ribosomal subunit. The present communication describes an in vitro assay which can detect message discriminatory activities. A competitive situation is established by using reovirus and globin mRNAs, and then the specificity with which this competition is relieved by added components is measured. Among the various initiation factors surveyed with this assay, two have the properties expected of the mRNA discriminatory factor. These are eukaryotic initiation factor 4A and a "cap binding protein" complex. Inasmuch as the cap binding protein complex contains a subunit similar or identical to the initiation factor eIF-4A, it seems likely that only one form of the latter factor may be active in vivo. In vitro, both factors relieve competition among both capped and uncapped reovirus mRNAs according to similar hierarchies. These results suggest that some feature other than the m7G cap, such as nucleotide sequence or secondary structure, is recognized by the discriminatory factor.

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.

Encephalomyocarditis viral RNA can be translated under conditions of poliovirus-induced translation shutoff in vivo

Superinfection with poliovirus of HeLa cells already infected with encephalomyocarditis (EMC) virus does not inhibit translation of EMC viral mRNA, whereas residual host translation is completely inhibited. This result indicates that the cap recognition factors inactivated by poliovirus are not required for translation of EMC viral mRNA in vivo, in agreement with previous in vitro experiments. This raises the question of why EMC virus has evolved a capindependent translation mechanism.

Translational control of protein synthesis after infection by vesicular stomatitis virus

Four hours after infection of BHK cells by vesicular stomatitis virus (VSV), the rate of total protein synthesis was about 65% that of uninfected cells and synthesis of the 12 to 15 predominant cellular polypeptides was reduced to a level about 25% that of control cells. As determined by in vitro translation of isolated RNA and both one- and two-dimensional gel analyses of the products, all predominant cellular mRNA's remained intact and translatable after infection. The total amount of translatable mRNA per cell increased about threefold after infection; this additional mRNA directed synthesis of the five VSV structural proteins. To determine the subcellular localization of cellular and viral mRNA before and after infection, RNA from various sizes of polysomes and nonpolysomal ribonucleoproteins (RNPs) was isolated from infected and noninfected cells and translated in vitro. Over 80% of most predominant species of cellular mRNA was bound to polysomes in control cells, and over 60% was bound in infected cells. Only 2 of the 12 predominant species of translatable cellular mRNA's were localized to the RNP fraction, both in infected and in uninfected cells. The average size of polysomes translating individual cellular mRNA's was reduced about two- to threefold after infection. For example, in uninfected cells, actin (molecular weight 42,000) mRNA was found predominantly on polysomes with 12 ribosomes; after infection it was found on polysomes with five ribosomes, the same size of polysomes that were translating VSV N (molecular weight 52,000) and M (molecular weight 35,000) mRNA. We conclude that the inhibition of cellular protein synthesis after VSV infection is due, in large measure, to competition for ribosomes by a large excess of viral mRNA. The efficiency of initiation of translation on cellular and viral mRNA's is about the same in infected cells; cellular ribosomes are simply distributed among more mRNA's than are present in growing cells. About 20 to 30% of each of the predominant cellular and viral mRNA's were present in RNP particles in infected cells and were presumably inactive in protein synthesis. There was no preferential sequestration of cellular or viral mRNA's in RNPs after infection.

Shutoff of HeLa cell protein synthesis by encephalomyocarditis virus and poliovirus: a comparative study

Previous experimental results have suggested that poliovirus and encephalomyocarditis (EMC) virus employ very different mechanisms for shutting off host protein synthesis. However, this conclusion is suspect, inasmuch as different cell types were used for the two viruses; hence the apparent mechanistic differences might be specific for cell type and not virus type. To test this possibility we compared shutoff mechanisms in poliovirus- and EMC virus-infected HeLa cells. Striking differences were seen: poliovirus-induced shutoff was much more rapid and extensive than that induced by EMC virus; relative translation rates of certain host proteins were inhibited to different extents by the two viruses; initiation factors prepared from poliovirus-infected cells were specifically defective for translation of capped mRNA's in vitro, whereas those from EMC virus-infected cells were not. These results indicate that EMC virus and poliovirus employ different mechanisms for the shutoff of HeLa cell protein synthesis. This conclusion is consistent with much earlier work and indicates that many differences previously reported are specific to virus type.

Theoretical mechanisms for synthesis of carcinogen-induced embryonic proteins: IV. The viruses

Speculations are developed for a mechanism by which oncogenic viruses can induce alterations in cells allowing them to express embryonic genes. It is suggested that if viral deoxyribonucleic acid, directly or via ribonucleic acid directed deoxyribonucleic acid polymerase activity becomes inserted at particular euchromatin - heterochromatin junctions of quasidifferentiated stem-like cells, then deheterochromatization may result, causing in turn derepression of genes for acidic protein phosphokinases. This sets into motion a series of events including altered acid protein repressors of embryonic genes which are repressed by uniquely weak type repressors. This explains how viruses can act as specific embryonic gene-inducing agents similar to chemical inducing agents such as the hepatocarcinogen ethinine.

Comparison of initiation rates of encephalomyocarditis virus and host protein synthesis in infected cells

The relative initiation rates for encephalomyocarditis virus mRNA and host mRNA's in infected cells were measured using two independent techniques. In both cases the results showed that viral mRNA initiates at a much higher rate than host mRNA'S. This difference was observed midway in the infectious cycle, well before virus-induced cytopathic effects (leakage of low-molecular-weight metabolites, failure to exclude trypan blue) were apparent. These results confirm that encephalomyocarditis viral mRNA is a more efficient initiator than host mRNA's in vivo, as has previously been demonstrated in in vitro experiments.

Inhibition of translation by poliovirus: inactivation of a specific initiation factor

Translation of vesicular stomatitis virus (VSV) mRNA, like host mRNA translation, is inhibited in cells infected with poliovirus. To study the mechanism of poliovirus-induced inhibition of protein synthesis, we prepared extracts from poliovirus-infected and uninfected HeLa cells. Poliovirus mRNA was translated in lysates from both infected and uninfected cells, while VSV mRNA was translated only in the lysate from uninfected cells. Addition of purified translation initiation factors to the extract from infected cells showed that one factor, eIF-4B, could restore VSV mRNA translation in the infected lysate, but did not increase poliovirus mRNA translation. Further experiments involving translation of VSV mRNA in mixed extracts from poliovirus-infected and uninfected cells showed (i) that there was not an excess of an inhibitor of VSV mRNA translation in the infected lysate, but (ii) that an acitivity that caused a slow inactivation of eIF-4B was present in the infected lysate. Inactivation of eIF-4B appears to be the mechanism by which poliovirus infection causes a selective inhibition of translation.

Control of protein synthesis in extracts from poliovirus-infected cells. I. mRNA discrimination by crude initiation factors

By using cell-free systems prepared from uninfected and poliovirus-infected cells, we have been able to demonstrate that crude preparations of initiation factors from infected cells do not stimulate the initiation of translation by polyribosomes containing endogenous host cell mRNA. When tested with polysomes containing endogenous viral mRNA, however, they were able to stimulate initiation of translation nearly as well as uninfected cell initiation factors. The uninfected cell initiation factor preparations were able to stimulate initiation of translation of both cell and viral mRNA. The results indicate an mRNA-specific activity present in crude initiation factor preparations from infected cells. Furthermore, the ability of eIF2 from infected cells to form a ternary complex with GTP and formyl [35S]methionine-tRNAfmet, an mRNA-independent step in initiation, was found not to be deficient. Implications of these data for proposed mechanisms of poliovirus-induced host cell shutoff are discussed.

Selective translation of mengovirus RNA over Host mRNA in homologous, fractionated, cell-free translational systems from Ehrlich-ascites-tumor cells

The selective translation of viral RNA in mengovirus-infected Ehrlich ascites tumor cells was investigated using fractionated translational systems whose macromolecular components were derived entirely from uninfected or virus-infected cells. Both systems translate host mRNA from uninfected cells, host mRNA from virus-infected cells, and mengovirus RNA. In competition experiments, where viral RNA and host mRNA were translated together in systems from uninfected cells, the relative amounts of virus-specific and host-specific proteins synthesized were proportional to the relative concentrations of the RNA templates. In systems whose components were obtained from virus-infected cells, mengovirus RNA was preferentially translated. 70% of the selectivity found in the translational systems derived from infected cells was due to the initiation factor fraction, the remaining 30% to components of the pH 5 enzyme fraction. In addition, host mRNA isolated after virus infection is translated in vitro to a lower extent in the presence of mengovirus RNA than is host mRNA from uninfected cells.

Translation of ascites and mengovirus RNA in fractionated cell-free systems from uninfected and mengovirus-infected Ehrlich-ascites-tumor cells

We have prepared homologous, fractionated, cell-free translational systems from uninfected and mengovirus-infected Ehrlich ascites tumor cells in order to determine what alterations occur following virus infection in the translational machinery of the host cell. Two major differences distinguish the system developed from infected cells. First, it has a 40% lower rate of protein synthesis, primarily a consequence of the rate of chain elongation, which is depressed to 60 amino acids/min from 90 amino acids/min in the system from uninfected cells. Second, at supraoptimal concentrations of Mg2+ and K+ the system from virus-infected cells supports the translation of mengovirus RNA but not host mRNA. These differences between the two systems may reflect specific changes which are responsible for the selective translation of mengovirus RNA in the infected cell. In both systems the optimal concentrations of polyamines, monovalent and divalent cations, mRNA, and ribosomal subunits are the same for the translation of either host or viral RNA. This uniformity is useful in experiments, designed to investigate the selective translation of viral RNA, where various components of the two systems are interchanged.

Fate of histone messenger RNA in mengovirus-infected Ehrlich ascites tumor cells

Histone mRNA was isolated from mengovirus-infected Ehrlich ascites tumor cells at various times postinfection and quantitated in a reticulocyte cell-free protein-synthesizing system. The amount of translatable histone mRNA decreases during the first hour postinfection by 30%, rises during the following 1-1.5 h by 10-15%, drops progressively in the further course of infection, and reaches 20% of the control at the end of the infectious cycle (8-9 h postinfection). On the basis of the relative histone mRNA contents, the histone-synthesizing potentials of mengovirus-infected Ehrlich ascites tumor cells are substantially higher throughout infection than actually expressed in vivo. This result indicates that the virus-induced shutoff of histone synthesis is not directly the consequence of inactivation or degradation of histone mRNA. Most of the histone mRNA recovered from mengovirus-infected Ehrlich ascites tumor cells is bound to ribosomes. Late in infection, certain mRNAs are co-isolated with histone mRNAs, very likely due to loss or shortening of poly(A) occurring after release of the mRNAs from polyribosomes.

An investigation of the stability of messenger RNAs in cell-free, translational systems from uninfected and mengovirus-infected Ehrlich ascites tumor cells

The stabilities and translation of Ehrlich ascites tumor cell poly(A)-containing mRNA and mengovirus RNA in fractionated cell-free protein synthesizing systems from uninfected and mengovirus-infected Ehrlich ascites tumor cells were studied. During incubation of the systems about 20% of the input RNA is reduced in size and associated with ribosomes engaged in polypeptide synthesis; the remainder is rapidly degraded by RNases. At the end of active translation, both mRNA and nascent proteins are bound to polysomes which are of the same size as those formed during active protein synthesis. The kinetics of protein synthesis closely follow those of RNA hydrolysis. The stabilities of mengovirus RNA and poly(A)-containing mRNA from Ehrlich ascites tumor cells are the same in both systems.

Degradation of cellular mRNA during infection by herpes simplex virus

The fate of preexisting mRNA sequences was examined after infection by herpes simplex virus. Murine erythroid cells transformed by Friend leukemia virus were used as the host. Such cells, when exposed to 2% dimethyl sulfoxide, produce large amounts of globin and globin mRNA. The protein and its mRNA are easily recognized at 4 days by electrophoresis in high percentage acrylamide gels and by hybridization to cDNA, respectively. Herpes simplex virus replicates in these cells. By 2 hr after infection the rate of protein synthesis decreases to 30% of the level in mock-infected cells and only 49+/-8% (SEM) of the globin mRNA sequences present prior to infection could be detected by hybridization to cDNA. At 4 hr after infection, when the rate of protein synthesis in infected cells is at a maximum, only about 15% of the globin mRNA sequences remained. Control experiments support the hypothesis that globin mRNA sequences are degraded after infection by herpes simplex virus.

Inhibition of ribonucleases by ribonucleotides and transition state analogs in cell-free extracts from Ehrlich ascites tumor cells

We investigated the ribonucleolytic breakdown of poly(U), poly(A), RNA trascribed from calf thymus DNA with E. coli RNA polymerase, ribosomal RNA, tRNA and mengovirus RNA by an enzyme fraction obrained from a postribosomal supernatant of Ehrlich ascites tumor cells. The single-stranded homopolyribonucleotides are preferentially degraded by the enzyme fraction with the production of ribonucleoside 5'-monophosphates. The RNase activity is completely dependent on the presence of Mg2+ ions and is highest at Mg2+ and K+ concentrations optimal for cell-free protein synthesis. Ribonucleoside 5'-monophosphates, ribonucleoside 2'(3')-monophosphates, ribonucleoside 2'(3'),5'-bisphosphates and transition state analogs consisting of vanadyl sulfate and either ribonucleosides or ribonucleoside 5'-monophosphates in a molar ratio 1:1 inhibit the ribonucleolytic activity of the enzyme fraction. The ribonucleoside 2'(3'),5'-bisphosphates and the transition state analogs are the most effective inhibitors. However, only in the presence of ribonucleoside 2'(3'),5'-bisphosphates a concomitant stimulation by 50 to 60% of poly(U)-directed polyphenylalanine synthesis is observed; all the other RNase inhibitors tested also inhibit polypeptide synthesis. The results of preliminary experiments show that poly(U) and ribonucleoside 2'(3'),5'-bisphosphates are well suited as ligands for affinity chromatography of ribonucleases from Ehrlich ascites tumor cells.

Inhibition of cellular and viral protein synthesis by 3-methyleneoxindole

The effect of 3-methyleneoxindole (MO) on mengovirus and L-cell protein synthesis was investigated. MO was found to inhibit mengovirus multiplication and the incorporation of radioactive amino acids into both viral and cellular proteins. These results suggest that the antiviral effect of this compound is not specific but rather stems from its inhibition of the cellular translational machinery upon which mengovirus depends. We have also found that MO inhibits natural messenger ribonucleic acid (mengovirus and globin messenger ribonucleic acid) translation in cell-free extracts from Ehrlich ascites tumor cells but has no significant effect on polyuridylic acid translation. Additional data which suggest that MO inhibits protein synthesis at the level of initiation are shown.

Processing of mengovirus precursor polypeptides in the presence of zinc ions and sulfhydryl compounds

The effect of zinc ions on the post-translational cleavage of mengovirus polypeptides has been examined. The cleavage of the "A" precursor, which gives rise to the capsid proteins, was the most sensitive at concentrations of zinc chloride from 0.1 to 1.0 mM. Beta-mercaptoethanol and dithiothreitol antagonized the zinc-promoted inhibtion of clevage. Our results indicate that zinc ions interfere with the proper folding of the nascent polypeptide precursor rather than inhibit the proteases responsible for the cleavages. Thus, proper folding of mengovirus polypeptide "A" appears to be necessary for subsequent processing by proteases.