Hepatitis C virus IRES RNA-induced changes in the conformation of the 40s ribosomal subunit

Novel viral translation strategies

Viral genomes are compact and encode a limited number of proteins. Because they do not encode components of the translational machinery, viruses exhibit an absolute dependence on the host ribosome and factors for viral messenger RNA (mRNA) translation. In order to recruit the host ribosome, viruses have evolved unique strategies to either outcompete cellular transcripts that are efficiently translated by the canonical translation pathway or to reroute translation factors and ribosomes to the viral genome. Furthermore, viruses must evade host antiviral responses and escape immune surveillance. This review focuses on some recent major findings that have revealed unconventional strategies that viruses utilize, which include usurping the host translational machinery, modulating canonical translation initiation factors to specifically enhance or repress overall translation for the purpose of viral production, and increasing viral coding capacity. The discovery of these diverse viral strategies has provided insights into additional translational control mechanisms and into the viral host interactions that ensure viral protein synthesis and replication. WIREs RNA 2014, 5:779–801. doi: 10.1002/wrna.1246

This article is categorized under: 1

Translation > Translation Mechanisms

2

Translation > Translation Regulation

Nanopore-based conformational analysis of a viral RNA drug target

Nanopores are single-molecule sensors that show exceptional promise as a biomolecular analysis tool by enabling label-free detection of small amounts of sample. In this paper, we demonstrate that nanopores are capable of detecting the conformation of an antiviral RNA drug target. The hepatitis C virus uses an internal ribosome entry site (IRES) motif in order to initiate translation by docking to ribosomes in its host cell. The IRES is therefore a viable and important drug target. Drug-induced changes to the conformation of the HCV IRES motif, from a bent to a straight conformation, have been shown to inhibit HCV replication. However, there is presently no straightforward method to analyze the effect of candidate small-molecule drugs on the RNA conformation. In this paper, we show that RNA translocation dynamics through a 3 nm diameter nanopore is conformation-sensitive by demonstrating a difference in transport times between bent and straight conformations of a short viral RNA motif. Detection is possible because bent RNA is stalled in the 3 nm pore, resulting in longer molecular dwell times than straight RNA. Control experiments show that binding of a weaker drug does not produce a conformational change, as consistent with independent fluorescence measurements. Nanopore measurements of RNA conformation can thus be useful for probing the structure of various RNA motifs, as well as structural changes to the RNA upon small-molecule binding.

Measurement of the change in twist at a helical junction in RNA using the orientation dependence of FRET

Indocarbocyanine fluorophores attached via the 5' terminus of double-stranded nucleic acids have a strong propensity to stack onto the terminal basepair. We previously demonstrated that the efficiency of fluorescence resonance energy transfer between cyanine 3 and 5 terminally attached to duplex species exhibits a pronounced modulation with helix length. This results from a systematic variation in the orientation parameter κ(2) as the relative rotation of the fluorophore transition moments changes due to the helical geometry. Analysis of such profiles provides a rich source of orientational information. In this work, we applied this methodology to the structure of a three-way helical junction that plays an important role in the hepatitis C virus internal ribosome entry site. By comparing matched pairs of duplex and junction species, we were able to measure the change in rotation at the junction. The data reveal a 29.5° overwinding and a small axial extension. This shows the power of this approach for measuring orientational information in biologically important RNA junctions.

Structure-function relationship in viral RNA genomes: The case of hepatitis C virus

The acquisition of a storage information system beyond the nucleotide sequence has been a crucial issue for the propagation and dispersion of RNA viruses. This system is composed by highly conserved, complex structural units in the genomic RNA, termed functional RNA domains. These elements interact with other regions of the viral genome and/or proteins to direct viral translation, replication and encapsidation. The genomic RNA of the hepatitis C virus (HCV) is a good model for investigating about conserved structural units. It contains functional domains, defined by highly conserved structural RNA motifs, mostly located in the 5’-untranslatable regions (5’UTRs) and 3’UTR, but also occupying long stretches of the coding sequence. Viral translation initiation is mediated by an internal ribosome entry site located at the 5’ terminus of the viral genome and regulated by distal functional RNA domains placed at the 3’ end. Subsequent RNA replication strongly depends on the 3’UTR folding and is also influenced by the 5’ end of the HCV RNA. Further increase in the genome copy number unleashes the formation of homodimers by direct interaction of two genomic RNA molecules, which are finally packed and released to the extracellular medium. All these processes, as well as transitions between them, are controlled by structural RNA elements that establish a complex, direct and long-distance RNA-RNA interaction network. This review summarizes current knowledge about functional RNA domains within the HCV RNA genome and provides an overview of the control exerted by direct, long-range RNA-RNA contacts for the execution of the viral cycle.

The intrinsic disorder status of the human hepatitis C virus proteome

Many viral proteins or their biologically important regions are disordered as a whole, or contain long disordered regions. These intrinsically disordered proteins/regions do not possess unique structures and possess functions that complement the functional repertoire of "normal" ordered proteins and domains, with many protein functional classes being heavily dependent on the intrinsic disorder. Viruses commonly use these highly flexible regions to invade the host organisms and to hijack various host systems. These disordered regions also help viruses in adapting to their hostile habitats and to manage their economic usage of genetic material. In this article, we focus on the structural peculiarities of proteins from human hepatitis C virus (HCV) and use a wide spectrum of bioinformatics techniques to evaluate the abundance of intrinsic disorder in the completed proteomes of several human HCV genotypes, to analyze the peculiarities of disorder distribution within the individual HCV proteins, and to establish potential roles of the structural disorder in functions of ten HCV proteins. We show that the intrinsic disorder or increased flexibility is not only abundant in these proteins, but is also absolutely necessary for their functions, playing a crucial role in the proteolytic processing of the HCV polyprotein, the maturation of the individual HCV proteins, and being related to the posttranslational modifications of these proteins and their interactions with DNA, RNA, and various host proteins.

Establishment of chronic hepatitis C virus infection: Translational evasion of oxidative defence

Hepatitis C virus (HCV) causes a clinically important disease affecting 3% of the world population. HCV is a single-stranded, positive-sense RNA virus belonging to the genus Hepacivirus within the Flaviviridae family. The virus establishes a chronic infection in the face of an active host oxidative defence, thus adaptation to oxidative stress is key to virus survival. Being a small RNA virus with a limited genomic capacity, we speculate that HCV deploys a different strategy to evade host oxidative defence. Instead of counteracting oxidative stress, it utilizes oxidative stress to facilitate its own survival. Translation is the first step in the replication of a plus strand RNA virus so it would make sense if the virus can exploit the host oxidative defence in facilitating this very first step. This is particularly true when HCV utilizes an internal ribosome entry site element in translation, which is distinctive from that of cap-dependent translation of the vast majority of cellular genes, thus allowing selective translation of genes under conditions when global protein synthesis is compromised. Indeed, we were the first to show that HCV translation was stimulated by an important pro-oxidant-hydrogen peroxide in hepatocytes, suggesting that HCV is able to adapt to and utilize the host anti-viral response to facilitate its own translation thus allowing the virus to thrive under oxidative stress condition to establish chronicity. Understanding how HCV translation is regulated under oxidative stress condition will advance our knowledge on how HCV establishes chronicity. As chronicity is the initiator step in disease progression this will eventually lead to a better understanding of pathogenicity, which is particularly relevant to the development of anti-virals and improved treatments of HCV patients using anti-oxidants.

Sequence-based design of bioactive small molecules that target precursor microRNAs

Oligonucleotides are designed to target RNA using base pairing rules, but they can be hampered by poor cellular delivery and nonspecific stimulation of the immune system. Small molecules are preferred as lead drugs or probes but cannot be designed from sequence. Herein, we describe an approach termed Inforna that designs lead small molecules for RNA from solely sequence. Inforna was applied to all human microRNA hairpin precursors, and it identified bioactive small molecules that inhibit biogenesis by binding nuclease-processing sites (44% hit rate). Among 27 lead interactions, the most avid interaction is between a benzimidazole (1) and precursor microRNA-96. Compound 1 selectively inhibits biogenesis of microRNA-96, upregulating a protein target (FOXO1) and inducing apoptosis in cancer cells. Apoptosis is ablated when FOXO1 mRNA expression is knocked down by an siRNA, validating compound selectivity. Markedly, microRNA profiling shows that 1 only affects microRNA-96 biogenesis and is at least as selective as an oligonucleotide.

Hepatitis C virus translation inhibitors targeting the internal ribosomal entry site

The internal ribosome entry site (IRES) in the 5' untranslated region (UTR) of the hepatitis C virus (HCV) genome initiates translation of the viral polyprotein precursor. The unique structure and high sequence conservation of the 5' UTR render the IRES RNA a potential target for the development of selective viral translation inhibitors. Here, we provide an overview of approaches to block HCV IRES function by nucleic acid, peptide, and small molecule ligands. Emphasis will be given to the IRES subdomain IIa, which currently is the most advanced target for small molecule inhibitors of HCV translation. The subdomain IIa behaves as an RNA conformational switch. Selective ligands act as translation inhibitors by locking the conformation of the RNA switch. We review synthetic procedures for inhibitors as well as structural and functional studies of the subdomain IIa target and its ligand complexes.

Hepatitis-C-virus-like internal ribosome entry sites displace eIF3 to gain access to the 40S subunit

Hepatitis C virus (HCV) and classical swine fever virus (CSFV) messenger RNAs contain related (HCV-like) internal ribosome entry sites (IRESs) that promote 5'-end independent initiation of translation, requiring only a subset of the eukaryotic initiation factors (eIFs) needed for canonical initiation on cellular mRNAs. Initiation on HCV-like IRESs relies on their specific interaction with the 40S subunit, which places the initiation codon into the P site, where it directly base-pairs with eIF2-bound initiator methionyl transfer RNA to form a 48S initiation complex. However, all HCV-like IRESs also specifically interact with eIF3 (refs 2, 5-7, 9-12), but the role of this interaction in IRES-mediated initiation has remained unknown. During canonical initiation, eIF3 binds to the 40S subunit as a component of the 43S pre-initiation complex, and comparison of the ribosomal positions of eIF3 and the HCV IRES revealed that they overlap, so that their rearrangement would be required for formation of ribosomal complexes containing both components. Here we present a cryo-electron microscopy reconstruction of a 40S ribosomal complex containing eIF3 and the CSFV IRES. Remarkably, although the position and interactions of the CSFV IRES with the 40S subunit in this complex are similar to those of the HCV IRES in the 40S-IRES binary complex, eIF3 is completely displaced from its ribosomal position in the 43S complex, and instead interacts through its ribosome-binding surface exclusively with the apical region of domain III of the IRES. Our results suggest a role for the specific interaction of HCV-like IRESs with eIF3 in preventing ribosomal association of eIF3, which could serve two purposes: relieving the competition between the IRES and eIF3 for a common binding site on the 40S subunit, and reducing formation of 43S complexes, thereby favouring translation of viral mRNAs.

Unmasking the information encoded as structural motifs of viral RNA genomes: a potential antiviral target

RNA viruses show enormous capacity to evolve and adapt to new cellular and molecular contexts, a consequence of mutations arising from errors made by viral RNA-dependent RNA polymerase during replication. Sequence variation must occur, however, without compromising functions essential for the completion of the viral cycle. RNA viruses are safeguarded in this respect by their genome carrying conserved information that does not code only for proteins but also for the formation of structurally conserved RNA domains that directly perform these critical functions. Functional RNA domains can interact with other regions of the viral genome and/or proteins to direct viral translation, replication and encapsidation. They are therefore potential targets for novel therapeutic strategies. This review summarises our knowledge of the functional RNA domains of human RNA viruses and examines the achievements made in the design of antiviral compounds that interfere with their folding and therefore their function.

Hepatitis C virus NS3 inhibitors: current and future perspectives

Currently, hepatitis C virus (HCV) infection is considered a serious health-care problem all over the world. A good number of direct-acting antivirals (DAAs) against HCV infection are in clinical progress including NS3-4A protease inhibitors, RNA-dependent RNA polymerase inhibitors, and NS5A inhibitors as well as host targeted inhibitors. Two NS3-4A protease inhibitors (telaprevir and boceprevir) have been recently approved for the treatment of hepatitis C in combination with standard of care (pegylated interferon plus ribavirin). The new therapy has significantly improved sustained virologic response (SVR); however, the adverse effects associated with this therapy are still the main concern. In addition to the emergence of viral resistance, other targets must be continually developed. One such underdeveloped target is the helicase portion of the HCV NS3 protein. This review article summarizes our current understanding of HCV treatment, particularly with those of NS3 inhibitors.

The role of microRNAs in hepatitis C virus RNA replication

Replication of hepatitis C virus (HCV) RNA is influenced by a variety of microRNAs, with the main player being the liver-specific microRNA-122 (miR-122). Binding of miR-122 to two binding sites near the 5' end of the 5' untranslated region (UTR) of the HCV genomic RNA results in at least two different effects. On the one hand, binding of miR-122 and the resulting recruitment of protein complexes containing Argonaute (Ago) proteins appears to mask the viral RNA's 5' end and stabilizes the viral RNA against nucleolytic degradation. On the other hand, this interaction of miR-122 with the 5'-UTR also stimulates HCV RNA translation directed by the internal ribosome entry site (IRES) located downstream of the miR-122 binding sites. However, it is suspected that additional, yet undefined roles of miR-122 in HCV replication may also contribute to HCV propagation. Accordingly, miR-122 is considered to contribute to the liver tropism of the virus. Besides miR-122, let-7b, miR-196, miR-199a* and miR-448 have also been reported to interact directly with the HCV RNA. However, the latter microRNAs inhibit HCV replication, and it has been speculated that miR-199a* contributes indirectly to HCV tissue tropism, since it is mostly expressed in cells other than hepatocytes. Other microRNAs influence HCV replication indirectly. Some of those are advantageous for HCV propagation, while others suppress HCV replication. Consequently, HCV up-regulates or down-regulates, respectively, the expression of most of these miRNAs.

Structure of the full-length HCV IRES in solution

The 5'-untranslated region of the hepatitis C virus genome contains an internal ribosome entry site (IRES) that initiates cap-independent translation of the viral RNA. Until now, the structural characterization of the entire (IRES) remained limited to cryo-electron microscopy reconstructions of the (IRES) bound to different cellular partners. Here we report an atomic model of free full-length hepatitis C virus (IRES) refined by selection against small-angle X-ray scattering data that incorporates the known structures of different fragments. We found that an ensemble of conformers reproduces small-angle X-ray scattering data better than a single structure suggesting in combination with molecular dynamics simulations that the hepatitis C virus (IRES) is an articulated molecule made of rigid parts that move relative to each other. Principal component analysis on an ensemble of physically accessible conformers of hepatitis C virus (IRES) revealed dominant collective motions in the molecule, which may underlie the conformational changes occurring in the (IRES) molecule upon formation of the initiation complex.

A human proteome microarray identifies that the heterogeneous nuclear ribonucleoprotein K (hnRNP K) recognizes the 5' terminal sequence of the hepatitis C virus RNA

Stem-loop I (SL1) located in the 5' untranslated region of the hepatitis C virus (HCV) genome initiates binding to miR-122, a microRNA required for hepatitis HCV replication. However, proteins that bind SL1 remain elusive. In this study, we employed a human proteome microarray, comprised of ∼17,000 individually purified human proteins in full-length, and identified 313 proteins that recognize HCV SL1. Eighty-three of the identified proteins were annotated as liver-expressing proteins, and twelve of which were known to be associated with hepatitis virus. siRNA-induced silencing of eight out of 12 candidate genes led to at least 25% decrease in HCV replication efficiency. In particular, knockdown of heterogeneous nuclear ribonucleoprotein K (hnRNP K) reduced HCV replication in a concentration-dependent manner. Ultra-violet-crosslinking assay also showed that hnRNP K, which functions in pre-mRNA processing and transport, showed the strongest binding to the HCV SL1. We observed that hnRNP K, a nuclear protein, is relocated in the cytoplasm in HCV-expressing cells. Immunoprecipitation of the hnRNP K from Huh7.5 cells stably expressing HCV replicon resulted in the co-immunoprecipitation of SL1. This work identifies a cellular protein that could have an important role in the regulation of HCV RNA gene expression and metabolism.

Emerging treatments for hepatitis C

INTRODUCTION

About 2.35% of the world population can be infected with hepatitis C virus (HCV) responsible for chronic hepatitis, liver cirrhosis and hepatocellular carcinoma. Currently available interferon-based medication is successful in up to 75% of the patients infected with HCV genotypes 1, 2 or 3 and lower efficacy in other genotypes. Unfortunately sustained virologic response (SVR) rate in genotype 1 infected non-responders to previous therapy with advanced hepatic fibrosis even after retreatment with the first generation direct acting antivirals (DAA) is about 40% only.

AREAS COVERED

The second generation DAA, which have recently been submitted for registration (Sofosbuvir and Simeprevir) still need combination with PegIFNα and RBV in patients infected with HCV genotype 1. There is a need for more effective antiviral therapy for difficult to treat patients who are interferon intolerant, developed liver cirrhosis or non-responders to previous therapies. Therefore, IFN-free regimens are step for future therapies consisting of combinations of novel investigational DAA and host targeting agents.

EXPERT OPINION

The introduction of novel DAA with a good safety profile and high barrier to resistance can lead to SVR rates exceeding 90% in treatment naïve patients and non-responders to previous therapy infected with different genotypes.

Structure of the mammalian ribosomal 43S preinitiation complex bound to the scanning factor DHX29

Eukaryotic translation initiation begins with assembly of a 43S preinitiation complex. First, methionylated initiator methionine transfer RNA (Met-tRNAi(Met)), eukaryotic initiation factor (eIF) 2, and guanosine triphosphate form a ternary complex (TC). The TC, eIF3, eIF1, and eIF1A cooperatively bind to the 40S subunit, yielding the 43S preinitiation complex, which is ready to attach to messenger RNA (mRNA) and start scanning to the initiation codon. Scanning on structured mRNAs additionally requires DHX29, a DExH-box protein that also binds directly to the 40S subunit. Here, we present a cryo-electron microscopy structure of the mammalian DHX29-bound 43S complex at 11.6 Å resolution. It reveals that eIF2 interacts with the 40S subunit via its α subunit and supports Met-tRNAi(Met) in an unexpected P/I orientation (eP/I). The structural core of eIF3 resides on the back of the 40S subunit, establishing two principal points of contact, whereas DHX29 binds around helix 16. The structure provides insights into eukaryote-specific aspects of translation, including the mechanism of action of DHX29.

HCV IRES interacts with the 18S rRNA to activate the 40S ribosome for subsequent steps of translation initiation

Previous analyses of complexes of 40S ribosomal subunits with the hepatitis C virus (HCV) internal ribosome entry site (IRES) have revealed contacts made by the IRES with ribosomal proteins. Here, using chemical probing, we show that the HCV IRES also contacts the backbone and bases of the CCC triplet in the 18S ribosomal RNA (rRNA) expansion segment 7. These contacts presumably provide interplay between IRES domain II and the AUG codon close to ribosomal protein S5, which causes a rearrangement of 18S rRNA structure in the vicinity of the universally conserved nucleotide G1639. As a result, G1639 becomes exposed and the corresponding site of the 40S subunit implicated in transfer RNA discrimination can select . These data are the first demonstration at nucleotide resolution of direct IRES–rRNA interactions and how they induce conformational transition in the 40S subunit allowing the HCV IRES to function without AUG recognition initiation factors.

Hepatitis C virus 3'UTR regulates viral translation through direct interactions with the host translation machinery

The 3′ untranslated region (3′UTR) of hepatitis C virus (HCV) messenger RNA stimulates viral translation by an undetermined mechanism. We identified a high affinity interaction, conserved among different HCV genotypes, between the HCV 3′UTR and the host ribosome. The 3′UTR interacts with 40S ribosomal subunit proteins residing primarily in a localized region on the 40S solvent-accessible surface near the messenger RNA entry and exit sites. This region partially overlaps with the site where the HCV internal ribosome entry site was found to bind, with the internal ribosome entry site-40S subunit interaction being dominant. Despite its ability to bind to 40S subunits independently, the HCV 3′UTR only stimulates translation in cis, without affecting the first round translation rate. These observations support a model in which the HCV 3′UTR retains ribosome complexes during translation termination to facilitate efficient initiation of subsequent rounds of translation.

Two RNA-binding motifs in eIF3 direct HCV IRES-dependent translation

The initiation of protein synthesis plays an essential regulatory role in human biology. At the center of the initiation pathway, the 13-subunit eukaryotic translation initiation factor 3 (eIF3) controls access of other initiation factors and mRNA to the ribosome by unknown mechanisms. Using electron microscopy (EM), bioinformatics and biochemical experiments, we identify two highly conserved RNA-binding motifs in eIF3 that direct translation initiation from the hepatitis C virus internal ribosome entry site (HCV IRES) RNA. Mutations in the RNA-binding motif of subunit eIF3a weaken eIF3 binding to the HCV IRES and the 40S ribosomal subunit, thereby suppressing eIF2-dependent recognition of the start codon. Mutations in the eIF3c RNA-binding motif also reduce 40S ribosomal subunit binding to eIF3, and inhibit eIF5B-dependent steps downstream of start codon recognition. These results provide the first connection between the structure of the central translation initiation factor eIF3 and recognition of the HCV genomic RNA start codon, molecular interactions that likely extend to the human transcriptome.

Screening for inhibitors of the hepatitis C virus internal ribosome entry site RNA

The highly conserved internal ribosome entry site (IRES) of hepatitis C virus (HCV) regulates translation of the viral RNA genome and is essential for the expression of HCV proteins in infected host cells. The structured subdomain IIa of the IRES element is the target site of recently discovered benzimidazole inhibitors that selectively block viral translation through capture of an extended conformation of an RNA internal loop. Here, we describe the development of a FRET-based screening assay for similarly acting HCV translation inhibitors. The assay relies on monitoring fluorescence changes that indicate rearrangement of the RNA target conformation upon ligand binding. Screening of a small pilot set of potential RNA binders identified a benzoxazole scaffold as a ligand that bound selectively to IIa IRES target and was confirmed as an inhibitor of in vitro viral translation. The screening approach outlined here provides an efficient method to discover HCV translation inhibitors that may provide leads for the development of novel antiviral therapies directed at the highly conserved IRES RNA.

Discovering new medicines targeting helicases: challenges and recent progress

Helicases are ubiquitous motor proteins that separate and/or rearrange nucleic acid duplexes in reactions fueled by adenosine triphosphate (ATP) hydrolysis. Helicases encoded by bacteria, viruses, and human cells are widely studied targets for new antiviral, antibiotic, and anticancer drugs. This review summarizes the biochemistry of frequently targeted helicases. These proteins include viral enzymes from herpes simplex virus, papillomaviruses, polyomaviruses, coronaviruses, the hepatitis C virus, and various flaviviruses. Bacterial targets examined include DnaB-like and RecBCD-like helicases. The human DEAD-box protein DDX3 is the cellular antiviral target discussed, and cellular anticancer drug targets discussed are the human RecQ-like helicases and eIF4A. We also review assays used for helicase inhibitor discovery and the most promising and common helicase inhibitor chemotypes, such as nucleotide analogues, polyphenyls, metal ion chelators, flavones, polycyclic aromatic polymers, coumarins, and various DNA binding pharmacophores. Also discussed are common complications encountered while searching for potent helicase inhibitors and possible solutions for these problems.

The genetic code as expressed through relationships between mRNA structure and protein function

Structured RNA elements within messenger RNA often direct or modulate the cellular production of active proteins. As reviewed here, RNA structures have been discovered that govern nearly every step in protein production: mRNA production and stability; translation initiation, elongation, and termination; protein folding; and cellular localization. Regulatory RNA elements are common within RNAs from every domain of life. This growing body of RNA-mediated mechanisms continues to reveal new ways in which mRNA structure regulates translation. We integrate examples from several different classes of RNA structure-mediated regulation to present a global perspective that suggests that the secondary and tertiary structure of RNA ultimately constitutes an additional level of the genetic code that both guides and regulates protein biosynthesis.

Modulation of mitogen-activated protein kinase-activated protein kinase 3 by hepatitis C virus core protein

Hepatitis C virus (HCV) is highly dependent on cellular proteins for its own propagation. In order to identify the cellular factors involved in HCV propagation, we performed protein microarray assays using the HCV core protein as a probe. Of ~9,000 host proteins immobilized in a microarray, approximately 100 cellular proteins were identified as HCV core-interacting partners. Of these candidates, mitogen-activated protein kinase-activated protein kinase 3 (MAPKAPK3) was selected for further characterization. MAPKAPK3 is a serine/threonine protein kinase that is activated by stress and growth inducers. Binding of HCV core to MAPKAPK3 was confirmed by in vitro pulldown assay and further verified by coimmunoprecipitation assay. HCV core protein interacted with MAPKAPK3 through amino acid residues 41 to 75 of core and the N-terminal half of kinase domain of MAPKAPK3. In addition, both RNA and protein levels of MAPKAPK3 were elevated in both HCV subgenomic replicon cells and cell culture-derived HCV (HCVcc)-infected cells. Silencing of MAPKAPK3 expression resulted in decreases in both protein and HCV infectivity levels but not in the intracellular HCV RNA level. We showed that MAPKAPK3 increased HCV IRES-mediated translation and MAPKAPK3-dependent HCV IRES activity was further increased by core protein. These data suggest that HCV core may modulate MAPKAPK3 to facilitate its own propagation.

Systematic analysis of enhancer and critical cis-acting RNA elements in the protein-encoding region of the hepatitis C virus genome

Hepatitis C virus (HCV) causes chronic hepatitis, cirrhosis, and liver cancer. cis-acting RNA elements of the HCV genome are critical for translation initiation and replication of the viral genome. We hypothesized that the coding regions of nonstructural proteins harbor enhancer and essential cis-acting replication elements (CRE). In order to experimentally identify new cis RNA elements, we utilized an unbiased approach to introduce synonymous substitutions. The HCV genome coding for nonstructural proteins (nucleotide positions 3872 to 9097) was divided into 17 contiguous segments. The wobble nucleotide positions of each codon were replaced, resulting in 33% to 41% nucleotide changes. The HCV genome containing one of each of 17 mutant segments (S1 to S17) was tested for genome replication and infectivity. We observed that silent mutations in segment 13 (S13) (nucleotides [nt] 7457 to 7786), S14 (nt 7787 to 8113), S15 (nt 8114 to 8440), S16 (nt 8441 to 8767), and S17 (nt 8768 to 9097) resulted in impaired genome replication, suggesting CRE structures are enriched in the NS5B region. Subsequent high-resolution mutational analysis of NS5B (nt 7787 to 9289) using approximately 51-nucleotide contiguous subsegment mutant viruses having synonymous mutations revealed that subsegments SS8195-8245, SS8654-8704, and SS9011-9061 were required for efficient viral growth, suggesting that these regions act as enhancer elements. Covariant nucleotide substitution analysis of a stem-loop, JFH-SL9098, revealed the formation of an extended stem structure, which we designated JFH-SL9074. We have identified new enhancer RNA elements and an extended stem-loop in the NS5B coding region. Genetic modification of enhancer RNA elements can be utilized for designing attenuated HCV vaccine candidates.

Unconventional miR-122 binding stabilizes the HCV genome by forming a trimolecular RNA structure

MicroRNAs (miRNAs) typically downregulate protein expression from target mRNAs through limited base-pairing interactions between the 5′ ‘seed’ region of the miRNA and the mRNA 3′ untranslated region (3′UTR). In contrast to this established mode of action, the liver-specific human miR-122 binds at two sites within the hepatitis C viral (HCV) 5′UTR, leading to increased production of infectious virions. We show here that two copies of miR-122 interact with the HCV 5′UTR at partially overlapping positions near the 5′ end of the viral transcript to form a stable ternary complex. Both miR-122 binding sites involve extensive base pairing outside of the seed sequence; yet, they have substantially different interaction affinities. Structural probing reveals changes in the architecture of the HCV 5′UTR that occur on interaction with miR-122. In contrast to previous reports, however, results using both the recombinant cytoplasmic exonuclease Xrn1 and liver cell extracts show that miR-122-mediated protection of the HCV RNA from degradation does not correlate with stimulation of viral propagation in vivo. Thus, the miR-122:HCV ternary complex likely functions at other steps critical to the viral life cycle.

MicroRNA-122 dependent binding of Ago2 protein to hepatitis C virus RNA is associated with enhanced RNA stability and translation stimulation

Translation of Hepatitis C Virus (HCV) RNA is directed by an internal ribosome entry site (IRES) in the 5′-untranslated region (5′-UTR). HCV translation is stimulated by the liver-specific microRNA-122 (miR-122) that binds to two binding sites between the stem-loops I and II near the 5′-end of the 5′-UTR. Here, we show that Argonaute (Ago) 2 protein binds to the HCV 5′-UTR in a miR-122-dependent manner, whereas the HCV 3′-UTR does not bind Ago2. miR-122 also recruits Ago1 to the HCV 5’-UTR. Only miRNA duplex precursors of the correct length stimulate HCV translation, indicating that the duplex miR-122 precursors are unwound by a complex that measures their length. Insertions in the 5′-UTR between the miR-122 binding sites and the IRES only slightly decrease translation stimulation by miR-122. In contrast, partially masking the miR-122 binding sites in a stem-loop structure impairs Ago2 binding and translation stimulation by miR-122. In an RNA decay assay, also miR-122-mediated RNA stability contributes to HCV translation stimulation. These results suggest that Ago2 protein is directly involved in loading miR-122 to the HCV RNA and mediating RNA stability and translation stimulation.

Ribosomal protein S25 dependency reveals a common mechanism for diverse internal ribosome entry sites and ribosome shunting

During viral infection or cellular stress, cap-dependent translation is shut down. Proteins that are synthesized under these conditions use alternative mechanisms to initiate translation. This study demonstrates that at least two alternative translation initiation routes, internal ribosome entry site (IRES) initiation and ribosome shunting, rely on ribosomal protein S25 (RPS25). This suggests that they share a mechanism for initiation that is not employed by cap-dependent translation, since cap-dependent translation is not affected by the loss of RPS25. Furthermore, we demonstrate that viruses that utilize an IRES or a ribosome shunt, such as hepatitis C virus, poliovirus, or adenovirus, have impaired amplification in cells depleted of RPS25. In contrast, viral amplification of a virus that relies solely on cap-dependent translation, herpes simplex virus, is not hindered. We present a model that explains how RPS25 can be a nexus for multiple alternative translation initiation pathways.

HCV IRES manipulates the ribosome to promote the switch from translation initiation to elongation

The hepatitis C virus (HCV) internal ribosome entry site (IRES) drives non-canonical initiation of protein synthesis necessary for viral replication. HCV IRES functional studies have focused on 80S ribosome formation, but have not explored roles after the 80S ribosome is poised at the start codon. Here, we report that mutations of an IRES domain that docks in the 40S subunit’s decoding groove and cause only a local perturbation in IRES structure result in conformational changes in the IRES-rabbit 40S subunit complex. Functionally, we find the mutation decreases IRES activity by inhibiting the first ribosome translocation event, and modeling suggests that this effect is through an interaction with a single ribosomal protein. The HCV IRES’ ability to manipulate the ribosome provides insight into how the ribosome’s structure and function can be altered by bound RNAs, including those derived from cellular invaders.

Cryo-EM structure of the mammalian eukaryotic release factor eRF1-eRF3-associated termination complex

Eukaryotic translation termination results from the complex functional interplay between two eukaryotic release factors, eRF1 and eRF3, and the ribosome, in which GTP hydrolysis by eRF3 couples codon recognition with peptidyl-tRNA hydrolysis by eRF1. Here, using cryo-electron microscopy (cryo-EM) and flexible fitting, we determined the structure of eRF1-eRF3-guanosine 5'-[β,γ-imido]triphosphate (GMPPNP)-bound ribosomal pretermination complex (pre-TC), which corresponds to the initial, pre-GTP hydrolysis stage of factor attachment. Our results show that eukaryotic translation termination involves a network of interactions between the two release factors and the ribosome. Our structure provides mechanistic insight into the coordination between GTP hydrolysis by eRF3 and subsequent peptide release by eRF1.

RNA structural elements of hepatitis C virus controlling viral RNA translation and the implications for viral pathogenesis

Hepatitis C virus (HCV) genome multiplication requires the concerted action of the viral RNA, host factors and viral proteins. Recent studies have provided information about the requirement of specific viral RNA motifs that play an active role in the viral life cycle. RNA regulatory motifs controlling translation and replication of the viral RNA are mostly found at the 5' and 3' untranslated regions (UTRs). In particular, viral protein synthesis is under the control of the internal ribosome entry site (IRES) element, a complex RNA structure located at the 5'UTR that recruits the ribosomal subunits to the initiator codon. Accordingly, interfering with this RNA structural motif causes the abrogation of the viral cycle. In addition, RNA translation initiation is modulated by cellular factors, including miRNAs and RNA-binding proteins. Interestingly, a RNA structural motif located at the 3'end controls viral replication and establishes long-range RNA-RNA interactions with the 5'UTR, generating functional bridges between both ends on the viral genome. In this article, we review recent advances on virus-host interaction and translation control modulating viral gene expression in infected cells.

Severe acute respiratory syndrome coronavirus protein nsp1 is a novel eukaryotic translation inhibitor that represses multiple steps of translation initiation

Severe acute respiratory syndrome (SARS) coronavirus nonstructural protein 1 (nsp1) binds to the 40S ribosomal subunit and inhibits translation, and it also induces a template-dependent endonucleolytic cleavage of host mRNAs. nsp1 inhibits the translation of cap-dependent and internal ribosome entry site (IRES)-driven mRNAs, including SARS coronavirus mRNAs, hepatitis C virus (HCV), and cricket paralysis virus (CrPV) IRES-driven mRNAs that are resistant to nsp1-induced RNA cleavage. We used an nsp1 mutant, nsp1-CD, lacking the RNA cleavage function, to delineate the mechanism of nsp1-mediated translation inhibition and identify the translation step(s) targeted by nsp1. nsp1 and nsp1-CD had identical inhibitory effects on mRNA templates that are resistant to nsp1-induced RNA cleavage, implying the validity of using nsp1-CD to dissect the translation inhibition function of nsp1. We provide evidence for a novel mode of action of nsp1. nsp1 inhibited the translation initiation step by targeting at least two separate stages: 48S initiation complex formation and the steps involved in the formation of the 80S initiation complex from the 48S complex. nsp1 had a differential, mRNA template-dependent, inhibitory effect on 48S and 80S initiation complex formation. nsp1 inhibited different steps of translation initiation on CrPV and HCV IRES, both of which initiate translation via an IRES-40S binary complex intermediate; nsp1 inhibited binary complex formation on CrPV IRES and 48S complex formation on HCV IRES. Collectively, the data revealed that nsp1 inhibited translation by exerting its effect on multiple stages of translation initiation, depending on the mechanism of initiation operating on the mRNA template.

Aberrant expression of c-Jun in glioblastoma by internal ribosome entry site (IRES)-mediated translational activation

Although the protooncogene c-Jun plays a critical role in cell proliferation, cell death, and malignant transformation, DNA microarray screens have identified only a few human cancer types with aberrant expression of c-Jun. Here, we show that c-Jun accumulation is robustly elevated in human glioblastoma and that this increase contributes to the malignant properties of the cells. Most importantly, the increase in c-Jun protein accumulation occurs with no corresponding increase in c-Jun mRNA or the half-life of the c-Jun protein but, rather, in the translatability of the transcript. The c-Jun 5'UTR harbors a potent internal ribosomal entry site (IRES) with a virus-like IRES domain that directs cap-independent translation in glioblastoma cells. Accumulation of c-Jun is not dependent on MAPK activity but can be stimulated by a cytoskeleton-dependent pathway. Our findings provide evidence that human c-Jun is an IRES-containing cellular transcript that contributes to cancer development through translational activation. This previously undescribed mechanism of c-Jun regulation might also be relevant to other types of human cancer and offers unique potential targets for therapy.

An RNA Mapping DataBase for curating RNA structure mapping experiments

SUMMARY

We have established an RNA mapping database (RMDB) to enable structural, thermodynamic and kinetic comparisons across single-nucleotide-resolution RNA structure mapping experiments. The volume of structure mapping data has greatly increased since the development of high-throughput sequencing techniques, accelerated software pipelines and large-scale mutagenesis. For scientists wishing to infer relationships between RNA sequence/structure and these mapping data, there is a need for a database that is curated, tagged with error estimates and interfaced with tools for sharing, visualization, search and meta-analysis. Through its on-line front-end, the RMDB allows users to explore single-nucleotide-resolution mapping data in heat-map, bar-graph and colored secondary structure graphics; to leverage these data to generate secondary structure hypotheses; and to download the data in standardized and computer-friendly files, including the RDAT and community-consensus SNRNASM formats. At the time of writing, the database houses 53 entries, describing more than 2848 experiments of 1098 RNA constructs in several solution conditions and is growing rapidly.

AVAILABILITY

Freely available on the web at http://rmdb.stanford.edu.

CONTACT

rhiju@stanford.edu.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics Online.

Tricks an IRES uses to enslave ribosomes

In eukaryotes, mRNAs are primarily translated through a cap-dependent mechanism whereby initiation factors recruit the 40S ribosomal subunit to a cap structure at the 5' end of the mRNA. However, some viral and cellular messages initiate protein synthesis without a cap. They use a structured RNA element termed an internal ribosome entry site (IRES) to recruit the 40S ribosomal subunit. IRESs were discovered over 20 years ago, but only recently have studies using a model IRES from dicistroviruses expanded our understanding of how a 3D RNA structure can capture and manipulate the ribosome to initiate translation.

Targeting ribosome assembly on the HCV RNA using a small RNA molecule

Translation initiation of hepatitis C Virus (HCV) RNA is the initial obligatory step of the viral life cycle, mediated through the Internal Ribosome Entry Site (IRES) present in the 5'-untranslated region (UTR). Initiation on the HCV IRES is mediated by multiple structure-specific interactions between IRES RNA and host 40S ribosomal subunit. In the present study we demonstrate that the SLIIIef domain, in isolation from other structural elements of HCV IRES, retain the ability to interact with 40S ribosome subunit. A small RNA SLRef, mimicking the SLIIIef domain was found to interact specifically with human La protein and the ribosomal protein S5 and selectively inhibit HCV RNA translation. More importantly, SLRef RNA showed significant suppression of replication in HCV monocistronic replicon and decrease of negative strand synthesis in HCV cell culture system. Finally, using Sendai virus based virosome, the targeted delivery of SLRef RNA into mice liver succeeded in selectively inhibiting HCV IRES mediated translation in vivo.

Modulation of translation initiation efficiency in classical swine fever virus

Modulation of translation initiation efficiency on classical swine fever virus (CSFV) RNA can be achieved by targeted mutations within the internal ribosome entry site (IRES). In this study, cDNAs corresponding to the wild-type (wt) or mutant forms of the IRES of CSFV strain Paderborn were amplified and inserted into dicistronic reporter plasmids encoding Fluc and Rluc under the control of a T7 promoter. The mutations were within domains II, IIId(1), and IIIf of the IRES. The plasmids were transfected into baby hamster kidney (BHK) cells infected with recombinant vaccinia virus vTF7-3, which expresses the T7 RNA polymerase. IRES mutants with different levels of IRES activity were identified and then introduced by homologous recombination into bacterial artificial chromosomes (BACs) containing CSFV Paderborn cDNA downstream of a T7 promoter. From the wt and mutant BACs, full-length CSFV RNA transcripts were produced in vitro and electroporated into porcine PK15 cells. Rescued mutant viruses were obtained from RNAs that contained mutations within domain IIIf which retained more than 75% of the wt translation efficiency. Sequencing of cDNA generated from these rescued viruses verified the maintenance of the introduced changes within the IRES. The growth characteristics of each rescued mutant virus were compared to those of the wt virus. It was shown that viable mutant viruses with reduced translation initiation efficiency can be designed and generated and that viruses containing mutations within domain IIIf of the IRES have reduced growth in cell culture compared to the wt virus.

The complex of tmRNA-SmpB and EF-G on translocating ribosomes

Bacterial ribosomes stalled at the 3' end of malfunctioning messenger RNAs can be rescued by transfer-messenger RNA (tmRNA)-mediated trans-translation. The SmpB protein forms a complex with the tmRNA, and the transfer-RNA-like domain (TLD) of the tmRNA then enters the A site of the ribosome. Subsequently, the TLD-SmpB module is translocated to the P site, a process that is facilitated by the elongation factor EF-G, and translation is switched to the mRNA-like domain (MLD) of the tmRNA. Accurate loading of the MLD into the mRNA path is an unusual initiation mechanism. Despite various snapshots of different ribosome-tmRNA complexes at low to intermediate resolution, it is unclear how the large, highly structured tmRNA is translocated and how the MLD is loaded. Here we present a cryo-electron microscopy reconstruction of a fusidic-acid-stalled ribosomal 70S-tmRNA-SmpB-EF-G complex (carrying both of the large ligands, that is, EF-G and tmRNA) at 8.3 Å resolution. This post-translocational intermediate (TI(POST)) presents the TLD-SmpB module in an intrasubunit ap/P hybrid site and a tRNA(fMet) in an intrasubunit pe/E hybrid site. Conformational changes in the ribosome and tmRNA occur in the intersubunit space and on the solvent side. The key underlying event is a unique extra-large swivel movement of the 30S head, which is crucial for both tmRNA-SmpB translocation and MLD loading, thereby coupling translocation to MLD loading. This mechanism exemplifies the versatile, dynamic nature of the ribosome, and it shows that the conformational modes of the ribosome that normally drive canonical translation can also be used in a modified form to facilitate more complex tasks in specialized non-canonical pathways.

The structure and function of the eukaryotic ribosome

Structures of the bacterial ribosome have provided a framework for understanding universal mechanisms of protein synthesis. However, the eukaryotic ribosome is much larger than it is in bacteria, and its activity is fundamentally different in many key ways. Recent cryo-electron microscopy reconstructions and X-ray crystal structures of eukaryotic ribosomes and ribosomal subunits now provide an unprecedented opportunity to explore mechanisms of eukaryotic translation and its regulation in atomic detail. This review describes the X-ray crystal structures of the Tetrahymena thermophila 40S and 60S subunits and the Saccharomyces cerevisiae 80S ribosome, as well as cryo-electron microscopy reconstructions of translating yeast and plant 80S ribosomes. Mechanistic questions about translation in eukaryotes that will require additional structural insights to be resolved are also presented.

A new framework for understanding IRES-mediated translation

Studies over the past 5 or so years have indicated that the traditional clustering of mechanisms for translation initiation in eukaryotes into cap-dependent and cap-independent (or IRES-mediated) is far too narrow. From individual studies of a number of mRNAs encoding proteins that are regulatory in nature (i.e. likely to be needed in small amounts such as transcription factors, protein kinases, etc.), it is now evident that mRNAs exist that blur these boundaries. This review seeks to set the basic ground rules for the analysis of different initiation pathways that are associated with these new mRNAs as well as related to the more traditional mechanisms, especially the cap-dependent translational process that is the major route of initiation of mRNAs for housekeeping proteins and thus, the bulk of protein synthesis in most cells. It will become apparent that a mixture of descriptions is likely to become the norm in the near future (i.e. m(7)G-assisted internal initiation).

Structural and functional topography of the human ribosome

This review covers data on the structural organization of functional sites in the human ribosome, namely, the messenger RNA binding center, the binding site of the hepatitis C virus RNA internal ribosome entry site, and the peptidyl transferase center. The data summarized here have been obtained primarily by means of a site-directed cross-linking approach with application of the analogs of the respective ribosomal ligands bearing cross-linkers at the designed positions. These data are discussed taking into consideration available structural data on ribosomes from various kingdoms obtained with the use of cryo-electron microscopy, X-ray crystallography, and other approaches.

Structure of a hepatitis C virus RNA domain in complex with a translation inhibitor reveals a binding mode reminiscent of riboswitches

The internal ribosome entry site (IRES) in the hepatitis C virus (HCV) RNA genome is essential for the initiation of viral protein synthesis. IRES domains adopt well-defined folds that are potential targets for antiviral translation inhibitors. We have determined the three-dimensional structure of the IRES subdomain IIa in complex with a benzimidazole translation inhibitor at 2.2 Å resolution. Comparison to the structure of the unbound RNA in conjunction with studies of inhibitor binding to the target in solution demonstrate that the RNA undergoes a dramatic ligand-induced conformational adaptation to form a deep pocket that resembles the substrate binding sites in riboswitches. The presence of a well-defined ligand-binding pocket within the highly conserved IRES subdomain IIa holds promise for the development of unique anti-HCV drugs with a high barrier to resistance.

The structures of nonprotein-coding RNAs that drive internal ribosome entry site function

Internal ribosome entry sites (IRESs) are RNA sequences that can recruit the translation machinery independent of the 5' end of the messenger RNA. IRESs are found in both viral and cellular RNAs and are important for regulating gene expression. There is great diversity in the mechanisms used by IRESs to recruit the ribosome and this is reflected in a variety of RNA sequences that function as IRESs. The ability of an RNA sequence to function as an IRES is conferred by structures operating at multiple levels from primary sequence through higher-order three-dimensional structures within dynamic ribonucleoproteins (RNPs). When these diverse structures are compared, some trends are apparent, but overall it is not possible to find universal rules to describe IRES structure and mechanism. Clearly, many different sequences and structures have evolved to perform the function of recruiting, positioning, and activating a ribosome without using the canonical cap-dependent mechanism. However, as our understanding of the specific sequences, structures, and mechanisms behind IRES function improves, more common features may emerge to link these diverse RNAs.

Alternative Mechanisms to Initiate Translation in Eukaryotic mRNAs

The composition of the cellular proteome is under the control of multiple processes, one of the most important being translation initiation. The majority of eukaryotic cellular mRNAs initiates translation by the cap-dependent or scanning mode of translation initiation, a mechanism that depends on the recognition of the m(7)G(5')ppp(5')N, known as the cap. However, mRNAs encoding proteins required for cell survival under stress bypass conditions inhibitory to cap-dependent translation; these mRNAs often harbor internal ribosome entry site (IRES) elements in their 5'UTRs that mediate internal initiation of translation. This mechanism is also exploited by mRNAs expressed from the genome of viruses infecting eukaryotic cells. In this paper we discuss recent advances in understanding alternative ways to initiate translation across eukaryotic organisms.

Crystal structure of the HCV IRES central domain reveals strategy for start-codon positioning

Translation of hepatitis C viral proteins requires an internal ribosome entry site (IRES) located in the 5' untranslated region of the viral mRNA. The core domain of the hepatitis C virus (HCV) IRES contains a four-way helical junction that is integrated within a predicted pseudoknot. This domain is required for positioning the mRNA start codon correctly on the 40S ribosomal subunit during translation initiation. Here, we present the crystal structure of this RNA, revealing a complex double-pseudoknot fold that establishes the alignment of two helical elements on either side of the four-helix junction. The conformation of this core domain constrains the open reading frame's orientation for positioning on the 40S ribosomal subunit. This structure, representing the last major domain of HCV-like IRESs to be determined at near-atomic resolution, provides the basis for a comprehensive cryoelectron microscopy-guided model of the intact HCV IRES and its interaction with 40S ribosomal subunits.