Mutation of lysine residues in the nucleotide binding segments of the poliovirus RNA-dependent RNA polymerase

Insights into In Vitro Adaptation of EV71 and Analysis of Reduced Virulence by In Silico Predictions

EV-A71 is a common viral pathogen that causes hand, foot and mouth disease. It is a single-stranded RNA virus that has a low fidelity RNA polymerase and, as a result, spontaneous mutations frequently occur in the EV-A71 genome. The mutations within the genome give rise to quasispecies within the viral population that could be further defined by haplotypes. In vitro virulence of EV-A71 was shown by plaque size in Rhabdomyosarcoma (RD) cells, which was substantiated by in vitro characterizations of growth, RNA replication, binding, attachment and host cell internalization. Viruses could exhibit different host cell adaptations in different cell lines during viral passaging. The EV-A71/WT (derived from EV-A71 subgenotype B4) was shown to comprise six haplotypes through next-generation sequencing, where only EV-A71/Hap2 was found to be cultivable in RD cells, while EV-A71/Hap4 was the only cultivable haplotype in Vero cells. The EV-A71/WT produced plaques of four different sizes (small, medium, big, huge) in RD cells, while only two plaque variants (small, medium) were present in Vero cells. The small plaque variant isolated from RD cells displayed lower RNA replication rates, slower in vitro growth kinetics, higher TCID50 and lower attachment, binding and entry ability when compared against EV-A71/WT due to the mutation at 3D-S228P that disrupted the active site of the RNA polymerase, resulting in low replication and growth of the variant.

Conformational Ensemble of the Poliovirus 3CD Precursor Observed by MD Simulations and Confirmed by SAXS: A Strategy to Expand the Viral Proteome?

The genomes of RNA viruses are relatively small. To overcome the small-size limitation, RNA viruses assign distinct functions to the processed viral proteins and their precursors. This is exemplified by poliovirus 3CD protein. 3C protein is a protease and RNA-binding protein. 3D protein is an RNA-dependent RNA polymerase (RdRp). 3CD exhibits unique protease and RNA-binding activities relative to 3C and is devoid of RdRp activity. The origin of these differences is unclear, since crystal structure of 3CD revealed "beads-on-a-string" structure with no significant structural differences compared to the fully processed proteins. We performed molecular dynamics (MD) simulations on 3CD to investigate its conformational dynamics. A compact conformation of 3CD was observed that was substantially different from that shown crystallographically. This new conformation explained the unique properties of 3CD relative to the individual proteins. Interestingly, simulations of mutant 3CD showed altered interface. Additionally, accelerated MD simulations uncovered a conformational ensemble of 3CD. When we elucidated the 3CD conformations in solution using small-angle X-ray scattering (SAXS) experiments a range of conformations from extended to compact was revealed, validating the MD simulations. The existence of conformational ensemble of 3CD could be viewed as a way to expand the poliovirus proteome, an observation that may extend to other viruses.

Distinct conformations of a putative translocation element in poliovirus polymerase

The mechanism whereby RNA is translocated by the single subunit viral RNA-dependent RNA polymerases is not yet understood. These enzymes lack homologs of the "O-helix" structures and associated fingers domain movements thought to be responsible for translocation in many DNA-templated polymerases. The structures of multiple picornavirus polymerase elongation complexes suggest that these enzymes use a different molecular mechanism where translocation is not strongly coupled to the opening of the active site following catalysis. Here we present the 2.0- to 2.6-Å-resolution crystal structures and biochemical data for 12 poliovirus polymerase mutants that together show how proper enzyme functions and translocation activity requires conformational flexibility of a loop sequence in the palm domain B-motif. Within the loop, the Ser288-Gly289-Cys290 sequence is shown to play a major role in the catalytic cycle based on RNA binding, processive elongation activity, and single nucleotide incorporation assays. The structures show that Ser288 forms a key hydrogen bond with Asp238, the backbone flexibility of Gly289 is required for translocation competency, and Cys290 modulates the overall elongation activity of the enzyme. Some conformations of the loop represent likely intermediates on the way to forming the catalytically competent closed active site, while others are consistent with a role in promoting translocation of the nascent base pair out of the active site. The loop structure and key residues surrounding it are highly conserved, suggesting that the structural dynamics we observe in poliovirus 3D(pol) are a common feature of viral RNA-dependent RNA polymerases.

StralSV: assessment of sequence variability within similar 3D structures and application to polio RNA-dependent RNA polymerase

Background

Most of the currently used methods for protein function prediction rely on sequence-based comparisons between a query protein and those for which a functional annotation is provided. A serious limitation of sequence similarity-based approaches for identifying residue conservation among proteins is the low confidence in assigning residue-residue correspondences among proteins when the level of sequence identity between the compared proteins is poor. Multiple sequence alignment methods are more satisfactory--still, they cannot provide reliable results at low levels of sequence identity. Our goal in the current work was to develop an algorithm that could help overcome these difficulties by facilitating the identification of structurally (and possibly functionally) relevant residue-residue correspondences between compared protein structures.

Results

Here we present StralSV (structure-alignment sequence variability), a new algorithm for detecting closely related structure fragments and quantifying residue frequency from tight local structure alignments. We apply StralSV in a study of the RNA-dependent RNA polymerase of poliovirus, and we demonstrate that the algorithm can be used to determine regions of the protein that are relatively unique, or that share structural similarity with proteins that would be considered distantly related. By quantifying residue frequencies among many residue-residue pairs extracted from local structural alignments, one can infer potential structural or functional importance of specific residues that are determined to be highly conserved or that deviate from a consensus. We further demonstrate that considerable detailed structural and phylogenetic information can be derived from StralSV analyses.

Conclusions

StralSV is a new structure-based algorithm for identifying and aligning structure fragments that have similarity to a reference protein. StralSV analysis can be used to quantify residue-residue correspondences and identify residues that may be of particular structural or functional importance, as well as unusual or unexpected residues at a given sequence position. StralSV is provided as a web service at http://proteinmodel.org/AS2TS/STRALSV/.

The crystal structure of coxsackievirus B3 RNA-dependent RNA polymerase in complex with its protein primer VPg confirms the existence of a second VPg binding site on Picornaviridae polymerases

The RNA-dependent RNA polymerase (RdRp) is a central piece in the replication machinery of RNA viruses. In picornaviruses this essential RdRp activity also uridylates the VPg peptide, which then serves as a primer for RNA synthesis. Previous genetic, binding, and biochemical data have identified a VPg binding site on poliovirus RdRp and have shown that is was implicated in VPg uridylation. More recent structural studies have identified a topologically distinct site on the closely related foot-and-mouth disease virus RdRp supposed to be the actual VPg-primer-binding site. Here, we report the crystal structure at 2.5-A resolution of active coxsackievirus B3 RdRp (also named 3D(pol)) in a complex with VPg and a pyrophosphate. The pyrophosphate is situated in the active-site cavity, occupying a putative binding site either for the coproduct of the reaction or an incoming NTP. VPg is bound at the base of the thumb subdomain, providing first structural evidence for the VPg binding site previously identified by genetic and biochemical methods. The binding mode of VPg to CVB3 3D(pol) at this site excludes its uridylation by the carrier 3D(pol). We suggest that VPg at this position is either uridylated by another 3D(pol) molecule or that it plays a stabilizing role within the uridylation complex. The CVB3 3D(pol)/VPg complex structure is expected to contribute to the understanding of the multicomponent VPg-uridylation complex essential for the initiation of genome replication of picornaviruses.

Crystal structure of poliovirus 3CD protein: virally encoded protease and precursor to the RNA-dependent RNA polymerase

Poliovirus 3CD is a multifunctional protein that serves as a precursor to the protease 3C(pro) and the viral polymerase 3D(pol) and also plays a role in the control of viral replication. Although 3CD is a fully functional protease, it lacks polymerase activity. We have solved the crystal structures of 3CD at a 3.4-A resolution and the G64S fidelity mutant of 3D(pol) at a 3.0-A resolution. In the 3CD structure, the 3C and 3D domains are joined by a poorly ordered polypeptide linker, possibly to facilitate its cleavage, in an arrangement that precludes intramolecular proteolysis. The polymerase active site is intact in both the 3CD and the 3D(pol) G64S structures, despite the disruption of a network proposed to position key residues in the active site. Therefore, changes in molecular flexibility may be responsible for the differences in fidelity and polymerase activities. Extensive packing contacts between symmetry-related 3CD molecules and the approach of the 3C domain's N terminus to the VPg binding site suggest how 3D(pol) makes biologically relevant interactions with the 3C, 3CD, and 3BCD proteins that control the uridylylation of VPg during the initiation of viral replication. Indeed, mutations designed to disrupt these interfaces have pronounced effects on the uridylylation reaction in vitro.

Mutagenesis analysis of the rGTP-specific binding site of hepatitis C virus RNA-dependent RNA polymerase

Hepatitis C virus (HCV) nonstructural protein 5B (NS5B) is the virus-encoded RNA-dependent RNA polymerase (RdRp) essential for HCV RNA replication. An earlier crystallographic study identified a rGTP-specific binding site lying at the surface between the thumb domain and the fingertip about 30 A away from the active site of the HCV RdRp (S. Bressanelli, L. Tomei, F. A. Rey, and R. De Francesco, J. Virol 76:3482-3492, 2002). To determine its physiological importance, we performed a systematic mutagenesis analysis of the rGTP-specific binding pocket by amino acid substitutions. Effects of mutations of the rGTP-specific binding site on enzymatic activity were determined by an in vitro RdRp assay, while effects of mutations on HCV RNA replication were examined by cell colony formation, as well as by transient replication of subgenomic HCV RNAs. Results derived from these studies demonstrate that amino acid substitutions of the rGTP-specific binding pocket did not significantly affect the in vitro RdRp activity of purified recombinant NS5B proteins, as measured by their abilities to synthesize RNA on an RNA template containing the 3' untranslated region of HCV negative-strand RNA. However, most mutations of the rGTP-specific binding site either impaired or completely ablated the ability of subgenomic HCV RNAs to induce cell colony formation. Likewise, these mutations caused either reduction in or lethality to transient replication of the human immunodeficiency virus Tat-expressing HCV replicon RNAs in the cell. Collectively, these findings demonstrate that the rGTP-specific binding site of the HCV NS5B is not required for in vitro RdRp activity but is important for HCV RNA replication in vivo.

Differential rescue of poliovirus RNA replication functions by genetically modified RNA polymerase precursors

We have previously described the RNA replication properties of poliovirus transcripts harboring chimeric RNA polymerase sequences representing suballelic exchanges between poliovirus type 1 (PV1) and coxsackievirus B3 (CVB3) utilizing an in vitro translation and RNA replication assay (C. Cornell, R. Perera, J. E. Brunner, and B. L. Semler, J. Virol. 78:4397-4407, 2004). We showed that three of the seven chimeras were capable of RNA replication in vitro, although replication levels were greatly reduced compared to that of wild-type transcripts. Interestingly, one of the replication-competent transcripts displayed a strand-specific RNA synthesis defect suggesting (i) a differential replication complex assembly mechanism involving 3D and/or precursor molecules (i.e., 3CD) required for negative- versus positive-strand RNA synthesis or (ii) effect(s) on the ability of the 3D polymerase to form higher-ordered structures required for positive-strand RNA synthesis. In this study, we have attempted to rescue defective RNA replication in vitro by cotranslating nonstructural proteins from a transcript encoding a large precursor polyprotein (P3) to complement 3D polymerase and/or precursor polypeptide functions altered in each of the chimeric constructs. Utilization of a wild-type P3 construct revealed that all transcripts containing chimeric PV1/CVB3 polymerase sequences can be complemented in trans for both negative- and positive-strand RNA synthesis. Furthermore, data from experiments utilizing genetically modified forms of the P3 polyprotein, containing mutations within 3C or 3D sequences, strongly suggest the existence of different protein-protein and protein-RNA interactions required for positive- versus negative-strand RNA synthesis. These results, combined with data from in vitro RNA elongation assays, indicate that the delivery of active 3D RNA polymerase to replication complexes requires a series of macromolecular interactions that rely on the presence of specific 3D amino acid sequences.

Structural basis for proteolysis-dependent activation of the poliovirus RNA-dependent RNA polymerase

The active RNA-dependent RNA polymerase of poliovirus, 3Dpol, is generated by cleavage of the 3CDpro precursor protein, a protease that has no polymerase activity despite containing the entire polymerase domain. By intentionally disrupting a known and persistent crystal packing interaction, we have crystallized the poliovirus polymerase in a new space group and solved the complete structure of the protein at 2.0 A resolution. It shows that the N-terminus of fully processed 3Dpol is buried in a surface pocket where it makes hydrogen bonds that act to position Asp238 in the active site. Asp238 is an essential residue that selects for the 2' OH group of substrate rNTPs, as shown by a 2.35 A structure of a 3Dpol-GTP complex. Mutational, biochemical, and structural data further demonstrate that 3Dpol activity is exquisitely sensitive to mutations at the N-terminus. This sensitivity is the result of allosteric effects where the structure around the buried N-terminus directly affects the positioning of Asp238 in the active site.

Membrane requirements for uridylylation of the poliovirus VPg protein and viral RNA synthesis in vitro

Efficient translation of poliovirus (PV) RNA in uninfected HeLa cell extracts generates all of the viral proteins required to carry out viral RNA replication and encapsidation and to produce infectious virus in vitro. In infected cells, viral RNA replication occurs in ribonucleoprotein complexes associated with clusters of vesicles that are formed from preexisting intracellular organelles, which serve as a scaffold for the viral RNA replication complex. In this study, we have examined the role of membranes in viral RNA replication in vitro. Electron microscopic and biochemical examination of extracts actively engaged in viral RNA replication failed to reveal a significant increase in vesicular membrane structures or the protective aggregation of vesicles observed in PV-infected cells. Viral, nonstructural replication proteins, however, bind to heterogeneous membrane fragments in the extract. Treatment of the extracts with nonionic detergents, a membrane-altering inhibitor of fatty acid synthesis (cerulenin), or an inhibitor of intracellular membrane trafficking (brefeldin A) prevents the formation of active replication complexes in vitro, under conditions in which polyprotein synthesis and processing occur normally. Under all three of these conditions, synthesis of uridylylated VPg to form the primer for initiation of viral RNA synthesis, as well as subsequent viral RNA replication, was inhibited. Thus, although organized membranous structures morphologically similar to the vesicles observed in infected cells do not appear to form in vitro, intact membranes are required for viral RNA synthesis, including the first step of forming the uridylylated VPg primer for RNA chain elongation.

A single mutation in poliovirus RNA-dependent RNA polymerase confers resistance to mutagenic nucleotide analogs via increased fidelity

Ribavirin is a nucleotide analog that can be incorporated by viral polymerases, causing mutations by allowing base mismatches. It is currently used therapeutically as an antiviral drug during hepatitis C virus infections. During the amplification of poliovirus genomic RNA or hepatitis C replicons, error frequency is known to increase upon ribavirin treatment. This observation has led to the hypothesis that ribavirin's antiviral activity results from error catastrophe caused by increased mutagenesis of viral genomes. Here, we describe the generation of ribavirin-resistant poliovirus by serial viral passage in the presence of increasing concentrations of the drug. Ribavirin resistance can be caused by a single amino acid change, G64S, in the viral polymerase in an unresolved portion of the fingers domain. Compared with wild-type virus, ribavirin-resistant poliovirus displays increased fidelity of RNA synthesis in the absence of ribavirin and increased survival both in the presence of ribavirin and another mutagen, 5-azacytidine. Ribavirin-resistant poliovirus represents an unusual class of viral drug resistance: resistance to a mutagen through increased fidelity.

Manganese-dependent polioviruses caused by mutations within the viral polymerase

Viral RNA-dependent RNA polymerases exhibit great sequence diversity. Only six core amino acids are conserved across all polymerases of positive-strand RNA viruses of eukaryotes. While exploring the function of one of these completely conserved residues, asparagine 297 in the prototypic poliovirus polymerase 3D(pol), we identified three viable mutants with noncanonical amino acids at this conserved position. Although asparagine 297 could be replaced by glycine or alanine in these mutants, the viruses exhibited Mn(2+)-dependent RNA replication and viral growth. All known RNA polymerases and replicative polymerases of bacterial, eukaryotic, and viral organisms are thought to be magnesium dependent in vivo, and therefore these mutant polioviruses may represent the first viruses with a requirement for an alternative polymerase cation. These results demonstrate the extreme functional flexibility of viral RNA-dependent RNA polymerases. Furthermore, the finding that strictly conserved residues in the nucleotide binding pocket of the polymerase can be altered in a manner that supports virus production suggests that drugs targeting this region of the enzyme will still be susceptible to the problem of drug-resistant escape mutants.

Selective stimulation of hepatitis C virus and pestivirus NS5B RNA polymerase activity by GTP

NS5B of the hepatitis C virus is an RNA template-dependent RNA polymerase and therefore the key player of the viral replicase complex. Using a highly purified enzyme expressed with recombinant baculoviruses in insect cells, we demonstrate a stimulation of RNA synthesis up to 2 orders of magnitude by high concentrations of GTP but not with ATP, CTP, UTP, GDP, or GMP. Enhancement of RNA synthesis was found with various heteropolymeric RNA templates, with poly(C)-oligo(G)12 but not with poly(A)-oligo(U)12. Several amino acid substitutions in polymerase motifs B, C, and D previously shown to be crucial for RdRp activity were tested for GTP stimulation of RNA synthesis. Most of these mutations, in particular those affecting the GDD motif (motif C) strongly reduced or completely abolished activation by GTP, suggesting that the same NTP-binding site is used for stimulation and RNA synthesis. Since GTP did not affect the overall RNA binding properties or the elongation rate, high concentrations of GTP appear to accelerate a rate-limiting step at the level of initiation of RNA synthesis. Finally, enhancement of RNA synthesis by high GTP concentrations was also found with NS5B of the pestivirus classical swine fever virus, but not with the 3D polymerase of poliovirus. Thus, stimulation of RdRp activity by GTP is evolutionarily conserved between the closely related hepaciviruses and pestiviruses but not between these and the more distantly related picornaviruses.

Analysis of RNA-dependent RNA polymerase structure and function as guided by known polymerase structures and computer predictions of secondary structure

RNA-dependent RNA polymerases (RdRps) function as the catalytic subunit of the viral replicase required for the replication of all positive strand RNA viruses. The vast majority of RdRps have been identified solely on the basis of sequence similarity. Structural studies of RdRps have lagged behind those of the DNA-dependent DNA polymerases, DNA-dependent RNA polymerases, and reverse transcriptases until the recent report of the partial crystal structure of the poliovirus RdRp, 3Dpol [Hansen, J. L., et al. (1997). Structure 5, 1109-1122]. We seek to address whether all RdRps will have structures similar to those found in the poliovirus polymerase structure. Therefore, the PHD method of Rost and Sander [Rost, B., and Sander, C. (1993a). J. Mol. Biol. 232, 584-599; Rost, B., and Sander, C. (1994). Protein 19, 55-77] was used to predict the secondary structure of the RdRps from six different viral families: bromoviruses, tobamoviruses, tombusvirus, leviviruses, hepatitis C-like viruses, and picornaviruses. These predictions were compared with the known crystal structure of the poliovirus polymerase. The PHD method was also used to predict picornavirus structures in places in which the poliovirus crystal structure was disordered. All five families and the picornaviruses share a similar order of secondary structure elements present in their polymerase proteins. All except the leviviruses have the unique region observed in the poliovirus 3Dpol that is suggested to be involved in polymerase oligomerization. These structural predictions are used to explain the phenotypes of a collection of mutations that exist in several RNA polymerases. This analysis will help to guide further characterization of RdRps.

Rescue of defective poliovirus RNA replication by 3AB-containing precursor polyproteins

This study demonstrates the in vitro complementation of an RNA replication-defective lesion in poliovirus RNA by providing a replicase/polymerase precursor polypeptide [P3(wt) (wild type)] in trans. The replication-defective mutation was a phenylalanine-to-histidine change (F69H) in the hydrophobic domain of the membrane-associated viral protein 3AB. RNAs encoding wild-type forms of protein 3AB or the P3 precursor polypeptide were cotranslated with full-length poliovirus RNAs containing the F69H mutation in a HeLa cell-free translation/replication assay in an attempt to trans complement the RNA replication defect exhibited by the 3AB(F69H) lesion. Unexpectedly, generation of 3AB(wt) in trans was not able to efficiently complement the defective replication complex; however, cotranslation of the large P3(wt) precursor protein allowed rescue of RNA replication. Furthermore, P3 proteins harboring mutations that resulted in either an inactive polymerase or an inactive proteinase domain displayed differential abilities to trans complement the RNA replication defect. Our results indicate that replication proteins like 3AB may need to be delivered to the poliovirus replication complex in the form of a larger 3AB-containing protein precursor prior to complex assembly rather than as the mature viral cleavage product.

Genetic dissection of interaction between poliovirus 3D polymerase and viral protein 3AB

Poliovirus RNA-dependent RNA polymerase 3D and viral protein 3AB are both thought to be required for the initiation of RNA synthesis. These two proteins physically associate with each other and with viral RNA replication complexes found on virus-induced membranes in infected cells. An understanding of the interface between 3D and 3AB would provide a first step in visualizing the architecture of the multiprotein complex that is assembled during poliovirus infection to replicate and package the viral RNA genome. The identification of mutations in 3D that diminish 3D-3AB interactions without affecting other functions of 3D polymerase is needed to study the function of the 3D-3AB interaction in infected cells. We describe the use of the yeast two-hybrid system to isolate and characterize mutations in 3D polymerase that cause it to interact less efficiently with 3AB than wild-type polymerase. One mutation, a substitution of leucine for valine at position 391 (V391L), resulted in a 3AB-specific interaction defect in the two-hybrid system, causing a reduction in the interaction of 3D polymerase with 3AB but not with another viral protein or a host protein tested. In vitro, purified 3D-V391L polymerase bound to membrane-associated 3AB with reduced affinity. Poliovirus that contained the 3D-V391L mutation was temperature sensitive, displaying a pronounced conditional defect in RNA synthesis. We conclude that interaction between 3AB and 3D or 3D-containing polypeptides plays a role in RNA synthesis during poliovirus infection.

One of two NTP binding sites in poliovirus RNA polymerase required for RNA replication

The poliovirus RNA-dependent RNA polymerase (3Dpol) has been shown to contain two NTP binding sites by chemical cross-linking of oxidized nucleotide to the intact protein. Only one site (Lys-61) was shown to be essential for RNA chain elongation activity by purified enzyme; however, a full-length viral RNA, coding for an altered lysine residue (K276L) in the second site, generated virus with a minute plaque phenotype that rapidly reverted to a wild-type phenotype with Arg-276 replacing Leu-276 in 3D. Viruses with lysine to leucine substitutions in other positions of the second binding site of their polymerase proteins grew with wild-type phenotype. To test the significance of the second binding site, poliovirus 3Dpol was generated with lysine (wild-type), leucine, or arginine at residue 276 and tested for NTP cross-linking using 32P-oxidized GTP. Analysis of cyanogen bromide peptides of each 3D preparation showed that the second NTP binding site had severely reduced NTP binding in mu276(Leu) but not in the revertant mu276(Arg), despite the reported requirement for lysine in the cross-linking reaction. To eliminate the possibility that 32P-oxidized GTP cross-linked to Arg at residue 276, a model system was designed with unmodified amino acid or acetylated (alpha-amino) amino acid and 32P-oxidized GTP. Cross-linking to lysine, but not leucine or arginine, was observed thus eliminating the possibility that NTP could be cross-linked to residue 276 in 3D. We conclude that NTP binding at the second site in poliovirus 3D is at lysine residues at positions other than 276 (278 or 283), and nucleotide binding at these sites has no bearing on elongation activity or replication of the virus. Nucleotide binding only at the site including Lys-61 is essential for RNA replication.