Molecular mechanisms of translation initiation in eukaryotes

Translation initiation is a complex process in which initiator tRNA, 40S, and 60S ribosomal subunits are assembled by eukaryotic initiation factors (eIFs) into an 80S ribosome at the initiation codon of mRNA. The cap-binding complex eIF4F and the factors eIF4A and eIF4B are required for binding of 43S complexes (comprising a 40S subunit, eIF2/GTP/Met-tRNAi and eIF3) to the 5' end of capped mRNA but are not sufficient to promote ribosomal scanning to the initiation codon. eIF1A enhances the ability of eIF1 to dissociate aberrantly assembled complexes from mRNA, and these factors synergistically mediate 48S complex assembly at the initiation codon. Joining of 48S complexes to 60S subunits to form 80S ribosomes requires eIF5B, which has an essential ribosome-dependent GTPase activity and hydrolysis of eIF2-bound GTP induced by eIF5. Initiation on a few mRNAs is cap-independent and occurs instead by internal ribosomal entry. Encephalomyocarditis virus (EMCV) and hepatitis C virus epitomize distinct mechanisms of internal ribosomal entry site (IRES)-mediated initiation. The eIF4A and eIF4G subunits of eIF4F bind immediately upstream of the EMCV initiation codon and promote binding of 43S complexes. EMCV initiation does not involve scanning and does not require eIF1, eIF1A, and the eIF4E subunit of eIF4F. Initiation on some EMCV-like IRESs requires additional noncanonical initiation factors, which alter IRES conformation and promote binding of eIF4A/4G. Initiation on the hepatitis C virus IRES is even simpler: 43S complexes containing only eIF2 and eIF3 bind directly to the initiation codon as a result of specific interaction of the IRES and the 40S subunit.

La autoantigen enhances and corrects aberrant translation of poliovirus RNA in reticulocyte lysate

Translation initiation on poliovirus RNA occurs by internal binding of ribosomes to a sequence within the 5' untranslated region. We have previously characterized a HeLa cell protein, p52, that binds to a fragment of the poliovirus 5' untranslated region (K. Meerovitch, J. Pelletier, and N. Sonenberg, Genes Dev. 3:1026-1034, 1989). Here we report the purification of the HeLa p52. Protein microsequencing identified p52 as La autoantigen. The La protein is a human antigen that is recognized by antibodies from patients with autoimmune disorders such as systemic lupus erythematosus and Sjögren's syndrome. We show that the La protein stimulates translation of poliovirus RNA, but not brome mosaic virus, tobacco mosaic virus, and alfalfa mosaic virus 4 RNA, translation in a reticulocyte lysate. In addition, La corrects aberrant translation of poliovirus RNA in a reticulocyte lysate. Subcellular immunolocalization showed that La protein is mainly nuclear, but after poliovirus infection, La is redistributed to the cytoplasm. Our results suggest that La protein is involved in poliovirus internal initiation of translation and might function through a similar mechanism in the translation of cellular mRNAs.

Prokaryotic-like cis elements in the cap-independent internal initiation of translation on picornavirus RNA

Initiation of translation on picornavirus RNAs is accomplished through internal binding of ribosomes to a complex cis-acting element. Here we show that efficient function of this element involves two appropriately spaced smaller elements: UUUCC and an AUG. This conclusion emerged from analysis of the genome structures of spontaneous revertants of mutant polioviruses with extended insertions between the UUUCC and AUG motifs. It was confirmed by the results obtained with specially designed constructs. A similarity to the prokaryotic translation initiation mechanism, which involves the Shine-Dalgarno sequence, is emphasized, but in the picornavirus system the position of the UUUCC must be strictly fixed relative to upstream cis-acting elements, and the AUG may not necessarily serve as an initiation codon.

Conserved structural domains in the 5'-untranslated region of picornaviral genomes: an analysis of the segment controlling translation and neurovirulence

A model of secondary structure common for the central part (ca. 400 nucleotides) of the 5'-untranslated regions (5'-UTR) of all the so far sequenced genomes of polioviruses, coxsackieviruses, and rhinoviruses was derived on the basis of evolutionary and thermodynamic considerations. According to the model, this part of the genome comprises three domains, which appear to be involved, at least in the poliovirus genome, in the control of viral neurovirulence and in vitro translation. Some salient features of this model were supported by investigating RNAs of five poliovirus and one coxsackievirus strains with respect to their accessibility to modifications with dimethyl sulfate and sensitivity to single-strand- and double-strand-specific nucleases. In contrast to the previous suggestion, no major changes in the conformation of the Sabin vaccine poliovirus type 3 5'-UTR due to the transition in position 472 were observed. The biological relevance of the conserved primary and secondary structure elements in the picornaviral 5'-UTRs is discussed.

Conservation of the secondary structure elements of the 5'-untranslated region of cardio- and aphthovirus RNAs

An analysis of published nucleotide sequences of the 5'-untranslated region (5'-UTR) of 7 cardioviruses and 3 aphthoviruses has allowed us to derive a consensus secondary structure model that differs from that previously proposed for the 5'-UTR of entero- and rhinoviruses, though all these viruses belong to the same family, Picornaviridae. The theoretical model derived here was experimentally supported by investigating the accessibility of encephalomyocarditis virus RNA to modifications with dimethyl sulfate and its susceptibility to S1 and cobra venom nucleases. The possible involvement of the 5"-UTR secondary structure domains in the translational control is briefly discussed.

Apoptosis-inducing and apoptosis-preventing functions of poliovirus

Data showing that an apoptotic reaction (the exit into the cytoplasm and nucleolytic internucleosomal degradation of chromosomal DNA, compaction and fragmentation of chromatin, cellular shrinkage, and cytoplasmic blebbing) developed in a subline of HeLa-S3 cells upon nonpermissive poliovirus infection with either a guanidine-sensitive poliovirus in the presence of guanidine, a guanidine-dependent mutant in the absence of guanidine, or certain temperature-sensitive mutants at a restrictive temperature are presented. Essentially, no apoptotic reaction occurred upon permissive infection of these cells. Both permissive and nonpermissive infections resulted in the inhibition of host protein synthesis. Actinomycin D or cycloheximide also elicited a rapid apoptotic reaction in uninfected cells. However, preinfection or coinfection with poliovirus prevented the apoptotic response to the addition of actinomycin D, and preinfection blocked cycloheximide-induced apoptosis as well. These data fit a model in which the cells used are prepared to develop apoptosis, with their viability due to the presence of certain short-lived mRNA and protein species. Poliovirus infection turns on two oppositely directed sets of reactions. On the one hand, the balance is driven toward apoptosis, probably via the shutoff of host macromolecular synthesis. On the other hand, viral protein exhibits antiapoptotic activity, thereby preventing premature cell death. To our knowledge, this is the first description of an antiapoptotic function for an RNA virus.

The genomes of attenuated and virulent poliovirus strains differ in their in vitro translation efficiencies

In mRNA-dependent extracts of Krebs-2 cells, RNAs from attenuated strains of poliovirus type 1 and type 3 exhibited diminished template activity as compared to RNAs from the respective virulent counterparts. This defect appeared to be due to the impaired initiation of viral polyprotein synthesis as evidenced by a relatively low level of accumulation of polypeptide 1a (which corresponds to an NH2-terminal region of the polyprotein) in samples programmed with RNAs from attenuated strains. In reticulocyte lysates, where poliovirus RNA is translated predominantly from abnormal (internal) sites [Dorner et al. (1984) J. Virol. 50, 507-514], this difference in the overall template activity of the attenuated and virulent poliovirus genomes was less pronounced, but the correct initiation (as judged by polypeptide 1a accumulation) was again more efficient on RNAs from virulent strains. It is suggested that template deficiency is a factor contributing to the attenuated phenotype of poliovirus strains studied. A possible involvement of nucleotide sequences located far upstream from the initiator codon in the control of translation of poliovirus genome is briefly discussed.

Internal translation initiation on poliovirus RNA: further characterization of La function in poliovirus translation in vitro

Initiation of poliovirus RNA translation by internal entry of ribosomes is believed to require the participation of trans-acting factors. The mechanism of action of these factors is poorly defined. The limiting amount of one of these factors, La protein, in rabbit reticulocyte lysates (RRL) has been postulated to partially explain the inefficient translation of poliovirus RNA in this system. To further characterize La activity in translation and to identify other potential limiting factors, we assayed the ability of La protein as well as purified initiation factors, eIF-2, guanine nucleotide exchange factor (GEF), eIF-4A, eIF-4B, eIF-4F, and eIF-3, to stimulate the synthesis of P1, the capsid precursor protein, in poliovirus type 1 (Mahoney) RNA-programmed RRL. Of the proteins tested, only La, GEF, and to some extent eIF-2 stimulated the synthesis of P1. The enhanced translation of P1 in response to La occurred concomitantly with the inhibition of synthesis of most aberrant polypeptides, resulting from initiation in the middle of the genome. Deletion of the carboxy-terminal half (214 amino acids) of La did not decrease its binding to the poliovirus 5' untranslated region but abrogated the stimulatory and correcting activity in translation. In contrast to La, GEF and eIF-2 stimulated the overall translation and increased the synthesis of aberrant products as well as P1. Neither La, GEF, nor any other factor stimulated translation of encephalomyocarditis virus RNA in RRL. The implications of these findings for the mechanism of internal translation initiation on picornavirus RNAs are discussed.

Kissing of the two predominant hairpin loops in the coxsackie B virus 3' untranslated region is the essential structural feature of the origin of replication required for negative-strand RNA synthesis

Higher-order RNA structures in the 3' untranslated region (3'UTR) of enteroviruses are thought to play a pivotal role in viral negative-strand RNA synthesis. The structure of the 3'UTR was predicted by thermodynamic calculations using the STAR (structural analysis of RNA) computer program and experimentally verified using chemical and enzymatic probing of in vitro-synthesized RNA. A possible pseudoknot interaction between the 3D polymerase coding sequence and domain Y and a "kissing" interaction between domains X and Y was further studied by mutational analysis, using an infectious coxsackie B3 virus cDNA clone (domain designation as proposed by E. V. Pilipenko, S. V. Maslova, A. N. Sinyakov, and V.I. Agol (Nucleic Acids Res. 20:1739-1745, 1992). The higher-order RNA structure of the 3'UTR appeared to be maintained by an intramolecular kissing interaction between the loops of the two predominant hairpin structures (X and Y) within the 3'UTR. Disturbing this interaction had no effect on viral translation and processing of the polyprotein but exerted a primary effect on viral replication, as was demonstrated in a subgenomic coxsackie B3 viral replicon, in which the capsid P1 region was replaced by the luciferase gene. Mutational analysis did not support the existence of the pseudoknot interaction between hairpin loop Y and the 3D polymerase coding sequence. Based on these experiments, we constructed a three-dimensional model of the 3'UTR of coxsackie B virus that shows the kissing interaction as the essential structural feature of the origin of replication required for its functional competence.

Point mutations modify the response of poliovirus RNA to a translation initiation factor: a comparison of neurovirulent and attenuated strains

Upon translation of poliovirus RNA in reticulocyte lysates, initiation occurs largely "incorrectly," that is, at sites in the middle of the viral genome rather than at the beginning of the polyprotein reading frame; the anomaly appears to be due to an initiation factor deficiency. Here, a fraction which stimulated initiation at the correct site, provisionally called "initiation correcting factor" (ICF), was partially purified from Krebs-2 cells. The ICF activity appeared to copurify with a complex of initiation factors eIF-2 and eIF-2B. The ability of ICF to stimulate, in reticulocyte lysates, the correct initiation of translation on the RNAs from neurovirulent and attenuated type 1 and type 3 poliovirus strains was investigated. Like crude initiation factor preparations, ICF appeared to be relatively less active with the RNAs from attenuated strains, the difference being especially pronounced for the type 3 strains. For the latter strains, the data suggested an important role of the nucleotide at, and perhaps around, position 472 in determining a response to the addition of ICF. It is proposed that interaction of a specific segment of the viral RNA with one or more of initiation factors plays an important part in the mechanism of translation of the picornavirus genomes, poliovirus attenuation, and, possibly, pathogenesis of poliomyelitis.

Evolution of circulating wild poliovirus and of vaccine-derived poliovirus in an immunodeficient patient: a unifying model

We determined nucleotide sequences of the VP1 and 2AB genes and portions of the 2C and 3D genes of two evolving poliovirus lineages: circulating wild viruses of T geotype and Sabin vaccine-derived isolates from an immunodeficient patient. Different regions of the viral RNA were found to evolve nonsynchronously, and the rate of evolution of the 2AB region in the vaccine-derived population was not constant throughout its history. Synonymous replacements occurred not completely randomly, suggesting the need for conservation of certain rare codons (possibly to control translation elongation) and the existence of unidentified constraints in the viral RNA structure. Nevertheless the major contribution to the evolution of the two lineages came from linear accumulation of synonymous substitutions. Therefore, in agreement with current theories of viral evolution, we suggest that the majority of the mutations in both lineages were fixed as a result of successive sampling, from the heterogeneous populations, of random portions containing predominantly neutral and possibly adverse mutations. As a result of such a mode of evolution, the virus fitness may be maintained at a more or less constant level or may decrease unless more-fit variants are stochastically generated. The proposed unifying model of natural poliovirus evolution has important implications for the epidemiology of poliomyelitis.

Towards identification of cis-acting elements involved in the replication of enterovirus and rhinovirus RNAs: a proposal for the existence of tRNA-like terminal structures

On the basis of a comparative analysis of published sequences, models for the secondary structure of the 3'-terminal [poly(A)-preceding] untranslated region of the entero- and rhinovirus RNAs were worked out. The models for all these viruses share a common core element, but there are an extra enterovirus-specific element and still an additional element characteristic of a subset of enterovirus RNAs. The two latter models were verified for poliovirus and coxsackievirus B genomes by testing with single-strand and double-strand specific enzymatic and chemical probes. A tRNA-like tertiary structure model for the 3'-terminal folding of enterovirus RNAs was proposed. A similar folding was proposed for the 3' termini of the negative RNA strands as well as for the 5' termini of the positive strand of all entero- and rhinovirus RNAs. Implications of these data for template recognition during negative and positive RNA strands synthesis and for the evolution of the picornavirus genomes are discussed.

Poliovirus protein 3A inhibits tumor necrosis factor (TNF)-induced apoptosis by eliminating the TNF receptor from the cell surface

Viral infections often trigger host defensive reactions by activating intrinsic (intracellular) and extrinsic (receptor-mediated) apoptotic pathways. Poliovirus is known to encode an antiapoptotic function(s) suppressing the intrinsic pathway. Here, the effect of poliovirus nonstructural proteins on cell sensitivity to tumor necrosis factor (TNF)-induced (i.e., receptor-mediated) apoptosis was studied. This sensitivity is dramatically enhanced by the viral proteinase 2A, due, most likely, to inhibition of cellular translation. On the other hand, cells expressing poliovirus noncapsid proteins 3A and 2B exhibit strong TNF resistance. Expression of 3A neutralizes the proapoptotic activity of 2A and results in a specific suppression of TNF signaling, including the lack of activation of NF-kappaB, due to elimination of the TNF receptor from the cell surface. In agreement with this, poliovirus infection results in a dramatic decrease in TNF receptor abundance on the surfaces of infected cells as early as 4 h postinfection. Poliovirus proteins that confer resistance to TNF interfere with endoplasmic reticulum-Golgi protein trafficking, and their effect on TNF signaling can be imitated by brefeldin A, suggesting that the mechanism of poliovirus-mediated resistance to TNF is a result of aberrant TNF receptor trafficking.

Nonreplicative RNA recombination in poliovirus

Current models of recombination between viral RNAs are based on replicative template-switch mechanisms. The existence of nonreplicative RNA recombination in poliovirus is demonstrated in the present study by the rescue of viable viruses after cotransfections with different pairs of genomic RNA fragments with suppressed translatable and replicating capacities. Approximately 100 distinct recombinant genomes have been identified. The majority of crossovers occurred between nonhomologous segments of the partners and might have resulted from transesterification reactions, not necessarily involving an enzymatic activity. Some of the crossover loci are clustered. The origin of some of these "hot spots" could be explained by invoking structures similar to known ribozymes. A significant proportion of recombinant RNAs contained the entire 5' partner, if its 3' end was oxidized or phosphorylated prior to being mixed with the 3' partner. All of these observations are consistent with a mechanism that involves intermediary formation of the 2',3'-cyclic phosphate and 5'-hydroxyl termini. It is proposed that nonreplicative RNA recombination may contribute to evolutionarily significant RNA rearrangements.

The primary structure of crossover regions of intertypic poliovirus recombinants: a model of recombination between RNA genomes

The nucleotide sequence of crossover sites in the genome of four intertypic (type 3/type 1) poliovirus recombinants has been determined. The approximate boundaries of the crossover regions were first estimated by RNase T1 oligonucleotide mapping of the recombinant genomes; then appropriate regions were sequenced by the chain termination method using oligonucleotide-primed reverse transcription of the recombinant RNAs. The crossover sites (defined as the contiguous sequences shared by the recombinant and both parental genomes flanked, in the recombinant genome, by heterotypic RNA segments) are 5, 5, 7, and 11 nucleotides long, respectively. The recombination was precise and was not accompanied by any other genetic alterations. The recombination sites were found to be located within genome segments having a potential to form secondary structure elements. Based on this observation, a model of recombination between picornaviral RNA genomes has been proposed. The essence of this model consists in bringing together homologous regions of two recombining RNA genomes via formation of intermolecular duplexes, detachment of the nascent 3' end of the newly synthesized complementary RNA from a "parting" site on the first template and its subsequent "jumping" to the identical (or closely related) "anchoring" site on the other template. Features of this model are discussed in some detail.

Paradoxes of the replication of picornaviral genomes

A wealth of experimental data on the mechanism of the picornavirus genome replication has accumulated. Not infrequently, however, conclusions derived from these data appear to contradict each other. On the one hand, initiation of a complementary RNA strand can be demonstrated to occur in a solution containing only the poliovirus RNA polymerase, VPg, uridine triphosphate, poly(A) template and appropriate ions. On the other hand, convincing experiments suggest that efficient initiation of a viral complementary RNA strand requires complex cis-acting signals on the viral RNA template, additional viral and possibly cellular proteins as well as a membrane-containing environment. On the one hand, there is evidence that the viral RNA, in order to be replicated, should first be translated, but on the other hand, the viral RNA polymerase appears to be unable to overcome the ribosome barrier. Possible solutions for these and several other similar paradoxes are discussed, along with less contradictory results on the properties of the picornaviral replicative proteins. Recent results suggesting that recombination and other rearrangements of the viral RNA genomes may be accomplished not only by the replicative template switching but also by nonreplicative mechanisms are also briefly reviewed.

The 5'-untranslated region of picornaviral genomes

Picornaviruses are small naked icosahedral viruses with a single-stranded RNA genome of positive polarity. According to current taxonomy, the family includes four genera: Enterouirus (polioviruses, coxsackieviruses, echoviruses, and other enteroviruses), Rhinovirus, Curdiouirus [encephalomyocarditis virus (EMCV), mengovirus, Theiler's murine encephalomyelitis virus (TMEV)], and Aphthouirus [foot-and-mouth disease viruses (FMDV)]. There are also some, as yet, unclassified picornaviruses [e.g., hepatitis A virus (HAW] that should certainly be assessed as a separate genus. Studies on the molecular biology of picornaviruses might be divided into two periods: those before and after the first sequencing of the poliovirus genome. The 5'-untranslated region (5-UTR) of the viral genome was one of the unexpected problems. This segment proved to be immensely long: about 750 nucleotides or ∼10% of the genome length. There were also other unusual features (e.g., multiple AUG triplets preceding the single open reading frame (ORF) that encodes the viral polyprotein). This chapter shows that the picornaviral 5-UTRs are not only involved in such essential events as the synthesis of viral proteins and RNAs that could be expected to some extent, although some of the underlying mechanisms appeared to be quite a surprise, but also may determine diverse biological phenotypes from the plaque size or thermosensitivity of reproduction to attenuation of neurovirulence. Furthermore, a close inspection of the 5-UTR structure unravels certain hidden facets of the evolution of the picornaviral genome. Finally, the conclusions drawn from the experiments with the picornaviral5-UTRs provide important clues for understanding the functional capabilities of the eukaryotic ribosomes.

Two types of death of poliovirus-infected cells: caspase involvement in the apoptosis but not cytopathic effect

The death of poliovirus-infected cells may occur in two forms: canonical cytopathic effect (CPE) (on productive infections) or apoptosis (when the viral reproduction is hindered by certain drugs or some other restrictive conditions). Morphological manifestations of the CPE and apoptosis, being distinct, share some traits (e.g., chromatin condensation and nuclear deformation). It was shown here that a permeable caspase inhibitor, benzyloxycarbonyl-Val-Ala-Asp-(OMe) fluoromethyl ketone (zVAD.fmk), prevented the development of the poliovirus-induced apoptosis on abortive infection. The apoptotic pathway could be dissected by an inhibitor of chymotrypsin-like serine proteases, N-tosyl-l-phenylalanine chloromethyl ketone (TPCK), which prevented the cleavage of DNA to oligonucleosome-sized pieces and nuclear fragmentation but did not suppress cellular shrinkage, cytoplasmic blebbing, and partial chromatin condensation. These results demonstrate that caspase activation is involved in the execution phase of the viral apoptosis and suggest that a nuclear subset of the apoptotic program is under a separate control, involving a TPCK-sensitive event. Neither zVAD.fmk nor TPCK, at the concentrations affecting the apoptotic response, exerted appreciable influence on the virus growth or cellular pathological changes on productive infection, indicating that the pathways leading to the poliovirus-evoked CPE and apoptosis are different.

Competing death programs in poliovirus-infected cells: commitment switch in the middle of the infectious cycle

Productive poliovirus infection of HeLa cells leads to the canonical cytopathic effect (CPE), whereas certain types of abortive infection result in apoptosis. To define the time course of commitment to the different types of poliovirus-induced death, inhibitors of viral replication (guanidine HCl) or translation (cycloheximide) were added at different times postinfection (p.i.). Early in the infection (during the first approximately 2 h p.i.), predominantly proapoptotic viral function was expressed, rendering the cells committed to apoptosis, which developed several hours after viral expression was arrested. In the middle of infection, concomitantly with the onset of fast generation of viral progeny, the implementation of the viral apoptotic program was abruptly interrupted. In particular, activation of an Asp-Glu-Val-Asp (DEVD)-specific caspase(s) occurring in the apoptosis-committed cells was prevented by the ongoing productive infection. Simultaneously, the cells retaining normal or nearly normal morphology became committed to CPE, which eventually developed regardless of whether or not further viral expression was allowed to proceed. The implementation of the poliovirus-induced apoptotic program was suppressed in HeLa cells overexpressing the Bcl-2 protein, indicating that the fate of poliovirus-infected cells depends on the balance of host and viral pro- and antiapoptotic factors.

Genetic studies on the poliovirus 2C protein, an NTPase. A plausible mechanism of guanidine effect on the 2C function and evidence for the importance of 2C oligomerization

Poliovirus RNA replication is known to be inhibited by millimolar concentrations of guanidine. A variety of guanidine-resistant (gr) and guanidine-dependent (gd) poliovirus strains were selected, and mutations responsible for the phenotypic alterations were mapped to distinct loci of the viral NTP-binding pattern containing protein 2C. Together with already published results, our data have demonstrated that the overwhelming majority of guanidine mutants of poliovirus 2C can be assigned to one of the two classes, N (with a change in Asn179) or M (with a change in Met187). As inferred from the structure/function relations in other NTP-binding proteins, both these "main" mutations should reside in a loop adjoining the so-called B motif known to interact with the Mg2+ involved in the NTP splitting. In classes M (always) and N (not infrequently), these B motif mutations were combined with mutations in, or close to, motif A (involved in binding of the NTP phosphate moieties) and/or motif C (another conserved element of a subset of NTP-binding proteins). These data strongly support the notion that the region of polypeptide 2C involved in the NTP utilization is affected by the guanidine mutations and by the presence of the drug itself. The mutations, however, never altered highly conserved amino acid residues assumed to be essential for the NTP binding or splitting. These facts and some other considerations led us to propose that guanidine affects coupling between the NTP binding and/or splitting, on the one hand, and the 2C function (related to conformational changes), on the other. Both N and M classes of mutants contain gr and gd variants, and the gr/gd interconversion as well as modulations of the guanidine phenotype can be caused by additional mutations within each class; sometimes, these additional substitutions are located far away from the "main" mutations. It is suggested that the target for guanidine action involves long-range tertiary interactions. Under conditions restrictive for the individual growth of each parent, efficient reciprocal intra-allelic complementation between guanidine-sensitive (gs) and gd strains (of M or N classes) was observed. The complementation occurred at the level of viral RNA synthesis. These data allowed us to propose that oligomerization of polypeptide 2C is an essential step in the replication of viral genome.

Restricted growth of attenuated poliovirus strains in cultured cells of a human neuroblastoma

Cultured cells of a human neuroblastoma, SK-N-MC, were found to be highly resistant to Sabin attenuated poliovirus types 1 and 2 strains; no appreciable cytopathic effect was observed, and the total harvest was generally in the order of 1 PFU per cell or less. On the other hand, related neurovirulent strains of these antigenic types produced a relatively good (2 orders of magnitude higher) yield in a markedly protracted infectious cycle. The limited growth of the attenuated virus in the neuroblastoma cells appeared to be confined to a minor cell subpopulation. Experiments with intratypic (type 1) poliovirus recombinants suggested that the major genetic determinants limiting reproduction of the attenuated polioviruses in the neuroblastoma cells are located in the 5' half of the viral RNA, although the 3' half also appears to contribute somewhat to this phenotype. The possibility that neuroblastoma cells may represent an in vitro model for studying poliovirus neurovirulence is briefly discussed.

Studies on the recombination between RNA genomes of poliovirus: the primary structure and nonrandom distribution of crossover regions in the genomes of intertypic poliovirus recombinants

A series of intertypic (type 3/type 1) poliovirus recombinants was obtained whose crossover sites were expected to be located in the middle of the viral genome, between the loci encoding type-specific antigenic properties, on the 5' side, and an altered sensitivity to guanidine, on the 3' side. The primary structures of the crossover regions in the genomes of these recombinants were determined by the primer extension method. The length of the crossover sites (the uninterrupted sequences shared by the recombinant and both parental genomes that are flanked, in the recombinant RNAs, by two heterotypic segments) varied between 2 and 32 nucleotides, but the majority of the sites were 5 nucleotides long or shorter. The crossover sites were nonrandomly distributed over the presumably available genome region: only a single such site was found within the gene for polypeptide 2A, whereas an apparent clustering of the crossover sites was encountered in other genomic segments. When the crossover sites were superimposed on a model of the secondary structure of the relevant region of the viral RNA molecule, a pattern consistent with the previously proposed mechanism of poliovirus recombination (L.I. Romanova, V.M. Blinov, E.A. Tolskaya, E.G. Viktorova, M.S. Kolesnikova, E.I. Guseva, and V.I. Agol (1986) Virology 155, 202-213) was observed. It is suggested that the nonrandom distribution of the crossover sites in the genomes of intertypic poliovirus recombinants was due to two factors: the existence of preferred sites for recombination, and selection against recombinants with a lowered level of viability.

Analysis of the functional significance of amino acid residues in the putative NTP-binding pattern of the poliovirus 2C protein

The amino acid sequence of the poliovirus 2C protein contains two highly conserved stretches, GSPGTGKS136 and MDD177, which correspond to the consensus 'A' and 'B' motifs (GXXXXGKS/T and DD/E, respectively) found in nucleoside triphosphate-binding proteins. To assess the functional importance of these amino acid sequences, we changed conserved and non-conserved amino acids. The replacement of the non-conserved Thr133 residue with Ser or Ala did not markedly change the virus phenotype. Similarly, replacement of the non-conserved Pro131 residue by Ala did not abolish virus viability, but changes of this residue to Thr or Asn were not tolerated. No viable mutant could be isolated after transfection of cultured cells with transcripts mutated at the conserved Lys135, Ser136 or Asp177 residues. However, true revertants were selected from Arg135 and Ser135 mutants, from Glu177 and Gly177 mutants, and from Ala136 mutants. Thr136 mutants not only gave rise to true revertants, but also to two independent isolates of a suppressor mutant, Asn140----Tyr. All the lethal mutations resulted in severe inhibition of viral RNA synthesis in vivo, although no translational deficiency was detected in a cell-free system. This is the first direct evidence for the functional significance of the nucleoside triphosphate-binding pattern in the poliovirus 2C protein.

RNA duplex unwinding activity of poliovirus RNA-dependent RNA polymerase 3Dpol

The ability of highly purified preparations of poliovirus RNA-dependent RNA polymerase, 3Dpol, to unwind RNA duplex structures was examined during a chain elongation reaction in vitro. Using an antisense RNA prehybridized to an RNA template, we show that poliovirus polymerase can elongate through a highly stable RNA duplex of over 1,000 bp. Radiolabeled antisense RNA was displaced from the template during the reaction, and product RNAs which were equal in length to the template strand were synthesized. Unwinding did not occur in the absence of chain elongation and did not require hydrolysis of the gamma-phosphate of ATP. The rate of elongation through the duplex region was comparable to the rate of elongation on the single-stranded region of the template. Parallel experiments conducted with avian myeloblastosis virus reverse transcriptase showed that this enzyme was not able to unwind the RNA duplex, suggesting that strand displacement by poliovirus 3Dpol is not a property shared by all polymerases.