Human rhinovirus type 14 gain-of-function mutants for oriI utilization define residues of 3C(D) and 3Dpol that contribute to assembly and stability of the picornavirus VPg uridylylation complex

Structure of coxsackievirus cloverleaf RNA and 3Cpro dimer establishes the RNA-binding mechanism of enterovirus protease 3Cpro

In positive-strand RNA viruses, the genome serves as a template for both protein translation and negative-strand RNA synthesis. Enteroviruses use the cloverleaf RNA structure at the 5' end of the genome to balance these two processes. Cloverleaf acts as a promoter for RNA synthesis and forms a complex with viral 3CD protein, the precursor to 3Cpro protease, and 3Dpol polymerase. The interaction between cloverleaf and 3CD is mediated by the 3Cpro domain, yet how 3Cpro promotes specific RNA-binding is not clear. We report the structure of coxsackievirus cloverleaf RNA-3Cpro complex, wherein two 3Cpro molecules interact with cloverleaf stem-loop D. 3Cpro dimer mainly recognizes the shape of the dsRNA helix through symmetric interactions, suggesting that 3Cpro is a previously undiscovered type of RNA binding protein. We show that 3CD protein also dimerizes on cloverleaf RNA and binds the RNA with higher affinity than 3Cpro. The structure provides insight into the RNA-binding mechanism of 3Cpro or 3CD with other cis-acting replication elements.

Picornavirus 3C Proteins Intervene in Host Cell Processes through Proteolysis and Interactions with RNA

The Picornaviridae family comprises a large group of non-enveloped viruses with enormous impact on human and animal health. The picornaviral genome contains one open reading frame encoding a single polyprotein that can be processed by viral proteases. The picornaviral 3C proteases share similar three-dimensional structures and play a significant role in the viral life cycle and virus-host interactions. Picornaviral 3C proteins also have conserved RNA-binding activities that contribute to the assembly of the viral RNA replication complex. The 3C protease is important for regulating the host cell response through the cleavage of critical host cell proteins, acting to selectively 'hijack' host factors involved in gene expression, promoting picornavirus replication, and inactivating key factors in innate immunity signaling pathways. The protease and RNA-binding activities of 3C are involved in viral polyprotein processing and the initiation of viral RNA synthesis. Most importantly, 3C modifies critical molecules in host organelles and maintains virus infection by subtly subverting host cell death through the blocking of transcription, translation, and nucleocytoplasmic trafficking to modulate cell physiology for viral replication. Here, we discuss the molecular mechanisms through which 3C mediates physiological processes involved in promoting virus infection, replication, and release.

Picornavirus 3C - a protease ensuring virus replication and subverting host responses

The protease 3C is encoded by all known picornaviruses, and the structural features related to its protease and RNA-binding activities are conserved; these contribute to the cleavage of viral polyproteins and the assembly of the viral RNA replication complex during virus replication. Furthermore, 3C performs functions in the host cell through its interaction with host proteins. For instance, 3C has been shown to selectively 'hijack' host factors involved in gene expression, promoting picornavirus replication, and to inactivate key factors in innate immunity signaling pathways, inhibiting the production of interferon and inflammatory cytokines. Importantly, 3C maintains virus infection by subtly subverting host cell death and modifying critical molecules in host organelles. This Review focuses on the molecular mechanisms through which 3C mediates physiological processes involved in virus-host interaction, thus highlighting the picornavirus-mediated pathogenesis caused by 3C.

Structural Biology of the Enterovirus Replication-Linked 5'-Cloverleaf RNA and Associated Virus Proteins

Although enteroviruses are associated with a wide variety of diseases and conditions, their mode of replication is well conserved. Their genome is carried as a single, positive-sense RNA strand. At the 5' end of the strand is an approximately 90-nucleotide self-complementary region called the 5' cloverleaf, or the oriL. This noncoding region serves as a platform upon which host and virus proteins, including the 3B, 3C, and 3D virus proteins, assemble in order to initiate replication of a negative-sense RNA strand. The negative strand in turn serves as a template for synthesis of multiple positive-sense RNA strands. Building on structural studies of individual RNA stem-loops, the structure of the intact 5' cloverleaf from rhinovirus has recently been determined via nuclear magnetic resonance/small-angle X-ray scattering (NMR/SAXS)-based methods, while structures have also been determined for enterovirus 3A, 3B, 3C, and 3D proteins. Analysis of these structures, together with structural and modeling studies of interactions between host and virus proteins and RNA, has begun to provide insight into the enterovirus replication mechanism and the potential to inhibit replication by blocking these interactions.

Initiation of protein-primed picornavirus RNA synthesis

Plus strand RNA viruses use different mechanisms to initiate the synthesis of their RNA chains. The Picornaviridae family constitutes a large group of plus strand RNA viruses that possess a small terminal protein (VPg) covalently linked to the 5'-end of their genomes. The RNA polymerases of these viruses use VPg as primer for both minus and plus strand RNA synthesis. In the first step of the initiation reaction the RNA polymerase links a UMP to the hydroxyl group of a tyrosine in VPg using as template a cis-replicating element (cre) positioned in different regions of the viral genome. In this review we will summarize what is known about the initiation reaction of protein-primed RNA synthesis by the RNA polymerases of the Picornaviridae. As an example we will use the RNA polymerase of poliovirus, the prototype of Picornaviridae. We will also discuss models of how these nucleotidylylated protein primers might be used, together with viral and cellular replication proteins and other cis-replicating RNA elements, during minus and plus strand RNA synthesis.

Human rhinovirus VPg uridylylation AlphaScreen for high-throughput screening

As an obligate step for picornaviruses to replicate their genome, the small viral peptide VPg must first be specifically conjugated with uridine nucleotides at a conserved tyrosine hydroxyl group. The resulting VPg-pUpU serves as the primer for genome replication. The uridylylation reaction requires the coordinated activity of many components, including the viral polymerase, a conserved internal RNA stem loop structure, and additional viral proteins. Formation of this complex and the resulting conjugation reaction catalyzed by the polymerase, offers a number of biochemical targets for inhibition of an essential process in the viral life cycle. Therefore, an assay recapitulating uridylylation would provide multiple opportunities for discovering potential antiviral agents. Our goal was to identify inhibitors of human rhinovirus (HRV) VPg uridylylation, which might ultimately be useful to reduce or prevent HRV-induced lower airway immunologic inflammatory responses, a major cause of asthma and chronic obstructive pulmonary disease exacerbations. We have reconstituted the complex uridylylation reaction in an AlphaScreen suitable for high-throughput screening, in which a rabbit polyclonal antiserum specific for uridylylated VPg serves as a key reagent. Assay results were validated by quantitative mass spectrometric detection of uridylylation.

A general synthetic method toward uridylylated picornavirus VPg proteins

Small proteins called viral protein genome-linked (VPg), attached to the 5'-end of the viral RNA genome are found as common structure in the large family of picornaviruses. The replication of these viruses is primed by this VPg protein linked to a single uridylyl residue. We report a general procedure to obtain such nucleoproteins employing a pre-uridylylated tyrosine building block in an on-line solid phase-based approach.

Enterovirus 71 VPg uridylation uses a two-molecular mechanism of 3D polymerase

VPg uridylylation is essential for picornavirus RNA replication. The VPg uridylylation reaction consists of the binding of VPg to 3D polymerase (3D(pol)) and the transfer of UMP by 3D(pol) to the hydroxyl group of the third amino acid Tyr of VPg. Previous studies suggested that different picornaviruses employ distinct mechanisms during VPg binding and uridylylation. Here, we report a novel site (Site-311, located at the base of the palm domain of EV71 3D(pol)) that is essential for EV71 VPg uridylylation as well as viral replication. Ala substitution of amino acids (T313, F314, and I317) at Site-311 reduced the VPg uridylylation activity of 3D(pol) by >90%. None of the Site-311 mutations affected the RNA elongation activity of 3D(pol), which indicates that Site-311 does not directly participate in RNA polymerization. However, mutations that abrogated VPg uridylylation significantly reduced the VPg binding ability of 3D(pol), which suggests that Site-311 is a potential VPg binding site on enterovirus 71 (EV71) 3D(pol). Mutation of a polymerase active site in 3D(pol) and Site-311 in 3D(pol) remarkably enables trans complementation to restore VPg uridylylation. In contrast, two distinct Site-311 mutants do not cause trans complementation in vitro. These results indicate that Site-311 is a VPg binding site that stabilizes the VPg molecule during the VPg uridylylation process and suggest a two-molecule model for 3D(pol) during EV71 VPg uridylylation, such that one 3D(pol) presents the hydroxyl group of Tyr3 of VPg to the polymerase active site of another 3D(pol), which in turn catalyzes VPg→VPg-pU conversion. For genome-length RNA, the Site-311 mutations that reduced VPg uridylylation were lethal for EV71 replication, which indicates that Site-311 is a potential antiviral target.

Non-template functions of viral RNA in picornavirus replication

The genomic RNA of poliovirus and closely related picornaviruses perform template and non-template functions during viral RNA replication. The non-template functions are mediated by cis-active RNA sequences that bind viral and cellular proteins to form RNP complexes. The RNP complexes mediate temporally dynamic, long-range interactions in the viral genome and ensure the specificity of replication. The 5' cloverleaf (5' CL)-RNP complex serves as a key cis-active element in all of the non-template functions of viral RNA. The 5'CL-RNP complex is proposed to interact with the cre-RNP complex during VPgpUpU synthesis, the 3'NTR-poly(A) RNP complex during negative-strand initiation and the 30 end negative-strand-RNP complex during positive-strand initiation. Co-ordinating these long-range interactions is important in regulating each step in the replication cycle.

RNA binding by the NS3 protease of the hepatitis C virus

The hepatitis C virus (HCV) nonstructural protein 3 (NS3) is essential for the processing of the HCV polyprotein, the replication of HCV RNA, and to short circuit innate immunity signaling. NS3 contains an N-terminal domain with protease activity and a C-terminal domain with helicase activity. The two domains communicate with each other along with other HCV and cellular proteins. Herein we show that RNAs can bind directly to the active site cleft of the NS3 protease domain (NS3P) and inhibit proteolysis of peptide substrates. RNAs that are less apt to form intramolecular structures have a stronger inhibitory activity than RNAs with more stable base paired regions. Two mutations in the protease domain that resulted in decreased affinity to ssRNA were also defective in RNA-induced ATPase activity from the helicase domain of NS3. The coordinated effects on inhibition of protease activity and stimulation of ATPase activity raise the possibility that RNA serves as a regulatory switch for the two processes.

5-fluorouracil in lethal mutagenesis of foot-and-mouth disease virus

5-fluorouracil (FU) is a pyrimidine analogue extensively used in cancer chemotherapy. FU can be metabolized into 5-fluorouridine-triphosphate, which can be used as substrate for viral RNA-dependent RNA polymerases. This results in the incorporation of mutations into viral RNA. Accumulation of mutations may lead to loss of virus infectivity, in a process known as lethal mutagenesis. RNA virus pathogens are particularly difficult to control because they are highly mutable, and mutants resistant to antiviral agents are readily selected. Here, we review the basic principles of lethal mutagenesis as an antiviral approach, and the participation of FU in its development. Recent studies with foot-and-mouth disease virus indicate that FU can act both as an inhibitor and as a mutagen during foot-and-mouth disease virus replication. This dual activity renders FU an adequate drug for lethal mutagenesis. We suggest that structural and biochemical studies can contribute to the lead to new design of base or nucleoside analogues targeted specifically to viral polymerases.

Overall linkage map of the nonstructural proteins of Aichi virus

Aichi virus (AiV), which is associated with acute gastroenteritis in humans, is a member of the genus Kobuvirus of the family Picornaviridae. Picornavirus genome replication occurs in replication complexes that include viral nonstructural proteins, host proteins and viral RNA. In poliovirus, all nonstructural proteins are found in the replication complexes, suggesting the ability of the viral nonstructural proteins to interact with each other. In this study, we examined the interactions between the AiV nonstructural proteins using a mammalian two-hybrid system. The results showed that all of the tested proteins could interact with more than one protein. We observed homodimerization of five proteins, bidirectional heterodimerization of six protein pairs, and unidirectional heterodimerization of eighteen protein pairs. Among the interactions detected in this study, the 2A-2BC, 2A-2BC, 2A-2C, 2BC-3CD, 2BC-3C, 2C-3C, 2C-3CD and 3AB-3C interactions have not been observed in the previous two-hybrid studies with other picornaviruses. The strongest interaction was observed between 2A and 3CD. AiV 2A has already been shown to be involved in genome replication. Domain mapping of the 2A and 3CD interaction in mammalian two-hybrid analysis revealed that the C-terminal quarter of 2A is not required for the interaction with 3CD.

Insight into poliovirus genome replication and encapsidation obtained from studies of 3B-3C cleavage site mutants

A poliovirus (PV) mutant (termed GG), which is incapable of producing 3AB, VPg, and 3CD proteins due to a defective cleavage site between the 3B and 3C proteins, replicated, producing 3BC-linked RNA rather than the VPg-linked RNA produced by the wild type (WT). GG PV RNA is quasi-infectious. The yield of infectious GG PV relative to replicated RNA is reduced by almost 5 logs relative to that of WT PV. Proteolytic activity required for polyprotein processing is normal for the GG mutant. 3BC-linked RNA can be encapsidated as efficiently as VPg-linked RNA. However, a step after genome replication but preceding virus assembly that is dependent on 3CD and/or 3AB proteins limits production of infectious GG PV. This step may involve release of replicated genomes from replication complexes. A pseudorevertant (termed EG) partially restored cleavage at the 3B-3C cleavage site. The reduced rate of formation of 3AB and 3CD caused corresponding reductions in the observed rate of genome replication and infectious virus production by EG PV without impacting the final yield of replicated RNA or infectious virus relative to that of WT PV. Using EG PV, we showed that genome replication and encapsidation were distinct steps in the multiplication cycle. Ectopic expression of 3CD protein reversed the genome replication phenotype without alleviating the infectious-virus production phenotype. This is the first report of a trans-complementable function for 3CD for any picornavirus. This observation supports an interaction between 3CD protein and viral and/or host factors that is critical for genome replication, perhaps formation of replication complexes.

A picornaviral loop-to-loop replication complex

Picornaviruses replicate their RNA genomes through a highly conserved mechanism that involves an interaction between the principal viral protease (3C(pro)) and the 5'-UTR region of the viral genome. The 3C(pro) catalytic site is the target of numerous replication inhibitors. This paper describes the first structural model of a complex between a picornaviral 3C(pro) and a region of the 5'-UTR, stem-loop D (SLD). Using human rhinovirus as a model system, we have combined NMR contact information, small-angle X-ray scattering (SAXS) data, and previous mutagenesis results to determine the shape, position and relative orientation of the 3C(pro) and SLD components. The results clearly identify a 1:1 binding stoichiometry, with pronounced loops from each molecule providing the key binding determinants for the interaction. Binding between SLD and 3C(pro) induces structural changes in the proteolytic active site that is positioned on the opposite side of the protease relative to the RNA/protein interface, suggesting that subtle conformational changes affecting catalytic activity are relayed through the protein.

Cis-active RNA elements (CREs) and picornavirus RNA replication

Our understanding of picornavirus RNA replication has improved over the past 10 years, due in large part to the discovery of cis-active RNA elements (CREs) within picornavirus RNA genomes. CREs function as templates for the conversion of VPg, the Viral Protein of the genome, into VPgpUpU(OH). These so called CREs are different from the previously recognized cis-active RNA sequences and structures within the 5' and 3' NTRs of picornavirus genomes. Two adenosine residues in the loop of the CRE RNA structures allow the viral RNA-dependent RNA polymerase 3D(Pol) to add two uridine residues to the tyrosine residue of VPg. Because VPg and/or VPgpUpU(OH) prime the initiation of viral RNA replication, the asymmetric replication of viral RNA could not be explained without an understanding of the viral RNA template involved in the conversion of VPg into VPgpUpU(OH) primers. We review the growing body of knowledge regarding picornavirus CREs and discuss how CRE RNAs work coordinately with viral replication proteins and other cis-active RNAs in the 5' and 3' NTRs during RNA replication.

Picornavirus genome replication: roles of precursor proteins and rate-limiting steps in oriI-dependent VPg uridylylation

The 5' ends of all picornaviral RNAs are linked covalently to the genome-encoded peptide, VPg (or 3B). VPg linkage is thought to occur in two steps. First, VPg serves as a primer for production of diuridylylated VPg (VPg-pUpU) in a reaction catalyzed by the viral polymerase that is templated by an RNA element (oriI). It is currently thought that the viral 3AB protein is the source of VPg in vivo. Second, VPg-pUpU is transferred to the 3' end of plus- and/or minus-strand RNA and serves as primer for production of full-length RNA. Nothing is known about the mechanism of transfer. We present biochemical and biological evidence refuting the use of 3AB as the donor for VPg uridylylation. Our data are consistent with precursors 3BC and/or 3BCD being employed for uridylylation. This conclusion is supported by in vitro uridylylation of these proteins, the ability of a mutant replicon incapable of producing processed VPg to replicate in HeLa cells and cell-free extracts and corresponding precursor processing profiles, and the demonstration of 3BC-linked RNA in mutant replicon-transfected cells. These data permit elaboration of our model for VPg uridylylation to include the use of precursor proteins and invoke a possible mechanism for location of the diuridylylated, VPg-containing precursor at the 3' end of plus- or minus-strand RNA for production of full-length RNA. Finally, determinants of VPg uridylylation efficiency suggest formation and/or collapse or release of the uridylylated product as the rate-limiting step in vitro depending upon the VPg donor employed.

Identification of a conserved RNA replication element (cre) within the 3Dpol-coding sequence of hepatoviruses

Internally located, cis-acting RNA replication elements (cre) have been identified within the genomes of viruses representing each of the major picornavirus genera (Enterovirus, Rhinovirus, Aphthovirus, and Cardiovirus) except Hepatovirus. Previous efforts to identify a stem-loop structure with cre function in hepatitis A virus (HAV), the type species of this genus, by phylogenetic analyses or thermodynamic predictions have not succeeded. However, a region of markedly suppressed synonymous codon variability was identified in alignments of HAV sequences near the 5' end of the 3D(pol)-coding sequence of HAV, consistent with noncoding constraints imposed by an underlying RNA secondary structure. Subsequent MFOLD predictions identified a 110-nucleotide (nt) complex stem-loop in this region with a typical AAACA/G cre motif in its top loop. A potentially homologous RNA structure was identified in this region of the avian encephalitis virus genome, despite little nucleotide sequence relatedness between it and HAV. Mutations that disrupted secondary RNA structure or the AAACA/G motif, without altering the amino acid sequence of 3D(pol), ablated replication of a subgenomic HAV replicon in transfected human hepatoma cells. Replication competence could be rescued by reinsertion of the native 110-nt stem-loop structure (but not an abbreviated 45-nt stem-loop) upstream of the HAV coding sequence in the replicon. These results suggest that this stem-loop is functionally similar to cre elements of other picornaviruses and likely involved in templating VPg uridylylation as in other picornaviruses, despite its significantly larger size and lower free folding energy.

The cis-acting replication elements define human enterovirus and rhinovirus species

Replication of picornaviruses is dependent on VPg uridylylation, which is linked to the presence of the internal cis-acting replication element (cre). Cre are located within the sequence encoding polyprotein, yet at distinct positions as demonstrated for poliovirus and coxsackievirus-B3, cardiovirus, and human rhinovirus (HRV-A and HRV-B), overlapping proteins 2C, VP2, 2A, and VP1, respectively. Here we report a novel distinct cre element located in the VP2 region of the recently reported HRV-A2 species and provide evolutionary evidence of its functionality. We also experimentally interrogated functionality of recently identified HRV-B cre in the 2C region that is orthologous to the human enterovirus (HEV) cre and show that it is dispensable for replication and appears to be a nonfunctional evolutionary relic. In addition, our mutational analysis highlights two amino acids in the 2C protein that are crucial for replication. Remarkably, we conclude that each genetic clade of HRV and HEV is characterized by a unique functional cre element, where evolutionary success of a new genetic lineage seems to be associated with an invention of a novel cre motif and decay of the ancestral one. Therefore, we propose that cre element could be considered as an additional criterion for human rhinovirus and enterovirus classification.

Poliovirus cis-acting replication element-dependent VPg Uridylylation lowers the Km of the initiating nucleoside triphosphate for viral RNA replication

Initiation of RNA synthesis by RNA-dependent RNA polymerases occurs when a phosphodiester bond is formed between the first two nucleotides in the 5' terminus of product RNA. The concentration of initiating nucleoside triphosphates (NTPi) required for RNA synthesis is typically greater than the concentration of NTPs required for elongation. VPg, a small viral protein, is covalently attached to the 5' end of picornavirus negative- and positive-strand RNAs. A cis-acting replication element (CRE) within picornavirus RNAs serves as a template for the uridylylation of VPg, resulting in the synthesis of VPgpUpU(OH). Mutations within the CRE RNA structure prevent VPg uridylylation. While the tyrosine hydroxyl of VPg can prime negative-strand RNA synthesis in a CRE- and VPgpUpU(OH)-independent manner, CRE-dependent VPgpUpU(OH) synthesis is absolutely required for positive-strand RNA synthesis. As reported herein, low concentrations of UTP did not support negative-strand RNA synthesis when CRE-disrupting mutations prevented VPg uridylylation, whereas correspondingly low concentrations of CTP or GTP had no negative effects on the magnitude of CRE-independent negative-strand RNA synthesis. The experimental data indicate that CRE-dependent VPg uridylylation lowers the K(m) of UTP required for viral RNA replication and that CRE-dependent VPgpUpU(OH) synthesis was required for efficient negative-strand RNA synthesis, especially when UTP concentrations were limiting. By lowering the concentration of UTP needed for the initiation of RNA replication, CRE-dependent VPg uridylylation provides a mechanism for a more robust initiation of RNA replication.

Picornavirus genome replication. Identification of the surface of the poliovirus (PV) 3C dimer that interacts with PV 3Dpol during VPg uridylylation and construction of a structural model for the PV 3C2-3Dpol complex

Picornaviruses have a peptide termed VPg covalently linked to the 5'-end of the genome. Attachment of VPg to the genome occurs in at least two steps. First, Tyr-3 of VPg, or some precursor thereof, is used as a primer by the viral RNA-dependent RNA polymerase, 3Dpol, to produce VPg-pUpU. Second, VPg-pUpU is used as a primer to produce full-length genomic RNA. Production of VPg-pUpU is templated by a single adenylate residue located in the loop of an RNA stem-loop structure termed oriI by using a slide-back mechanism. Recruitment of 3Dpol to and its stability on oriI have been suggested to require an interaction between the back of the thumb subdomain of 3Dpol and an undefined region of the 3C domain of viral protein 3CD. We have performed surface acidic-to-alanine-scanning mutagenesis of 3C to identify the surface of 3C with which 3Dpol interacts. This analysis identified numerous viable poliovirus mutants with reduced growth kinetics that correlated to reduced kinetics of RNA synthesis that was attributable to a change in VPg-pUpU production. Importantly, these 3C derivatives were all capable of binding to oriI as well as wild-type 3C. Synthetic lethality was observed for these mutants when placed in the context of a poliovirus mutant containing 3Dpol-R455A, a residue on the back of the thumb required for VPg uridylylation. These data were used to guide molecular docking of the structures for a poliovirus 3C dimer and 3Dpol, leading to a structural model for the 3C(2)-3Dpol complex that extrapolates well to all picornaviruses.