A genetic locus in mutant poliovirus genomes involved in overproduction of RNA polymerase and 3C proteinase

AUF1 is recruited to the stress granules induced by coxsackievirus B3

Stress granules (SGs) are cytoplasmic granules that are formed in cells when stress occurs. In this study, we found that SGs formed in cells infected with coxsackievirus B3 (CVB3), evidenced with the co-localization of some accepted SG markers in the viral infection-induced granules. We further discovered that adenosine-uridine (AU)-rich element RNA binding factor 1 (AUF1), which can bind to mRNAs and regulate their translation, was recruited to the SGs in response to high dose of CVB3 by detecting the co-localization of AUF1 with SG markers. Similar results were also observed in the enterovirus 71 (EV71)-infected cells. Finally, we demonstrated that AUF1 was also recruited to arsenite-induced SGs, suggesting that the recruitment of AUF1 to SG is not a specific response to viral infection. In summary, our data indicate that both CVB3 and EV71 infections can induce SG formation, and AUF1 is a novel SG component upon the viral infections. Our findings may shed light on understanding the picornavirus-host interaction.

Mutational and fitness landscapes of an RNA virus revealed through population sequencing

RNA viruses exist as genetically diverse populations. It is thought that diversity and genetic structure of viral populations determine the rapid adaptation observed in RNA viruses and hence their pathogenesis. However, our understanding of the mechanisms underlying virus evolution has been limited by the inability to accurately describe the genetic structure of virus populations. Next-generation sequencing technologies generate data of sufficient depth to characterize virus populations, but are limited in their utility because most variants are present at very low frequencies and are thus indistinguishable from next-generation sequencing errors. Here we present an approach that reduces next-generation sequencing errors and allows the description of virus populations with unprecedented accuracy. Using this approach, we define the mutation rates of poliovirus and uncover the mutation landscape of the population. Furthermore, by monitoring changes in variant frequencies on serially passaged populations, we determined fitness values for thousands of mutations across the viral genome. Mapping of these fitness values onto three-dimensional structures of viral proteins offers a powerful approach for exploring structure-function relationships and potentially uncovering new functions. To our knowledge, our study provides the first single-nucleotide fitness landscape of an evolving RNA virus and establishes a general experimental platform for studying the genetic changes underlying the evolution of virus populations.

Viral proteinase requirements for the nucleocytoplasmic relocalization of cellular splicing factor SRp20 during picornavirus infections

Infection of mammalian cells by picornaviruses results in the nucleocytoplasmic redistribution of certain host cell proteins. These viruses interfere with import-export pathways, allowing for the cytoplasmic accumulation of nuclear proteins that are then available to function in viral processes. We recently described the cytoplasmic relocalization of cellular splicing factor SRp20 during poliovirus infection. SRp20 is an important internal ribosome entry site (IRES) trans-acting factor (ITAF) for poliovirus IRES-mediated translation; however, it is not known whether other picornaviruses utilize SRp20 as an ITAF and direct its cytoplasmic relocalization. Also, the mechanism by which poliovirus directs the accumulation of SRp20 in the cytoplasm of the infected cell is currently unknown. Work described in this report demonstrated that infection by another picornavirus (coxsackievirus B3) causes SRp20 to relocalize from the nucleus to the cytoplasm of HeLa cells, similar to poliovirus infection; however, SRp20 is relocalized to a somewhat lesser extent in the cytoplasm of HeLa cells during infection by yet another picornavirus (human rhinovirus 16). We show that expression of poliovirus 2A proteinase is sufficient to cause the nucleocytoplasmic redistribution of SRp20. Following expression of poliovirus 2A proteinase in HeLa cells, we detect cleavage of specific nuclear pore proteins known to be cleaved during poliovirus infection. We also find that expression of human rhinovirus 16 2A proteinase alone can cause efficient cytoplasmic relocalization of SRp20, despite the lower levels of SRp20 relocalization observed during rhinovirus infection compared to poliovirus. Taken together, these results further define the mechanism of SRp20 cellular redistribution during picornavirus infections, and they provide additional insight into some of the differences observed between human rhinovirus and other enterovirus infections.

Refined X-ray crystallographic structure of the poliovirus 3C gene product

The X-ray crystallographic structure of the recombinant poliovirus 3C gene product (Mahoney strain) has been determined by single isomorphous replacement and non-crystallographic symmetry averaging and refined at 2.1 A resolution. Poliovirus 3C is comprised of two six-stranded antiparallel beta-barrel domains and is structurally similar to the chymotrypsin-like serine proteinases. The shallow active site cleft is located at the junction of the two beta-barrel domains and contains a His40, Glu71, Cys147 catalytic triad. The polypeptide loop preceding Cys147 is flexible and likely undergoes a conformational change upon substrate binding. The specificity pockets for poliovirus 3C are well-defined and modeling studies account for the known substrate specificity of this proteinase. Poliovirus 3C also participates in the formation of the viral replicative initiation complex where it specifically recognizes and binds the RNA stem-loop structure in the 5' non-translated region of its own genome. The RNA recognition site of 3C is located on the opposite side of the molecule in relation to its proteolytic active site and is centered about the conserved KFRDIR sequence of the domain linker. The recognition site is well-defined and also includes residues from the amino and carboxy-terminal helices. The two molecules in the asymmetric unit are related by an approximate 2-fold, non-crystallographic symmetry and form an intermolecular antiparallel beta-sheet at their interface.

The picornaviral 3C proteinases: cysteine nucleophiles in serine proteinase folds

The 3C proteinases are a novel group of cysteine proteinases with a serine proteinase-like fold that are responsible for the bulk of polyprotein processing in the Picornaviridae. Because members of this viral family are to blame for several ongoing global pandemic problems (rhinovirus, hepatitis A virus) as well as sporadic outbreaks of more serious pathologies (poliovirus), there has been continuing interest over the last two decades in the development of antiviral therapies. The recent determination of the structure of two of the 3C proteinases by X-ray crystallography opens the door for the application of the latest advances in computer-assisted identification and design of anti-proteinase therapeutic/chemoprophylactic agents.

Expression of virus-encoded proteinases: functional and structural similarities with cellular enzymes

Many viruses express their genome, or part of their genome, initially as a polyprotein precursor that undergoes proteolytic processing. Molecular genetic analyses of viral gene expression have revealed that many of these processing events are mediated by virus-encoded proteinases. Biochemical activity studies and structural analyses of these viral enzymes reveal that they have remarkable similarities to cellular proteinases. However, the viral proteinases have evolved unique features that permit them to function in a cellular environment. In this article, the current status of plant and animal virus proteinases is described along with their role in the viral replication cycle. The reactions catalyzed by viral proteinases are not simple enzyme-substrate interactions; rather, the processing steps are highly regulated, are coordinated with other viral processes, and frequently involve the participation of other factors.

Purification, properties, and mutagenesis of poliovirus 3C protease

Poliovirus protease 3C, type 1 Mahoney strain, was expressed in Escherichia coli under phage T7 promoter control and purified to homogeneity from resolubilized inclusion bodies. The renatured protein was as enzymatically active as the protease found in the soluble portion of the bacterial lysate. Proteolytic activity was assayed using as substrate either [35S]methionine-labeled recombinant poliovirus proteins 2C3AB or a truncated version of 3ABC, or synthetic peptide 16-mers corresponding to the cleavage sites at 2C/3A and 3A/3B. Poliovirus protein 3CD (protease-polymerase) was also expressed in bacteria. About 25% of this protein apparently autodigested in vivo, releasing immunoprecipitable protein 3D (polymerase). No further autodigestion of 3CD could be detected in vitro, nor could addition of purified protein 3C effect digestion in trans. Both the serine protease inhibitors PMSF, TPCK, and 3,4-dichloroisocoumarin, and the cysteine protease inhibitors cystatin and zinc, were effective inhibitors of the 3C protease. Six new mutants of the protease, with altered or no enzymatic activity, were identified based on the observation that low level expression of wild type enzyme severely retards growth of bacterial colonies harboring the expression plasmid.

Flavivirus enzyme-substrate interactions studied with chimeric proteinases: identification of an intragenic locus important for substrate recognition

The proteins of flaviviruses are translated as a single long polyprotein which is co- and posttranslationally processed by both cellular and viral proteinases. We have studied the processing of flavivirus polyproteins in vitro by a viral proteinase located within protein NS3 that cleaves at least three sites within the nonstructural region of the polyprotein, acting primarily autocatalytically. Recombinant polyproteins in which part of the polyprotein is derived from yellow fever virus and part from dengue virus were used. We found that polyproteins containing the yellow fever virus cleavage sites were processed efficiently by the yellow fever virus enzyme, by the dengue virus enzyme, and by various chimeric enzymes. In contrast, dengue virus cleavage sites were cleaved inefficiently by the dengue virus enzyme and not at all by the yellow fever virus enzyme. Studies with chimeric proteinases and with site-directed mutants provided evidence for a direct interaction between the cleavage sites and the proposed substrate-binding pocket of the enzyme. We also found that the efficiency and order of processing could be altered by site-directed mutagenesis of the proposed substrate-binding pocket.

Analysis of putative active site residues of the poliovirus 3C protease

It was recently suggested that the picornavirus 3C proteases are homologous to the chymotrypsin-like serine proteases. The two structural models proposed differ in one of the postulated active site residues, Glu/Asp71 or Asp85. We changed Glu71 of the poliovirus type 1 protease to Asp or Gln and Asp85 to Glu by oligonucleotide-directed site-specific mutagenesis of an infectious cDNA, and attempted to recover virus after transfection. Both Glu71 changes were lethal for the virus and proteolytic activity was abolished in vitro with the exception of the primary cleavage event at the P2/P3 junction. In contrast, the Asp85----Glu virus was viable. This mutant was temperature-sensitive for growth at 39 degrees and exhibited a minute plaque phenotype at permissive temperature. This defect correlated with low levels of viral-specific RNA and protein syntheses and slow virus growth. Proteolytic processing at the COOH-terminus of 3C was impaired, reducing the production of mature 3C and the viral replicase 3D. In addition, 3C-mediated cleavage events within the P2 region of the polyprotein seemed to occur rather inefficiently. 3C-specific processing within P1 and elsewhere within P3 was unaffected. We suggest that Asp85 does not form part of the active site of 3C, but could be important for the specific recognition of cleavage sites within P2.

Synthesis and processing of the nonstructural polyproteins of several temperature-sensitive mutants of Sindbis virus

We have examined the synthesis and processing of nonstructural polyproteins by several temperature-sensitive mutants of Sindbis virus, representing the four known RNA-minus complementation groups. Four mutants that possess mutations in the C-terminal domain of nonstructural protein nsP2 all demonstrated aberrant processing patterns when cells infected with these mutants were shifted from a permissive (30 degrees) to a nonpermissive (40 degrees) temperature. Mutants ts17, ts18, and ts24 showed severe defects in processing of nonstructural polyproteins at 40 degrees, whereas ts7 showed only a minor defect. In each case, cleavage of the bond between nsP2 and nsP3 was greatly reduced whereas cleavage between nsP1 and nsP2 occurred almost normally, giving rise to a set of polyprotein precursors not seen in wild-type-infected cells at this stage of infection. The nsP1 produced by these mutants was unstable and only small amounts could be detected in infected cells at the nonpermissive temperature. Submolar quantities of nsP2 were also present. We suggest that nsP1 and nsP2 may function as a complex and that free nsP1, and possibly nsP2, is degraded. Cleavage between nsP3 and nsP4 appeared to be normal in the mutants except in the case of ts17, where upon shift to 40 degrees P34 was unstable and nsP4 accumulated. We propose that the change in the P34/nsP4 ratio upon shift is responsible for the previously observed temperature sensitivity of subgenomic 26 S RNA synthesis in ts17 and for the failure of the mutant to regulate minus strand synthesis at 40 degrees. Other mutations tested, including ts21, which is found in the N-terminal half of nsP2, ts11, which has a mutation in nsP1, and ts6, which has a mutation in nsP4, all demonstrated nonstructural polyprotein processing indistinguishable from that in wild-type-infected cells. These results support our conclusion, based upon deletion mapping studies, that the C-terminal domain of nsP2 contains the nonstructural proteinase activity.