Folding and processing of the capsid protein precursor P1 is kinetically retarded in neutralization site 3B mutants of poliovirus

Molecular epidemiology of echoviruses 11 and 30 in Russia: different properties of genotypes within an enterovirus serotype

Over 100 known enterovirus serotypes differ in their epidemiological and pathogenic properties. Much less is known about variation of these features on a sub-serotype level, such as genotypes. Echovirus 11 (E11) and E30 are amongst the most frequent causative agents of aseptic meningitis. We studied the molecular epidemiology of these pathogens to evaluate potential epidemiological and pathogenic dissimilarities of their genotypes. The complete VP1 genome region was sequenced for 97 E11 and 62 E30 isolates collected in Russia from 2008 to 2012, and they were studied in comparison with all 140 E11 and 432 E30 sequences available in GenBank. A geographic pattern of genotype prevalence was observed for both types. Russian E11 isolates belonged mainly to A genotype, which is common in Asia, and D5, which is predominant in Europe. For E30, genotype III by classification of Ke et al. (2011), also termed genotype a by Bailly et al. (2009), was endemic in Russia from 2003 to 2012, while it was not detected in Europe and North America during this time. The E30 genotypes VI-B, VI-G, and VI-H (e, f and h) were regularly introduced from different countries, became predominant and vanished after no more than 4years. In addition to geographic patterns, E11 genotypes also differed by isolation source. Genotype A2 viruses were significantly more often found in sewage, compared to genotype D5 that was isolated from both sewage and human samples. In addition, there was evidence of a different capacity for international transfers among E11 GtA subclusters.

Antigenic evolution of vaccine-derived polioviruses: changes in individual epitopes and relative stability of the overall immunological properties

The Sabin oral poliovirus vaccine (OPV) readily undergoes changes in antigenic sites upon replication in humans. Here, a set of antigenically altered descendants of the three OPV serotypes (76 isolates) was characterized to determine the driving forces behind these changes and their biological implications. The amino acid residues of OPV derivatives that lie within or close to the known antigenic sites exhibited a marked tendency to be replaced by residues characteristic of homotypic wild polioviruses, and these changes may occur very early in OPV evolution. The specific amino acid alterations nicely correlated with serotype-specific changes in the reactivity of certain individual antigenic sites, as revealed by the recently devised monoclonal antibody-based enzyme-linked immunosorbent assay. In comparison to the original vaccine, small changes, if any, in the neutralizing capacity of human or rabbit sera were observed in highly diverged vaccine polioviruses of three serotypes, in spite of strong alterations of certain epitopes. We propose that the common antigenic alterations in evolving OPV strains largely reflect attempts to eliminate fitness-decreasing mutations acquired either during the original selection of the vaccine or already present in the parental strains. Variability of individual epitopes does not appear to be primarily caused by, or lead to, a significant immune evasion, enhancing only slightly, if at all, the capacity of OPV derivatives to overcome immunity in human populations. This study reveals some important patterns of poliovirus evolution and has obvious implications for the rational design of live viral vaccines.

Molecular mechanisms of poliovirus variation and evolution

Replication of poliovirus RNA is accomplished by the error-prone viral RNA-dependent RNA polymerase and hence is accompanied by numerous mutations. In addition, genetic errors may be introduced by nonreplicative mechanisms. Resulting variability is manifested by point mutations and genomic rearrangements (e.g., deletions, insertions and recombination). After description of basic mechanisms underlying this variability, the review focuses on regularities of poliovirus evolution (mutation fixation) in tissue cultures, human organisms and populations.

Cholesterol removal by methyl-beta-cyclodextrin inhibits poliovirus entry

Upon binding to the poliovirus receptor (PVR), the poliovirus 160S particles undergo a conformational transition to generate 135S particles, which are believed to be intermediates in the virus entry process. The 135S particles interact with host cell membranes through exposure of the N termini of VP1 and the myristylated VP4 protein, and successful cytoplasmic delivery of the genomic RNA requires the interaction of these domains with cellular membranes whose identity is unknown. Because detergent-insoluble microdomains (DIMs) in the plasma membrane have been shown to be important in the entry of other picornaviruses, it was of interest to determine if poliovirus similarly required DIMs during virus entry. We show here that methyl-beta-cyclodextrin (MbetaCD), which disrupts DIMs by depleting cells of cholesterol, inhibits virus infection and that this inhibition was partially reversed by partially restoring cholesterol levels in cells, suggesting that MbetaCD inhibition of virus infection was mediated by removal of cellular cholesterol. However, fractionation of cellular membranes into DIMs and detergent-soluble membrane fractions showed that both PVR and poliovirus capsid proteins localize not to DIMs but to detergent-soluble membrane fractions during entry into the cells, and their localization was unaffected by treatment with MbetaCD. We further demonstrate that treatment with MbetaCD inhibits RNA delivery after formation of the 135S particles. These data indicate that the cholesterol status of the cell is important during the process of genome delivery and that these entry pathways are distinct from those requiring DIM integrity.

Is the 135S poliovirus particle an intermediate during cell entry?

Poliovirus binding to its receptor (PVR) on the cell surface induces a conformational transition which generates an altered particle with a sedimentation value of 135S versus the 160S of the native virion. A number of lines of evidence suggest that the 135S particle is a cell entry intermediate. However, the low infection efficiencies of the 135S particle and the absence of detectable 135S particles during infection at 26 degrees C by the cold-adapted mutants argue against a role for the 135S particle during the cell entry process. We show here that binding of 135S-antibody complexes to the Fc receptor (CDw32) increases the infectivity of these particles by 2 to 3 orders of magnitude. Thus, the low efficiency of infection by 135S particles is due in part to the low binding affinity of these particles. In addition, we show that there is an additional stage in the entry process that is associated with RNA release. This stage occurs after formation of the 135S particle, is rate limiting during infection at 37 degrees C, but not at 26 degrees C, and is PVR independent. The data also demonstrate that during infection at 26 degrees C, the rate-limiting step is the PVR-mediated conversion of wild-type 160S particles to 135S particles. This suggests that during infection at 26 degrees C by the cold-adapted viruses, 135S particles are formed, but they fail to accumulate to detectable levels because the subsequent post-135S particle events occur at a significantly faster rate than the initial conversion of 160S to 135S particles. These data support a model in which the 135S particle is an intermediate during poliovirus entry.

Poliovirus mutants at histidine 195 of VP2 do not cleave VP0 into VP2 and VP4

The final stage of poliovirus assembly is characterized by a cleavage of the capsid precursor protein VP0 into VP2 and VP4. This cleavage is thought to be autocatalytic and dependent on RNA encapsidation. Analysis of the poliovirus empty capsid structure has led to a mechanistic model for VP0 cleavage involving a conserved histidine residue that is present in the surrounding environment of the VP0 cleavage site. Histidine 195 of VP2 (2195H) is hypothesized to activate local water molecules, thus initiating a nucleophilic attack at the scissile bond. To test this hypothesis, 2195H mutants were constructed and their phenotypes were characterized. Consistent with the requirement of VP0 cleavage for poliovirus infectivity, all 2195H mutants were nonviable upon introduction of the mutant genomes into HeLa cells. Replacement of 2195H with threonine or arginine resulted in the assembly of a highly unstable 150S virus particle. Further analyses showed that these particles contain genomic RNA and uncleaved VP0, criteria associated with the provirion assembly intermediate. These data support the involvement of 2195H in mediating VP0 cleavage during the final stages of virus assembly.

RNA binding properties of poliovirus subviral particles

The mechanism of encapsidation of the RNA genome of poliovirus and other picornaviruses is unknown. To test whether any of the putative assembly intermediates of poliovirus could interact directly with the poliovirus RNA genome, poliovirus RNA was attached to magnetic streptavidin beads and incubated with partially purified extracts containing 35S-labeled 14S pentamer and 75S empty-capsid subviral particles from infected cells. The amount of labeled protein bound to the beads was monitored, thus testing the RNA-binding activities of only the labeled viral proteins in the preparations. In this assay, nonspecific RNA-binding activity was displayed by the 14S pentameric particles and mature virons. 75S empty capsids displayed no propensity to associate with RNA. 14S pentamers were demonstrated to form rapidly sedimenting complexes and to undergo a conformational alteration upon RNA binding. These findings are consistent with a direct role for the 14S pentameric particles in RNA packaging during poliovirus morphogenesis.

Poliovirus capsid proteins derived from P1 precursors with glutamine-valine cleavage sites have defects in assembly and RNA encapsidation

Assembly of poliovirus virions requires proteolytic cleavage of the P1 capsid precursor polyprotein between two separate glutamine-glycine (QG) amino acid pairs by the viral protease 3CD. In this study, we have investigated the effects on P1 polyprotein processing and subsequent assembly of processed capsid proteins caused by substitution of the glycine residue at the individual QG cleavage sites with valine (QG-->QV). P1 cDNAs encoding the valine substitutions were created by site-directed mutagenesis and were recombined into wild-type vaccinia virus to generate recombinant vaccinia viruses which expressed the mutant P1 precursors. The recombinant vaccinia virus-expressed mutant P1 polyproteins were analyzed for proteolytic processing defects in cells coinfected with a recombinant vaccinia virus (VVP3) that expresses the poliovirus 3CD protease and for processing and assembly defects by using a trans complementation system in which P1-expressing recombinant vaccinia viruses provide capsid precursor to a defective poliovirus genome that does not express functional capsid proteins (D. C. Ansardi, D. C. Porter, and C. D. Morrow, J. Virol. 67:3684-3690, 1993). The QV-substituted precursors were proteolytically processed at the altered sites both in cells coinfected with VVP3 and in cells coinfected with defective poliovirus, although the kinetics of cleavage at the altered sites were slower than those of cleavage at the wild-type QG site in the precursor. Completely processed capsid proteins VP0, VP3, and VP1 derived from the mutant precursor containing a valine at the amino terminus of VP3 (VP3-G001V) were unstable and failed to assemble stable subviral structures in cells coinfected with defective poliovirus. In contrast, capsid proteins derived from the P1 precursor with a valine substitution at the amino terminus of VP1 (VP1-G001V) assembled empty capsid particles but were deficient in assembling RNA-containing virions. The assembly characteristics of the VP1-G001V mutant were compared with those of a previously described VP3-VP1 cleavage site mutant (K. Kirkegaard and B. Nelsen, J. Virol. 64:185-194, 1990) which contained a deletion of the first four amino-terminal residues of VP1 (VP1-delta 1-4) and which was reconstructed for our studies into the recombinant vaccinia virus system. Complete proteolytic processing of the VP1-delta 1-4 precursor also occurred more slowly than complete cleavage of the wild-type precursor, and formation of virions was delayed; however, capsid proteins derived from the VP1-G001V mutant assembled RNA-containing virions less efficiently than those derived from the VP1-delta 1-4 precursor.(ABSTRACT TRUNCATED AT 400 WORDS)

Efficient analysis of nonviable poliovirus capsid mutants

Nonviable poliovirus capsid mutants were studied by an efficient infection-transfection system. Phenotypically, nonviable poliovirus capsid mutants appear to segregate into three classes: those that form only protomers, those that can form pentamers, and one that can form completed virions.