Inhibition of protein synthesis in reticulocyte lysates by a double-stranded RNA component in HeLa mRNA

Double-stranded RNA (dsRNA) inhibits protein synthesis initiation in rabbit reticulocyte lysates by the activation of a latent dsRNA-dependent cAMP-independent protein kinase which phosphorylates the α-subunit of the eukaryotic initiation factor eIF-2. In this study, we describe a dsRNA-like component which is present in preparations of HeLa mRNA (poly A⁺) isolated from total cytoplasmic RNA. The inhibitory species in the HeLa cytoplasmic mRNA was detected by (a) its ability to inhibit protein synthesis with biphasic kinetics in reticulocyte lysates translating endogenous globin mRNA, and (b) by the inefficient translation of HeLa cytoplasmic mRNA in a nuclease-treated mRNA-dependent reticulocyte lysate. The inhibitory component was characterized as dsRNA by several criteria including (i) the ability to activate the lysate dsRNA-dependent eIF-2α kinase (dsI); (ii) the prevention of both dsI activation and inhibition of protein synthesis by high levels of dsRNA or cAMP; (iii) the reversal of inhibition by eIF-2; and (iv) the inability to inhibit protein synthesis in wheat germ extracts which lack latent dsI. By the same criteria, the putative dsRNA component(s) appears to be absent from preparations of HeLa mRNA isolated exclusively from polyribosomes.     

Restriction of translation of capped mRNA in vitro as a model for poliovirus-induced inhibition of host cell protein synthesis: relationship to p220 cleavage.

Poliovirus infection of HeLa cells results in a rapid inhibition of host protein synthesis by a mechanism that does not affect the translation of poliovirus RNA. It has been suggested that this virus-induced translational control results from inactivation of the cap-binding protein complex, and it has been shown that the 220-kilodalton component(s) (p220) of the cap-binding protein complex is cleaved in infected HeLa cells to form antigenically related polypeptides of 100 to 130 kilodaltons. We have previously described an activity in infected cells that specifically restricts translation of capped mRNA in rabbit reticulocyte lysates. Here, we describe further refinements and characterization of restriction assay. We determined that the assay is a good in vitro model for study of host cell shutoff by several criteria: (i) translation was inhibited in both instances at the step involving mRNA binding to ribosomes; (ii) translation of capped mRNA was specifically inhibited, whereas translation of poliovirus RNA was not; (iii) restriction activity appeared in infected cells with kinetics which parallel host cell shutoff; and (iv) restriction activity, like the specific inhibition of host translation, appeared in cells infected in the presence of guanidine-HCl. The restricting activity was partially purified from poliovirus-infected cells and was compared with the virus-induced p220 cleavage activity. Both activities copurified through numerous cell fractionation and biochemical fractionation procedures. However, specific restriction of capped mRNA translation in reticulocyte lysates occurred without complete cleavage of the endogenous p220.     

Catalytic utilization of eIF-2 and mRNA binding proteins are limiting in lysates from vesicular stomatitis virus infected L cells

Infection of mouse L cells by vesicular stomatitis virus results in the inhibition of cellular protein synthesis. Lysates prepared from these infected cells are impaired in their ability to translate endogenous or exogenous cellular and viral mRNAs. The ability of initiation factors from rabbit reticulocytes to stimulate protein synthesis in these lysates was examined. Preparations of eukaryotic initiation factor 2 (eIF-2) and the guanine nucleotide exchange factor (GEF) stimulated protein synthesis strongly in L cell lysates from infected cells but only slightly in lysates from mock-infected cells. Maximal stimulation was obtained when a fraction containing eukaryotic initiation factors 4B (eIF-4B) and 4F (eIF-4F) was also present. In lysates from infected cells, these initiation factors increased endogenous cellular mRNA translation on the average 2-fold. In contrast, endogenous viral mRNA translation was increased to a much greater extent: the M protein was stimulated 8-fold, NS 5-fold, N 2.5-fold, and G 12-fold. When fractions containing eIF-4B, eIF-4F, or eIF-4A were added to these lysates in the presence of eIF-2, all three stimulated translation. Fractions containing rabbit reticulocyte initiation factors eIF-3 and eIF-6 had no effect on translation in either lysate. The results suggest that lysates from infected L cells are defective in the catalytic utilization of eIF-2 and deficient in mRNA binding protein activity.     

Cellular mRNA translation is blocked at both initiation and elongation after infection by influenza virus or adenovirus.

During influenza virus infection, protein synthesis is maintained at high levels and a dramatic switch from cellular to viral protein synthesis occurs despite the presence of high levels of functional cellular mRNAs in the cytoplasm of infected cells (M. G. Katze and R. M. Krug, Mol. Cell. Biol. 4:2198-2206, 1984). To determine the step at which the block in cellular mRNA translation occurs, we compared the polysome association of several representative cellular mRNAs (actin, glyceraldehyde-3-phosphate dehydrogenase, and pHe7 mRNAs) in infected and uninfected HeLa cells. We showed that most of these cellular mRNAs remained polysome associated after influenza viral infection, indicating that the elongation of the proteins encoded by these cellular mRNAs was severely inhibited. Because the polysomes containing these cellular mRNAs did not increase in size but either remained the same size or decreased in size, the initiation step in cellular protein synthesis must also have been defective. Several control experiments established that the cellular mRNAs sedimenting in the polysome region of sucrose gradients were in fact associated with polyribosomes. Most definitively, puromycin treatment of infected cells caused the dissociation of polysomes and the release of cellular, as well as viral, mRNAs from the polysomes, indicating that the cellular mRNAs were associated with polysomes that were capable of forming at least a single peptide bond. A similar analysis was performed with HeLa cells infected by adenovirus, which also dramatically shuts down cellular protein synthesis. Again, it was found that most of the cellular mRNAs, which were translatable in reticulocyte extracts, remained associated with polysomes and that there was a combined initiation-elongation block to cellular protein synthesis. In cells infected by both adenovirus and influenza virus, influenza viral mRNAs were on larger polysomes than were several late adenoviral mRNAs with comparably sized coding regions. In addition, after influenza virus superinfection of cells infected by the adenovirus mutant dl331, a situation in which there is a limitation in the amount of functional initiation factor eIF-2 (M. G. Katze, B. M. Detjen, B. Safer, and R. M. Krug, Mol. Cell. Biol. 6:1741-1750, 1986), influenza viral mRNAs, but not late adenoviral mRNAs, were on polysomes. These results indicate that influenza viral mRNAs are better initiators of translation than are late adenoviral mRNAs.     

Inhibition of HeLa cell protein synthesis following poliovirus infection correlates with the proteolysis of a 220,000-dalton polypeptide associated with eucaryotic initiation factor 3 and a cap binding protein complex

Following poliovirus infection of HeLa cells, the synthesis of cellular proteins is inhibited but translation of poliovirus mRNA proceeds. The defect in the recognition of host cell mRNA may be due to a change in a cap recognition complex which, when added to an infected cell lysate, restores the ability to translate capped mRNAs. We employed immunoblotting techniques to examine initiation factors in crude lysates from uninfected and poliovirus-infected HeLa cells. Using an antiserum against eucaryotic initiation factor 3, we detected an antigen of approximate molecular weight 220,000 in uninfected cell lysates but not in infected cell lysates. Antigenically related polypeptides of 100,000 to 130,000 daltons, presumably degradation products, were detected in the infected cell lysate. The time course for degradation of the 220,000-dalton polypeptide correlates with that for inhibition of cellular protein synthesis in vivo. A portion of the population of 220,000-dalton polypeptides apparently associates with initiation factor eIF3 but is readily dissociated in buffers containing high salt. Affinity-purified antibodies against the polypeptide recognize a protein of the same size in a purified preparation of a cap binding protein complex obtained by cap-affinity chromatography. We postulate that the 220,000-dalton polypeptide is an essential component of the cap recognition complex and that its degradation in poliovirus-infected cells results in the inhibition of host cell translation. These results are in the first demonstration of a specific structural defect in an initiation factor resulting from poliovirus infection.     

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.     

Inhibition of viral mRNA translation in interferon-treated L cells infected with reovirus.

Murine L cells were treated with interferon (IFN) concentrations which reduced by 75 to 80% the synthesis of viral mRNA after infection with reovirus. Protein synthesis was not inhibited in these cells up to 6 h after infection, but a large fraction of the viral mRNA was not associated with polyribosomes and sedimented at about 50S. In contrast, most of the reovirus mRNA was associated with polyribosomes in control infected cells. This mRNA was of similar size to non-polyribosomal mRNA from IFN-treated cells when analyzed by Northern blot hybridization with a cloned cDNA for the s2 reovirus mRNA, indicating that the non-polyribosomal mRNA was not appreciably degraded. Viral mRNA was labeled with [3H]uridine and the non-polyribosomal mRNA was isolated from IFN-treated cells. This mRNA could quantitatively bind to 80S initiation complexes when incubated in a rabbit reticulocyte cell-free system. These findings indicated that the non-polyribosomal RNA was translatable, but that its binding to functional initiation complexes was inhibited in IFN-treated cells by a discriminatory mechanism, which did not affect translation of cellular mRNA. Previous experiments showed that mRNA is blocked in 48S complexes when the alpha subunit of initiation factor eIF-2 is phosphorylated by the double-stranded RNA-dependent protein kinase induced by IFN. A localized activation of this kinase could explain the block of viral mRNA in 48S complexes. By labeling the phosphoproteins of IFN-treated cells with 32P, eIF-2 (alpha P) was shown to cosediment with non-polyribosomal mRNA, presumably in 48S complexes.     

Translational control by influenza virus. Selective and cap-dependent translation of viral mRNAs in infected cells.

In cells infected by influenza virus type A, host protein synthesis undergoes a rapid and dramatic shutoff. To define the molecular mechanisms underlying this selective translation, a transfection/infection protocol was developed utilizing viral and cellular cDNA clones. When COS-1 cells were transfected with cDNAs encoding nonviral genes and subsequently infected with influenza virus, protein expression from the exogenous genes was diminished, similar to the endogenous cellular genes. However, when cells were transfected with a truncated influenza viral nucleocapsid protein (NP-S) gene, the NP-S protein was made as efficiently in influenza virus infected cells as in uninfected cells, showing that the NP-S mRNA, although expressed independently of the influenza virus replication machinery, was still recognized as a viral and not a cellular mRNA. Northern blot analysis demonstrated that the selective blocks to nonviral protein synthesis were at the level of translation. Moreover, polysome experiments revealed that the translational blocks occurred at both the initiation and elongation stages of cellular protein synthesis. Finally, we utilized this transfection/infection system as well as double infection experiments to demonstrate that the translation of influenza viral mRNAs probably occurred in a cap-dependent manner as poliovirus infection inhibited influenza viral mRNA translation.     

Minimum internal ribosome entry site required for poliovirus infectivity.

Translation initiation by internal ribosome binding is a recently discovered mechanism of eukaryotic viral and cellular protein synthesis in which ribosome subunits interact with the mRNAs at internal sites in the 5' untranslated RNA sequences and not with the 5' methylguanosine cap structure present at the extreme 5' ends of mRNA molecules. Uncapped poliovirus mRNAs harbor internal ribosome entry sites (IRES) in their long and highly structured 5' noncoding regions. Such IRES sequences are required for viral protein synthesis. In this study, a novel poliovirus was isolated whose genomic RNA contains two gross deletions removing approximately 100 nucleotides from the predicted IRES sequences within the 5' noncoding region. The deletions originated from previously in vivo-selected viral revertants displaying non-temperature-sensitive phenotypes. Each revertant had a different predicted stem-loop structure within the 5' noncoding region of their genomic RNAs deleted. The mutant poliovirus (Se1-5NC-delta DG) described in this study contains both stem-loop deletions in a single RNA genome, thereby creating a minimum IRES. Se1-5NC-delta DG exhibited slow growth and a pinpoint plaque phenotype following infection of HeLa cells, delayed onset of protein synthesis in vivo, and defective initiation during in vitro translation of the mutated poliovirus mRNAs. Interestingly, the peak levels of viral RNA synthesis in cells infected with Se1-5NC-delta DG occurred at slightly later times in infection than those achieved by wild-type poliovirus, but these mutant virus RNAs accumulated in the host cells during the late phases of virus infection. UV cross-linking assays with the 5' noncoding regions of wild-type and mutated RNAs were carried out in cytoplasmic extracts from HeLa cells and neuronal cells and in reticulocyte lysates to identify the cellular factors that interact with the putative IRES elements. The cellular proteins that were cross-linked to the minimum IRES may represent factors playing an essential role in internal translation initiation of poliovirus mRNAs.     

Eukaryotic protein synthesis initiation factor eIF-3: determination of concentration and association with ribosomes in rabbit reticulocyte and HeLa cell lysates

Monoclonal and polyclonal antibodies against eukaryotic protein synthesis initiation factor eIF-3 were produced and used to determine the factor concentration and its association with ribosomes in rabbit reticulocyte and HeLa cell lysates. In rabbit reticulocyte lysate we found 3-5 micrograms eIF-3 per mg total protein and in HeLa cell lysate 8-15 micrograms eIF-3 per mg total protein. The initiation factor eIF-3 was found both associated with 40 S ribosomal subunits and free in the post-ribosomal supernatant. However, no eIF-3 could be detected on mono- or polyribosomes.     

Encephalomyocarditis Virus Infection of Mouse Plasmacytoma Cells I. Inhibition of Cellular Protein Synthesis

Mouse plasmacytoma ascites tumor cells (MOPC 460) were efficiently infected with encephalomyocarditis virus. Inhibition of host protein synthesis was evident after 2 h and complete by 4 h postinfection. The mechanism by which virus infection results in inhibition of host cell protein synthesis was studied in vitro. Cell-free protein-synthesizing systems, prepared from uninfected and infected cells, were found to be equally active with respect to their abilities to translate cellular and viral mRNAs. The plasmacytoma cell-free system was also shown to be insensitive to the addition of double-stranded viral RNA. Host cellular mRNA was isolated from uninfected and infected cells. No difference in the amount or size distribution of the mRNA was detected. However, the mRNA from infected cells was translated only 46 to 49% as actively as that from uninfected cells. mRNA isolated from cells in which initiation of protein synthesis was inhibited with pactamycin was similarly inactivated. Simultaneous addition of viral RNA and cellular mRNA to the plasmacytoma cell-free system resulted in a complete suppression of the translation of the cellular message, whereas viral RNA was translated normally.     

Cap-Poly(A) synergy in mammalian cell-free extracts. Investigation of the requirements for poly(A)-mediated stimulation of translation initiation

The 5' cap and 3' poly(A) tail of eukaryotic mRNAs cooperate to stimulate synergistically translation initiation in vivo, a phenomenon observed to date in vitro only in translation systems containing endogenous competitor mRNAs. Here we describe nuclease-treated rabbit reticulocyte lysates and HeLa cell cytoplasmic extracts that reproduce cap-poly(A) synergy in the absence of such competitor RNAs. Extracts were rendered poly(A)-dependent by ultracentrifugation to partially deplete them of ribosomes and associated initiation factors. Under optimal conditions, values for synergy in reticulocyte lysates approached 10-fold. By using this system, we investigated the molecular mechanism of poly(A) stimulation of translation. Maximal cap-poly(A) cooperativity required the integrity of the eukaryotic initiation factor 4G-poly(A)-binding protein (eIF4G-PABP) interaction, suggesting that synergy results from mRNA circularization. In addition, polyadenylation stimulated uncapped cellular mRNA translation and that driven by the encephalomyocarditis virus internal ribosome entry segment (IRES). These effects of poly(A) were also sensitive to disruption of the eIF4G-PABP interaction, suggesting that 5'-3' end cross-talk is functionally conserved between classical mRNAs and an IRES-containing mRNA. Finally, we demonstrate that a rotaviral non-structural protein that evicts PABP from eIF4G is capable of provoking the shut-off of host cell translation seen during rotavirus infection.     

Studies on the biosynthesis of neurofilament proteins

To determine whether the triplet polypeptides of neurofilaments arise by degradation of precursor, we studied the biosynthesis of neurofilament polypeptides both in vivo and in cell-free systems. Neurofilament-enriched fractions and polyribosomes were prepared from the same rabbit spinal cord homogenates. At 1 h after intracisternal administration of [34S]methionine, radiolabeled neurofilament proteins were detected in spinal cord homogenates as well as in isolated filaments. When polyribosomes from rabbit spinal cord were allowed to incorporate [35S]methionine into protein, triplet polypeptides were among the proteins labeled. Addition of spinal cord polyribosomes to rabbit reticulocyte lysates led to several cycles of translation of the spinal cord mRNA; the three neurofilament polypeptides were among the proteins synthesized in this system. The results demonstrate that the triplet polypeptides of neurofilaments are synthesized as such in the course of individual translational events and do not arise from degradation of P200 or a larger precursor.     

On the association of mRNA with the cytoskeleton in uninfected and adenovirus-infected human KB cells

Triton-insoluble cytoskeletons were prepared from uninfected and adenovirus-infected KB cells. Gradient analysis showed that all cellular polyribosomes were present in the cytoskeletons. After disaggregation of the polyribosomes, in vivo or in vitro, most of the messenger RNA (mRNA) remained associated with the cytoskeletal framework. Translation experiments showed that most mRNA species were present in a bound (cytoskeletal), as well as in an unbound state. However, whereas some mRNA species were predominant as unbound mRNP particles, other mRNA species were almost exclusively found in polyribosomes associated with the cytoskeletal framework. Incubation of cytoskeletons in an mRNA-dependent reticulocyte cell-free system revealed synthesis of the same set of polypeptides as took place when using whole cells. Furthermore, the gradual shift from translation of cellular to translation of viral mRNA species during late phase of productive infection with adenovirus could also be followed when cytoskeletons were translated in the cell-free system. These results support the hypothesis that Triton X-100 extraction does not remove actively translating mRNA from the cells, thus suggesting a functional relationship between mRNA translation and mRNA binding to a cytoskeletal framework.     

A segment of the 5'' nontranslated region of encephalomyocarditis virus RNA directs internal entry of ribosomes during in vitro translation.

Picornavirus RNAs are uncapped messengers and have unusually long 5' nontranslated regions (5'NTRs) which contain many noninitiating AUG triplets. The translational efficiency of different picornavirus RNAs varies between different cell-free extracts and even in the same extract, such as micrococcal nuclease-treated rabbit reticulocyte lysates. The effect of the poliovirus 5'NTR on in vitro translation was compared with that of the 5'NTR of encephalomyocarditis virus by the use of synthetic mRNAs, micrococcal nuclease-treated HeLa cell extracts, and rabbit reticulocyte lysates. Artificial mono- and dicistronic mRNAs synthesized with T7 RNA polymerase were used to investigate whether the 5'NTR of encephalomyocarditis virus RNA contains a potential internal ribosomal entry site. The sequence between nucleotides 260 and 484 in the 5'NTR of encephalomyocarditis RNA was found to play a critical role in the efficient translation in both mono- and dicistronic mRNAs. Our data suggest that an internal ribosomal entry site resides in this region.     

Reversion by hypotonic medium of the shutoff of protein synthesis induced by encephalomyocarditis virus.

Infection of human HeLa cells by picornaviruses produces a drastic inhibition of host protein synthesis. Treatment of encephalomyocarditis virus-infected HeLa cells with hypotonic medium reversed this inhibition; no viral protein synthesis was detected. The blockade of viral translation by hypotonic conditions was observed for a wide range of multiplicities of infection. However, only with low virus-to-cell ratios did cellular protein synthesis resume. The ratio of cellular to viral mRNA translation was strongly influenced by the concentration of monovalent ions present in the culture medium: a high concentration of NaCl or KCl favored the translation of viral mRNA and strongly inhibited cellular protein synthesis, whereas the opposite was true when NaCl was omitted from the culture medium. Once viral protein synthesis had been blocked by hypotonic medium treatment, it resumed when the infected cells were placed in a normal or hypertonic medium, indicating that the viral components synthesized in the infected cells were not destroyed by this treatment. These observations reinforced the idea that ions play a role in discriminating between viral and cellular mRNA translation in virus-infected animal cells.     

Structural difference between the 5'' termini of viral and cellular mRNA in poliovirus-infected cells: possible basis for the inhibition of host protein synthesis.

Host protein synthesis in poliovirus-infected HeLa cells is interrupted, but the host mRNA appears to remain completely intact and unmodified. The average size and poly (A) content of host mRNA was previously known to be unchanged (Koschel, 1974; Leibowitz and Penman, 1971), and this was confirmed. In addition, the 5' terminal methylated "cap" structures remained intact, and no further base modifications at the level of 1 base in 1,000 could be detected. Poliovirus RNA from viruses was previously shown not to have "caps" (Wimmer, 1972), and in this work poliovirus RNA from polyribosomes was found to have pUp at its 5' end. Since, initiation of protein synthesis is probably the basis for the inhibition of cellular protein synthesis in infected cells, the difference in the 5' ends of the host cell and viral RNA could be the basis of selective translation of viral RNA during infection.