The cellular polypeptide p57 (pyrimidine tract-binding protein) binds to multiple sites in the poliovirus 5' nontranslated region

Initiation of translation of poliovirus RNA by ribosomal entry into an internal segment of the 742-nucleotide (nt)-long 5' nontranslated region involves trans-acting factors, including p57, a 57-kDa polypeptide which has been identified as the pyrimidine tract-binding protein (PTB). A UV cross-linking assay was used to compare the RNA-binding properties of the p57 present in various mammalian cytoplasmic extracts with those of purified murine p57 and recombinant human PTB. Three noncontiguous p57-binding sites were located within the poliovirus 5' nontranslated region, between nt 70 and 288, and 443 and 539 (domain V), and 630 and 730. With the same assay, a novel 34-kDa polypeptide was identified that bound nt 1 to 629 specifically. A single A-->G substitution of nt 480 which attenuates poliovirus did not alter UV cross-linking of p57 to domain V. Although UV cross-linking of p57 to the internal ribosome entry site was specifically reduced by competition with poly(U) but not by competition with poly(C), poly(G), and poly(A) homoribopolymers, the presence of a polyuridine tract was not a sufficient determinant for binding of RNA to the p57 present in cytoplasmic extracts, nor was the polypyrimidine tract downstream of domain V necessary for binding to this site.

Properties of purified recombinant poliovirus protein 3aB as substrate for viral proteinases and as co-factor for RNA polymerase 3Dpol

The poliovirus-specific polypeptide 3AB (B = VPg) was expressed in Escherichia coli and purified to near homogeneity. Corresponding to its known association with membranes in poliovirus-infected HeLa cells, 3AB expressed in E. coli was also membrane-associated, and it could be solubilized only in detergent-containing buffers. In soluble form, 3AB was resistant to digestion with the virus-specific proteinases 3Cpro and 3CDpro. However, it was cleaved by these enzymes to 3A and VPg when bound to the bacterial membranes, an observation suggesting that 3AB may deliver the genome-linked protein VPg to the membrane-associated poliovirus replication complex. The specific activity of 3CDpro in processing 3AB was significantly higher than that of 3Cpro. Soluble 3AB was found to stimulate nearly 100-fold poly (A)-dependent, primer-dependent poly(U) synthesis, catalyzed by purified poliovirus RNA polymerase 3Dpol. We propose that 3AB has a dual function in poliovirus genome replication: as a precursor for VPg, and as a co-factor for 3Dpol.

IRES-controlled protein synthesis and genome replication of poliovirus

Initiation of translation of the single-stranded genomic RNAs of picornaviruses such as poliovirus (PV) and encephalomyocarditis virus (EMCV) is cap-independent and controlled by a long segment within the 5' non-translated region (5'NTR), termed internal ribosomal entry site (IRES). Cellular RNA-binding proteins have been identified that are involved in IRES function in trans. One of these proteins (p57) has been found to be identical to the polypyrimidine tract binding protein (pPTB), a nuclear protein implicated in various processes involving pre-mRNA. Anti-pPTB antibodies inhibit picornavirus mRNA, but not globin mRNA translation, in vitro. Proof for the 5'-independent initiation of translation in vivo was obtained by inserting the EMCV IRES into the ORF of PV thereby constructing a dicistronic, viable poliovirus with the genotype [PV] 5'NTR-P1-[EMCV] IRES-[PV] P2-P3-3'NTR. Dicistronic polioviruses were also constructed that served as novel expression vectors where a foreign gene has been inserted into the PV genome. Incubation of poliovirus RNA in a HeLa cell-free extract leads to the synthesis and processing of viral proteins, viral RNA replication followed by formation of infectious virions. Cell-free synthesis of PV has nullified the dictum that no virus can multiply in a cell-free medium. The genome replication of poliovirus and the mechanism of recombination in poliovirus replication is still not fully understood. Biochemical evidence has been obtained that the conserved NTP-binding motif in PV protein 2C is essential for RNA replication and virus propagation. Finally by using genetic studies we found that during viral RNA synthesis a poliovirus containing two tandemly arranged VPgs (3A-VPg1-VPg2-3Cpro) led to the removal of the 3C-proximal VPg copy.

Molecular biology and cell-free synthesis of poliovirus

Poliovirus is a small icosahedral particle consisting of only five species of macromolecules: 60 copies each of the capsid protein VP1-4; and one copy of single-stranded RNA, approximately 7500 nt long. The genome, linked at the 5' end to a small protein VPg and 3' polyadenylylated, is of plus strand polarity. After receptor-mediated uptake of the virus and release of the RNA into the cytoplasm, the genome serves as mRNA, encoding only a single polypeptide, the polyprotein. The polyprotein is cleaved co-translationally into numerous polypeptides by its own, internal proteinases 2Apro, 3Cpro and 3CDpro. Initiation of translation is mediated by a novel genetic element, called internal ribosomal entry site (IRES). IRES elements, which are 400 nt long RNA segments located within the 5' non-translated region of the viral genome, are common to all picornaviruses. Their function renders translation of picornavirus mRNAs cap- and 5'-independent, an observation that has upset the dogma of cap-dependent translation in eukaryotic cells. IRES elements have also been used to genetically dissect the viral genome and to construct novel expression vectors. Genome replication is not fully understood, the major conundrum being the initiation of RNA synthesis by the primer-dependent viral RNA polymerase 3Dpol, a process leading to VPg-linked RNA products. Nearly all non-structural proteins appear to be involved in initiation, the proteinases 2Apro and 3CDpro included. A HeLa cell-free system has been developed that, on programming with plasmid-transcribed viral RNA, will perform viral translation, protein processing, RNA replication, and assembly of capsid protein and newly made genomic RNA. The final yield is infectious poliovirus.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies on dicistronic polioviruses implicate viral proteinase 2Apro in RNA replication

A dicistronic poliovirus W1-P1/E/P2,3-1 with the genotype [PV]5'NTR-P1-[EMCV]IRES-[PV]P2,3-3'NTR (Molla, Jang, Paul, Reuer, and Wimmer, 1992, Nature 356, 255) was used to investigate whether the viral proteinase 2Apro, whose primary function in proteolytic processing was erased through the insertion of an internal ribosomal entry site (IRES) element into the ORF of the polyprotein, had other function(s) in viral replication. Deletion of 2Apro from W1-P1/E/P2,3-1 rendered the corresponding transcripts unable to replicate whereas partial deletion of 2Apro or an exchange of Cys109 (an amino acid of the catalytic triad of the proteinase) to Ala reduced RNA replication. No cytopathic effects were observed after transfection with any of the three dicistronic constructs containing mutant 2A, and no virus was recovered after attempts to expand a possibly low yield of mutant virus. In contrast, insertion of the IRES of encephalomyocarditis virus (EMCV) into the ORF of the poliovirus polyprotein at the cleavage site between 2Apro and 2B yielded the novel dicistronic virus W1-P1,2A/E/2BC,P3-1 with the genotype [PV]5'NTR-P1-2A-[EMCV]IRES-[PV]2BC-P3-3'NTR, expressing a small plaque phenotype. These results indicate that neither the intact P2 polypeptide nor the cleavage fragment 2AB of P2 is required for viral proliferation. On the other hand, 2Apro appears to be an essential component in RNA replication as no viral RNA synthesis can be observed by reverse transcription/PCR in cells transfected with dicistronic RNA lacking this viral polypeptide.

Effects of temperature and lipophilic agents on poliovirus formation and RNA synthesis in a cell-free system

The translation and primary processing events of poliovirus polyproteins in HeLa cell extracts were more efficient at 34 degrees C than at 30 or 36 degrees C. The cleavage products of P2 such as 2Apro, 2BC, and 2C appeared early in the reaction before the appearance of the cleavage products of P1 and of 3CDpro, an observation suggesting that P2 was cleaved in cis by 3CDpro. Proteolytic processing of the capsid precursor P1 into VP0, VP1, and VP3 was also more efficient at 34 degrees C than at either 30 or 32 degrees C. Surprisingly, processing of 3CDpro to 3Cpro and 3Dpol was almost completely inhibited at 36 degrees C. The synthesis of virus in the cell extract was greatly enhanced at 34 degrees C over that at 30 or 32 degrees C, whereas incubation at 36 degrees C yielded very little virus. Cerulenin, an inhibitor of lipid synthesis, did not appear to affect virus-specific translation or protein processing, but it almost completely inhibited viral synthesis in vitro. Oleic acid drastically inhibited in vitro translation at 100 microM and in vitro poliovirus synthesis at 25 microM. Addition of HeLa cell smooth membranes partially restored translation but not virus formation. Our observations suggest that in vitro translation, proteolytic processing, and virus formation require intact membranes. Analysis of the in vitro translation products revealed that viral RNA polymerase activity increased linearly during incubation of the translation mixture. RNA polymerase in the crude mixture was inhibited by oleic acid but not by cerulenin. Surprisingly, oleic acid had no direct effect on oligo(U)-primed, poly(A)-dependent poly(U) synthesis catalyzed by purified 3Dpol.

Replication of poliovirus RNA containing two VPg coding sequences leads to a specific deletion event

Studies of the poliovirus genome-linked protein VPg have shown that this small viral protein is required for replication of virus-specific RNA (Q. Reuer, R. J. Kuhn, and E. Wimmer, J. Virol. 64:2967-2975, 1990). To understand the mechanism of RNA replication, we constructed a recombinant poliovirus genome encoding two tandemly arranged VPg coding sequences that were nearly identical in both nucleotide and amino acid sequence. Following transfection of this two-VPg-containing RNA into HeLa cells, we found a specific and selective deletion in the progeny virus genome. Sequence analysis of the recovered viral RNA indicated that the complete nucleotide sequence encoding the second (3C-proximal) VPg coding sequences was removed, restoring the authentic genome sequences in the poliovirus genome. Analysis of viral RNAs following transfection suggested that the deletion event occurred during genome replication. Deletion could have occurred via homologous recombination between two VPg sequences or via intramolecular deletion with loop-out of the template. In vitro translation of the two-VPg-containing transcript RNA indicated aberrant processing of the viral polyprotein. This result suggested that selection of the wild-type genotype in the transfected cells may occur at the level of viral protein synthesis.

Interaction of polypyrimidine tract binding protein with the encephalomyocarditis virus mRNA internal ribosomal entry site

Translation of encephalomyocarditis virus (EMCV) mRNA occurs in a cap-independent manner, requiring instead a cis-acting element termed the internal ribosomal entry site (IRES). Binding of a 57-kDa ribosome-associated protein (p57) to the EMCV IRES has been found to correlate with cap-independent translation. p57 has recently been reported to be very similar, if not identical, to the polypyrimidine tract binding protein (pPTB), a spliceosome-associated factor possibly involved in U2 snRNP/pre-mRNA complex formation of 3'-splice-site recognition. The interaction between purified pPTB and the EMCV IRES was characterized in this study using nitrocellulose filter binding and UV cross-linking assays. pPTB bound the EMCV IRES with high affinity (Kd = 40 nM at 25 degrees C, pH 5.5, 80 mM ionic strength). pPTB also bound strongly to RNA fragments containing either the 5'-end, 3'-end, or an internal stem-loop of the IRES. The binding properties of 16 RNA variants derived from the IRES revealed however that purified pPTB bound with less specificity than pPTB in a mixture of cytoplasmic HeLa cell polypeptides. The addition of HeLa extract to purified pPTB increased the binding specificity, suggesting that factors within the extract alter the binding specificity of pPTB. The binding of pPTB to the full-length IRES and three IRES-derived fragments was studied in detail. Complex formation was optimal at low pH and was driven entirely by entropy. As many as four ion pairs are formed upon binding, with electrostatic interactions accounting for approximately 35% of the total free energy of complex formation.

A cytoplasmic 57-kDa protein that is required for translation of picornavirus RNA by internal ribosomal entry is identical to the nuclear pyrimidine tract-binding protein

Initiation of translation of the RNA genomes of picornaviruses such as poliovirus and encephalomyocarditis virus is cap-independent and results from interaction of ribosomes with a segment of the 5' noncoding region of these mRNAs termed the internal ribosomal entry site. Genetic and biochemical studies have previously shown that a 57-kDa cytoplasmic RNA-binding protein (p57) plays an essential role in this translation mechanism. We have now found that p57 shares physical, biochemical, and antigenic properties with the pyrimidine tract-binding protein (PTB), a nuclear protein that has been implicated in various processes involving pre-mRNA. These data indicate that p57 and PTB are the same protein. Purified recombinant PTB bound specifically to a bulged hairpin within the internal ribosomal entry site of encephalomyocarditis virus and had a much lower affinity for a mutated derivative of this hairpin and for unrelated RNAs. Immunodepletion of p57/PTB from a HeLa cell-free lysate inhibited translation of poliovirus and encephalomyocarditis virus mRNAs but had no effect on translation of beta-globin mRNA, confirming the essential role of p57 in translation by internal ribosomal entry.

Poliovirus receptor on human blood cells: a possible extraneural site of poliovirus replication

In order to determine whether any primary human blood cells have the ability to replicate poliovirus (PV), peripheral blood cell components were isolated and analyzed for their cell surface expression of the poliovirus receptor (PVR). Following two-color immunfluoresence staining with lineage-specific markers, the cells were analyzed by flow-cytometric methods. PVR cell surface expression was detected on most mononuclear cells expressing CD14, a marker for mononuclear phagocytes. There was no PVR cell surface expression on platelets and extremely low levels on polymorphonuclear leukocytes. Mononuclear leukocytes from Ficoll density centrifugation were found to support PV replication. The finding of PVR on mononuclear phagocytes and the ability of primary human blood cells to support PV replication in the absence of cultivation has implications for both the normal and pathogenic role of PVR.

Inhibition of proteolytic activity of poliovirus and rhinovirus 2A proteinases by elastase-specific inhibitors

A polyprotein cleavage assay has been developed to assay the proteolytic activities in vitro of the 2A proteinases encoded by poliovirus and human rhinovirus 14, which are representative members of the Enterovirus and Rhinovirus genera of picornaviruses, respectively. The elastase-specific substrate-based inhibitors elastatinal and methoxysuccinyl-Ala-Ala-Pro-Val-chloromethylketone (MPCMK) inhibited both 2A proteinases in vitro. The electrophoretic mobilities of both 2A proteinases were reduced upon incubation with elastatinal, whereas the mobility of a Cys-109-->Ala poliovirus 2Apro mutant was unchanged, an observation suggesting that this inhibitor may have formed a covalent bond with the active-site Cys-109 nucleophile. Iodoacetamide, calpain inhibitor 1, and antipain inhibited poliovirus 2Apro. MPCMK caused a reduction in the yields of the enteroviruses poliovirus type 1 and coxsackievirus A21 and of human rhinovirus 2 in infected HeLa cells but did not affect the growth of encephalomyocarditis virus, a picornavirus of the Cardiovirus genus. MPCMK abrogated the shutoff of host cell protein synthesis that is induced by enterovirus and rhinovirus infection and reduced the synthesis of virus-encoded polypeptides in infected cells. These results indicate that the determinants of substrate recognition by 2A proteinases resemble those of pancreatic and leukocyte elastases. These results may be relevant to the development of broad-range chemotherapeutic agents against entero- and rhinoviruses.

Vectorial release of poliovirus from polarized human intestinal epithelial cells

Polarized epithelial cells represent the primary barrier to virus infection of the host, which must also be traversed prior to virus dissemination from the infected organism. Although there is considerable information available concerning the release of enveloped viruses from such cells, relatively little is known about the processes involved in the dissemination of nonenveloped viruses. We have used two polarized epithelial cell lines, Vero C1008 (African green monkey kidney epithelial cells) and Caco-2 (human intestinal epithelial cells), infected with poliovirus and investigated the process of virus release. Release of poliovirus was observed to occur almost exclusively from the apical cell surface in Caco-2 cells, whereas infected Vero C1008 cells exhibited nondirectional release. Structures consistent with the vectorial transport of virus contained within vesicles or viral aggregates were observed by electron microscopy. Treatment with monensin or ammonium chloride partially inhibited virus release from Caco-2 cells. No significant cell lysis was observed at the times postinfection when extracellular virus was initially detected, and transepithelial resistance and vital dye uptake measurements showed only a moderate decrease. Brefeldin A was found to significantly and specifically inhibit poliovirus biosynthetic processes by an as yet uncharacterized mechanism. The vectorial release of poliovirus from the apical (or luminal) surface of human intestinal epithelial cells has significant implications for viral pathogenesis in the human gut.

Viral proteases as targets for chemotherapeutic intervention

Many viruses encode proteinases that are essential for infectivity, and are consequently attractive chemotherapeutic targets. The biochemistry and structure of the human immunodeficiency virus proteinase have been characterized extensively, and potent peptide-mimetic inhibitors have been developed. Techniques and strategies used to improve the efficiency of these compounds are likely to be applicable to other viral proteinases.

Antibody-complexed foot-and-mouth disease virus, but not poliovirus, can infect normally insusceptible cells via the Fc receptor

Poliovirus and foot-and-mouth disease virus (FMDV) initiate infection by binding to specific cell surface receptors, which is followed by a poorly understood disassembly process. To probe these early steps of infection, the ability of poliovirus and FMDV to infect cells following binding through an alternative receptor was examined. For these studies, a Chinese hamster ovary (CHO) cell line expressing the B2 isoform of the murine Fc receptor (FcR) was used. Both viruses were able to bind to this cell line in an antibody-dependent manner, but only FMDV was able to productively infect these cells following binding through the FcR. These results suggest that the natural poliovirus receptor has dual functions in binding and destabilizing the virus particle, whereas the putative FMDV receptor may only be necessary for virion binding. These findings are consistent with differences in virion architecture which predict a more intimate virion-receptor association for poliovirus than for FMDV.

Bidirectional entry of poliovirus into polarized epithelial cells

The interactions of viruses with polarized epithelial cells are of some significance to the pathogenesis of disease because these cell types comprise the primary barrier to many virus infections and also serve as the sites for virus release from the host. Poliovirus-epithelial cell interactions are of particular interest since this virus is an important enteric pathogen and the host cell receptor has been identified. In this study, poliovirus was observed to adsorb to both the apical and basolateral surfaces of polarized monkey kidney (Vero C1008) and human intestinal (Caco-2) epithelial cells but exhibited preferential binding to the basolateral surfaces of both cell types. Localization of the poliovirus receptor by a receptor-specific monoclonal antibody (D171) revealed a similar distribution predominantly on basolateral membranes, and treatment of cells with antibody D171 inhibited virus adsorption to both membrane surfaces. Poliovirus was able to initiate infection with similar efficiency following adsorption to either surface, and infection was blocked at both surfaces by D171, indicating that functional receptor molecules are expressed on both surfaces at sufficient density to mediate efficient infection at the apical and basolateral plasma membranes. Poliovirus infection resulted in a decrease in transepithelial resistance which was inhibited by prior treatment with monoclonal antibody D171 and occurred prior to other visible cytopathic effects. These results have interesting implications for viral pathogenesis in the human gut.

N glycosylation of the virus binding domain is not essential for function of the human poliovirus receptor

The human poliovirus receptor (hPVR) is a glycoprotein with three immunoglobulin-like extracellular domains, of which the N-terminal domain (V-type domain) is necessary and sufficient for virus binding and uptake. The effect of N glycosylation of the V domain of hPVR on binding and entry of poliovirus was studied. Stable mouse L-cell lines were generated that express PVR-specific cDNA. One of the cell lines expressed a mutant of hPVR, in which both asparagine residues of the two N-glycosylation sites of the V domain were changed to aspartate (N105D) and serine (N120S), respectively. In the second mutant cell line, the portion of the cDNA encoding the V domain of hPVR was substituted by the homologous sequence of the recently isolated PVR cDNA from monkey cells. This V domain naturally lacks both N glycosylation sites and encodes D105 and S120 at the respective positions of the open reading frame. Absence of N glycosylation at these sites was demonstrated by in vitro translation of the two mutant coding sequences in the presence of microsomal membranes. Both PVR mutant cell lines were capable of poliovirus binding and replication. However, binding of anti-PVR monoclonal antibody D171 and protection from viral replication by this antibody were observed only with the glycosylation mutant carrying the human V domain. In contrast, infection of the cell line expressing the monkey-human hybrid receptor was not blocked even though monkey cells are fully protected by monoclonal antibody D171. The data suggest that N glycosylation of the V domain of hPVR is not essential for viral replication in human tissues and that differential glycosylation of hPVR at these sites is likely not a determinant of viral tissue tropism. Furthermore, the virus binding site and the epitope recognized by monoclonal antibody D171 do not appear to overlap.

Purification and characterization of poliovirus polypeptide 3CD, a proteinase and a precursor for RNA polymerase

A cDNA clone encoding the 3CD proteinase (3CDpro) of poliovirus type 2 (Sabin), the precursor to proteinase 3Cpro and RNA polymerase 3Dpol, was expressed in bacteria by using a T7 expression system. Site-specific mutagenesis of the 3C/3D cleavage site was performed to generate active proteolytic precursors impaired in their ability to process themselves to 3Cpro and 3Dpol. Of these mutations, the exchange of the Thr residue at the P4 position of the 3C/3D cleavage site for a Lys residue (3CDpro T181K) resulted in a mutant polypeptide exhibiting the smallest amount of autoprocessing. This mutant was purified to 86% homogeneity and used for subsequent proteolytic studies. Purified 3CDproM (M designates the cleavage site mutant 3CDpro T181K) was capable of cleaving the P1 capsid precursor, a peptide representing the 2BC cleavage site, and the 2BC precursor polypeptide. Purified 3CDproM demonstrated the same detergent sensitivity in processing experiments with the capsid precursor as was observed by using P1 and crude extracts of poliovirus-infected HeLa cell lysates. Purified 3CDproM did not have any detectable RNA polymerase activity, whereas 3Dpol, separated from 3CDproM by gel filtration in the last step of purification, did. We conclude that 3CDproM can process both structural and nonstructural precursors of the poliovirus polyprotein and that it is active against a synthetic peptide substrate. Moreover, cleavage of 3CD to 3Dpol is needed to activate the 3D RNA polymerase.

Mutational analysis of the proposed FG loop of poliovirus proteinase 3C identifies amino acids that are necessary for 3CD cleavage and might be determinants of a function distinct from proteolytic activity

Mutations were introduced into a cDNA clone of poliovirus resulting in single-amino-acid substitutions within the region of the proposed FG loop of proteinase 3C. RNAs were made by in vitro transcription with T7 RNA polymerase and used to transfect HeLa cells. Virus viability was assessed as indicated by cell lysis. In parallel, RNAs were translated in vitro by using a HeLa cell lysate, and the patterns of the processed poly-proteins were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Replacement of Lys-78, Arg-79, and Glu-81 had apparently no effect on virus viability and on proteolytic processing. In contrast, virus viability was abolished by mutation of Phe-83, Arg-84, Asp-85, Ile-86, and Arg-87. With respect to substitution of Phe-83, Asp-85, and Arg-87, these effects correlated with impaired processing of the 3CD cleavage site, separating 3C and 3D, and, to a lesser extent, of the P1 precursor. Replacement of Arg-84 and Ile-86, on the other hand, did not alter the processing pattern. Thus, the lethal effects in these mutant genomes may not have been caused by impaired processing. A special case was the mutant of Lys-82-Gln. Virus recovered from cells transfected with RNA carrying this mutation always contained an A-to-G transition which resulted in the replacement of glutamine for arginine. Our data suggest that residues in the proposed FG loop of proteinase 3C influence 3CD cleavage and that they are determinants of a function unrelated to proteolytic processing.

The soluble form of two N-terminal domains of the poliovirus receptor is sufficient for blocking viral infection

By means of deleting a C-terminal portion of the open reading frame of the poliovirus receptor cDNA, and by vaccinia virus-mediated overexpression we have produced a protein corresponding to the first two N-terminal Ig-like domains of the poliovirus receptor. This protein that lacked the third Ig-like domain, the transmembrane region and most of the intracellular C-terminal tail was detected in the medium of vaccinia virus infected cells. The properties of the truncated PVR cDNA were further characterized by in vitro translation and modification. The molecular weight of the unmodified protein was found to be 27 kDa; translation in the presence of dog pancreas microsomes led to an increase in molecular weights which we attribute to N-glycosylation. Upon incubation with poliovirus at 37 degrees C, the vaccinia-virus generated protein specifically reduced infectivity of poliovirus. Sucrose gradients of poliovirus particles derived after incubation with the protein showed the induction of a slower sedimenting particle (135S). Our experiments suggest that the two N-terminal domains of the poliovirus receptor in soluble form are sufficient for the conversion of poliovirus into a non-infectious particle.

Genetic analysis of an NTP-binding motif in poliovirus polypeptide 2C

Poliovirus polypeptide 2C is a nonstructural protein involved in replication of the viral genome. Analysis of the primary amino acid sequence of 2C shows homology to a family of proteins which contain a nucleoside-triphosphate (NTP)-binding motif. This motif consists of elements "A" (2/5 hydrophobic stretch) G/AXXGXGKS/T, where X stands for any amino acid, and "B" (3/5 hydrophobic stretch) D or DD/E. To assess the significance of the consensus sequence in 2C, we have engineered point mutations into the most conserved residues in the A and B sites and tested their effect on viral RNA replication in vivo and translation in vitro. Whereas in vitro translation of synthetic RNAs carrying mutations in the NTP-binding motif showed efficient processing of all viral proteins, indistinguishable from that of the parental strain, transfection of the RNAs into HeLa cells did not give rise to infectious virus. No viral RNA replication could be detected in cells transfected with mutant RNAs. However, revertants to the wild-type genotype in the A and B sites were obtained which gave rise to wild-type RNA synthesis, but pseudorevertants or second-site suppressors were not observed. Thus, viral RNA synthesis is greatly reduced but not entirely abolished in cells transfected with mutant RNAs. These results strongly suggest a functional role for the proposed NTP-binding motif of 2C in RNA replication and proliferation of poliovirus.

Determinants of substrate recognition by poliovirus 2A proteinase

Poliovirus proteinase 2A (2Apro) is autocatalytically released from the viral polyprotein by cleavage in cis of a Tyr-Gly dipeptide at its own amino terminus, resulting in separation of the P1 structural and P2-P3 nonstructural protein precursors. A second Ty-Gly dipeptide within 3D polymerase is cleaved by 2Apro in trans, but this is not essential for viral proliferation. The mechanism which limits cleavage to only 2 of the 10 Tyr-Gly dipeptides within the poliovirus polyprotein has not been characterized. We have therefore undertaken a systematic mutational analysis of the VP1-2A site to elucidate determinants of substrate recognition by 2Apro. The P2 and P1' positions are important determinants for cis cleavage of this site, whereas a variety of substituents could be tolerated at the P2', P1, and P3 positions. The requirements for trans cleavage of this site were more stringent. We found that the 2Apro of coxsackievirus type A21 and rhinoviruses 2 and 14 have stringent requirements similar to those of poliovirus 2Apro for cleavage in trans.

Infection of HeLa cells with poliovirus results in modification of a complex that binds to the rRNA promoter

In HeLa cells, RNA polymerase I (Pol I)-mediated transcription is severely inhibited soon after infection with poliovirus. We have developed a gel retardation assay to analyze DNA-protein complexes formed at the Pol I promoter. We show here that two complexes (A and C) formed by nuclear extracts from uninfected cells disappear after infection of cells with poliovirus. In contrast, a new, rapidly migrating complex (D) is formed in virus-infected cell extract. This change in the mobility of gel-retarded complexes correlates well with the kinetics of inhibition of rRNA transcription in virus-infected cells. Incubation of nuclear extracts from mock-infected cells with bacterially expressed, purified poliovirus protease 3C results in the disappearance of complexes A and C with concomitant generation of complex D. A partially purified transcription factor fraction derived from uninfected cells that contains complex A is able to restore Pol I transcription when added to virus-infected cell extracts, suggesting that this complex plays an important role in Pol I transcription. These results suggest that poliovirus proteinase 3C may have an important role in the shutoff of Pol I transcription in cells infected with poliovirus.

A chimeric poliovirus/CD4 receptor confers susceptibility to poliovirus on mouse cells

The human poliovirus receptor consists of three extracellular immunoglobulinlike domains, a transmembrane domain, and an intracytoplasmic domain. The amino-terminal variable-type domain (V domain) of the human poliovirus receptor is necessary and sufficient for its function as a viral receptor (H.-C. Selinka, A. Zibert, and E. Wimmer, Proc. Natl. Acad. Sci. USA 88:3598-3602, 1991). In this paper, data are presented showing that transfer of the putative poliovirus receptor-binding domain to a truncated receptor for the human immunodeficiency virus results in a functional receptor for poliovirus. After expression in mouse cells, this chimeric protein confers susceptibility to poliovirus. Thus, unlike human immunodeficiency virus, poliovirus can enter mouse cells by way of a truncated CD4 receptor if the specific binding domain for poliovirus is provided.