Internal Ribosome Entry Site Domain Research Articles

Inhibitor-induced structural change in the HCV IRES domain IIa RNA

Translation of the hepatitis C virus (HCV) RNA is initiated from a highly structured internal ribosomal entry site (IRES) in the 5' untranslated region (5' UTR) of the RNA genome. An important structural feature of the native RNA is an approximately 90 degrees helical bend localized to domain IIa that positions the apical loop of domain IIb of the IRES near the 40S ribosomal E-site to promote eIF2-GDP release, facilitating 80S ribosome assembly. We report here the NMR structure of a domain IIa construct in complex with a potent small-molecule inhibitor of HCV replication. Molecular dynamics refinement in explicit solvent and subsequent energetic analysis indicated that each inhibitor stereoisomer bound with comparable affinity and in an equivalent binding mode. The in silico analysis was substantiated by fluorescence-based assays showing that the relative binding free energies differed by only 0.7 kcal/mol. Binding of the inhibitor displaces key nucleotide residues within the bulge region, effecting a major conformational change that eliminates the bent RNA helical trajectory, providing a mechanism for the antiviral activity of this inhibitor class.

Rescue of internal initiation of translation by RNA complementation provides evidence for a distribution of functions between individual IRES domains

Picornavirus RNAs initiate translation using an internal ribosome entry site (IRES)-dependent mechanism. The IRES element of foot-and-mouth disease virus (FMDV) is organized in domains, being different from each other in RNA structure and RNA–protein interaction. Wild-type transcripts provided in trans rescue defective FMDV IRES mutants. Complementation, however, was partial since translation efficiency of the mutant RNAs was up to 10% of the wild type IRES. We report here that mutations diminishing the RNA–RNA interaction capacity induced a decrease in IRES rescue. On the other hand, IRES transcripts bearing mutations that reorganize the RNA structure of the apical region of central domain, although weakly, complement defective IRES that are unable to interact with the initiation factor eIF4G in a separate domain. Together, these results suggest that IRES rescue may involve RNA-mediated contacts between defective elements, each carrying a defect in a separate domain but having the complementing one with the appropriate structural orientation and/or ribonucleoprotein composition. Our observations further support the essential role of the central domain of the FMDV IRES during protein synthesis and demonstrate that there is a division of functions between the IRES domains.

Human initiation factor eIF3 subunit b interacts with HCV IRES RNA through its N-terminal RNA recognition motif

Many viral mRNAs contain a 5′-UTR RNA element called internal ribosome-entry site (IRES), which bypasses the requirement of some canonical initiation factors allowing cap-independent translation. The IRES of hepatitis-C virus drives translation by directly recruiting 40S ribosomal subunits and binds to eIF3 which plays a critical role in both cap-dependent and cap-independent translation. However, the molecular basis for eIF3 activity in either case remains enigmatic. Here we report that subunit b of the eIF3 complex directly binds to HCV IRES domain III via its N-terminal-RRM. Because eIF3b was previously shown to be involved in eIF3j binding, biological implications are discussed.

Structure of hepatitis C virus IRES subdomain IIa

The hepatitis C (HCV) internal ribosome entry site (IRES) element plays a central role in cap-independent translation of the viral genomic RNA. The unique conformation of IRES domain II is critical for 80S ribosomal assembly and initiation of viral translation. Here, the crystal structure of subdomain IIa of the HCV IRES has been determined at 2.3 A resolution, revealing the positions of divalent metal ions and complex inter-strand interactions that stabilize the L-shaped conformation of the RNA. The presence of divalent metal ions was necessary for crystal formation. Magnesium ions occupy specific sites that appear to be critical for the formation of the folded conformation. Subdomain IIa also was crystallized in the presence of strontium, which improved the diffraction quality of the crystals and the ability to identify interactions of the RNA with metal ions and tightly bound water molecules. The hinge region and noncanonical G-U base-pair motifs are stabilized by divalent metal ions and provide unique structural features that are potential interaction sites for small-molecule ligands. The information obtained from the crystal structure provides a basis for structure-guided design of HCV translation inhibitors targeting disruption of ribosomal assembly.

EIF2-dependent and eIF2-independent modes of initiation on the CSFV IRES: a common role of domain II

Specific interactions of the classical swine fever virus internal ribosomal entry site (IRES) with 40S ribosomal subunits and eukaryotic translation initiation factor (eIF)3 enable 43S preinitiation complexes containing eIF3 and eIF2-GTP-Met-tRNA(iMet) to bind directly to the initiation codon, yielding 48S initiation complexes. We report that eIF5B or eIF5B/eIF3 also promote Met-tRNA(iMet) binding to IRES-40S complexes, forming 48S complexes that can assemble elongation-competent ribosomes. Although 48S complexes assembled both by eIF2/eIF3- and eIF5B/eIF3-mediated Met-tRNA(iMet) recruitment were destabilized by eIF1, dissociation of 48S complexes formed with eIF2 could be out-competed by efficient subunit joining. Deletion of IRES domain II, which is responsible for conformational changes induced in 40S subunits by IRES binding, eliminated the sensitivity of 48S complexes assembled by eIF2/eIF3- and eIF5B/eIF3-mediated mechanisms to eIF1-induced destabilization. However, 48S complexes formed by the eIF5B/eIF3-mediated mechanism on the truncated IRES could not undergo efficient subunit joining, as reported previously for analogous complexes assembled with eIF2, indicating that domain II is essential for general conformational changes in 48S complexes, irrespective of how they were assembled, that are required for eIF5-induced hydrolysis of eIF2-bound GTP and/or subunit joining.

TRNA–mRNA mimicry drives translation initiation from a viral IRES

Internal ribosome entry site (IRES) RNAs initiate protein synthesis in eukaryotic cells by a noncanonical cap-independent mechanism. IRESes are critical for many pathogenic viruses, but efforts to understand their function are complicated by the diversity of IRES sequences as well as by limited high-resolution structural information. The intergenic region (IGR) IRESes of the Dicistroviridae viruses are powerful model systems to begin to understand IRES function. Here we present the crystal structure of a Dicistroviridae IGR IRES domain that interacts with the ribosome's decoding groove. We find that this RNA domain precisely mimics the transfer RNA anticodon-messenger RNA codon interaction, and its modeled orientation on the ribosome helps explain translocation without peptide bond formation. When combined with a previous structure, this work completes the first high-resolution description of an IRES RNA and provides insight into how RNAs can manipulate complex biological machines.

HCV and CSFV IRES domain II mediate eIF2 release during 80S ribosome assembly

Internal ribosome entry site (IRES) RNAs from the hepatitis C virus (HCV) and classical swine fever virus (CSFV) coordinate cap-independent assembly of eukaryotic 48S initiation complexes, consisting of the 40S ribosomal subunit, eukaryotic initiation factor (eIF) 3 and the eIF2/GTP/Met-tRNA(i)(Met) ternary complex. Here, we report that these IRESes also play a functional role during 80S ribosome assembly downstream of 48S complex formation, in promoting eIF5-induced GTP hydrolysis and eIF2/GDP release from the initiation complex. We show that this function is encoded in their independently folded IRES domain II and that it depends both on its characteristic bent conformation and two conserved RNA motifs, an apical hairpin loop and a loop E. Our data suggest a general mode of subunit joining in HCV and HCV-like IRESes.

Functional Architecture of HCV IRES Domain II Stabilized by Divalent Metal Ions in the Crystal and in Solution

The RNA genome of the hepatitis C virus (HCV) contains an internal ribosome entry site (IRES), which binds to the hostcell 40S ribosomal subunit and initiates protein translation in the absence of most initiation factors. Recruitment of the ribosomal subunit to the HCV RNA is driven by the high affinity of the IRES–40S interaction. The IRES sequence adopts a highly ordered secondary structure (Figure 1a). The three-dimensional architecture of the IRES is dominated by independently folding RNA domains. Structures of IRES–40S complexes have been studied by cryo-electron microscopy (cryo-EM) revealing the overall shape of the RNA. Higher-resolution structures of individual subdomains, including II and IIIa–e, have been determined by crystallography and NMR spectroscopy. Cryo-EM studies revealed that the domain II, which plays an important structural role in HCV translation, adopts an L-shaped conformation that directs the apical hairpin loop IIb to overlap with the ribosomal E site in the proximity of the P site. Binding of domain II induces a conformational change in the 40S head and closes the messenger RNA (mRNA) binding cleft. NMR studies suggested that subdomain IIa might be a flexible hinge whose bent state is stabilized by binding of divalent metal ions. To investigate the molecular architecture of the IRES domain II kink and its metal-ion-dependent stabilization, we have used X-ray crystallography and structure-guided incorporation of fluorescent labels. For X-ray crystal structure determination (see Figure S4 and Table S1 in the Supporting Information), an oligonucleotide (IIa-1) was used that contained residues 49–69 and 100–115 of the HCV IRES (Figure 1). The overall structure of the RNA revealed a bent architecture for the IIa subdomain. The two stems that are flanking the internal bulge are arranged at a right angle (Figure 1b). The upper stem (residues 58–69/100–110) forms a continuous helix that contains both standard and noncanonical base pairs that adopt cis-Watson–Crick geometries. U106 is looped out from the stem to allow continuous stacking of the flanking base pairs. The arrangement of residues of the internal bulge introduces a right-angled bend in the RNA

Molecular environment of the subdomain IIIe loop of the RNA IRES element of hepatitis C virus on the human 40S ribosomal subunit

The molecular environment of the internal ribosome entry site (IRES) element of hepatitis C viral (HCV) RNA in the binary complex with the human 40S ribosomal subunit was studied. To this end, RNA derivatives bearing mild UV-reactive perfluorophenylazide groups at nucleotide G87 in IRES domain II and at nucleotide A296 in the subdomain IIIe loop were used, which were prepared by the RNA complementarily-addressed modification with alkylating oligonucleotide derivatives. None of the RNA derivatives were shown to be crosslinked to the 18S rRNA of the 40S subunit. It was found that the photoreactive group of IRES nucleotide A296 was crosslinked to the 40S subunit S2/S3a, S5, and p40 (SOA) proteins. No protein crosslinking was observed for the RNA derivative containing the same photoreactive group in nucleotide G87. It was concluded that the subdomain IIIe loop of the HCV RNA IRES element in the complex with the 40S subunit is located on the outer subunit surface between the head and the body next to the "beak" near the entrance into the mRNA-binding channel. The English version of the paper: Russian Journal of Bioorganic Chemistry, 2006, vol. 32, no. 3; see also http://www.maik.ru.

Genetic Determinants of Cell Type-Specific Poliovirus Propagation in HEK 293 Cells

The ability of poliovirus to propagate in neuronal cells can be reduced by introducing appropriate nucleotide substitutions into the viral genome. Specific mutations scattered throughout the poliovirus genome yielded the live attenuated vaccine strains of poliovirus. Neuron-specific propagation deficits of the Sabin strains are partially encrypted within a confined region of the internal ribosomal entry site (IRES), which carries attenuating point mutations in all three serotypes. Recently, high levels of neurovirulence attenuation were achieved with genetically engineered polioviruses containing heterologous IRES elements. This is exemplified with poliovirus recombinants replicating under control of a human rhinovirus type 2 (HRV2) IRES element. We have carried out experiments delineating the genetic basis for neuronal IRES function. Neuronal dysfunction of the HRV2 IRES is determined mainly by IRES stem-loop domain V, the locus for attenuating point mutations within the Sabin strains. Neuronal incompetence associated with HRV2 IRES domain V is substantially more pronounced than that observed with the attenuating IRES point mutation of the Sabin serotype 1 vaccine strain. Mix-and-match recombination of polio and HRV2 IRES domain V suggests that the attenuation phenotype correlates with overall structural features rather than primary sequence. Our experiments have identified HEK 293 cells as a novel system for the study of neuron-specific replication phenotypes of poliovirus. This cell line, originally derived from embryonic human kidney, has recently been described to display neuronal characteristics. We report propagation properties in HEK 293 cells for poliovirus recombinants with attenuated neurovirulence in experimental animals that corroborate this observation.

A hepatitis C virus (HCV) internal ribosome entry site (IRES) domain III-IV-targeted aptamer inhibits translation by binding to an apical loop of domain IIId

The hepatitis C virus (HCV) has a positive single-stranded RNA genome, and translation starts within the internal ribosome entry site (IRES) in a cap-independent manner. The IRES is well conserved among HCV subtypes and has a unique structure consisting of four domains. We used an in vitro selection procedure to isolate RNA aptamers capable of binding to the IRES domains III–IV. The aptamers that were obtained shared the consensus sequence ACCCA, which is complementary to the apical loop of domain IIId that is known to be a critical region of IRES-dependent translation. This convergence suggests that domain IIId is preferentially selected in an RNA–RNA interaction. Mutation analysis showed that the aptamer binding was sequence and structure dependent. One of the aptamers inhibited translation both in vitro and in vivo. Our results indicate that domain IIId is a suitable target site for HCV blockage and that rationally designed RNA aptamers have great potential as anti-HCV drugs.

Interfering with hepatitis C virus IRES activity using RNA molecules identified by a novel in vitro selection method

Hepatitis C virus (HCV) infection is one of the world's major health problems, and the identification of efficient HCV inhibitors is a major goal. Here we report the isolation of efficient anti-HCV internal ribosome entry site (IRES) RNA molecules identified by a new in vitro selection method. The newly developed procedure consists of two sequential steps that use distinct criteria for selection: selection for binding and selection for cleaving. The selection protocol was applied to a population of more than 10(15) variants of an anti-hepatitis C virus ribozyme covalently linked to an aptamer motif. The ribozyme was directed against positions 357 to 369 of the HCV IRES, and the cleavage substrate was a 691-nucleotide-long RNA fragment that comprises the entire HCV IRES domain. After six selection cycles, seven groups of RNA variants were identified. A representative of each group was tested for its capacity to inhibit IRES activity using in vitro translation assays. All selected RNAs promoted significant inhibition, some by as much as 95%.

The Pathway of HCV IRES-Mediated Translation Initiation

The HCV internal ribosome entry site (IRES) directly regulates the assembly of translation initiation complexes on viral mRNA by a sequential pathway that is distinct from canonical eukaryotic initiation. The HCV IRES can form a binary complex with an eIF-free 40S ribosomal subunit. Next, a 48S-like complex assembles at the AUG initiation codon upon association of eIF3 and ternary complex. 80S complex formation is rate limiting and follows the GTP-dependent association of the 60S subunit. Efficient assembly of the 48S-like and 80S complexes on the IRES mRNA is dependent upon maintenance of the highly conserved HCV IRES structure. This revised model of HCV IRES translation initiation provides a context to understand the function of different HCV IRES domains during translation initiation.

Structure of HCV IRES domain II determined by NMR.

Complex RNA structures regulate many biological processes, but are often too large for structure determination by NMR methods. The 5' untranslated region (5' UTR) of the hepatitis C viral (HCV) RNA genome contains an internal ribosome entry site (IRES) that binds to 40S ribosomal subunits with high affinity and specificity to control translation. Domain II of the HCV IRES forms a 25-kDa folded subdomain that may alter ribosome conformation. We report here the structure of domain II as determined using an NMR approach that combines short- and long-range structural data. Domain II adopts a distorted L-shape structure, and its overall shape in the free form is markedly similar to its 40S subunit-bound form; this suggests how domain II may modulate 40S subunit conformation. The results show how NMR can be used for structural analysis of large biological RNAs.

Structure-inhibition analysis of RNA aptamers that bind to HCV IRES.

The translation of HCV starts at the internal ribosomal entry site (IRES) within the 5' untranslated region and IRES is well-conserved in HCV strains. We developed a novel selection strategy using biotinylated oligonucleotide probe and obtained RNA aptamers that bind HCV IRES domain II and domain III-IV, respectively. Selected aptamers specifically bound to target sequence via RNA-RNA interactions. These aptamers inhibited IRES-depend translation in vitro. Especially, 3-07 aptamer, which bound domain IIId, showed strong inhibition. Structure/function relationship of these aptamers was analyzed by mutagenesis, RNase mapping and binding kinetics.

Detection of tRNA-like Structure through RNase P Cleavage of Viral Internal Ribosome Entry Site RNAs Near the AUG Start Triplet

The 9600-base RNA genome of hepatitis C virus (HCV) has an internal ribosome entry site (IRES) in its first 370 bases, including the AUG start triplet at bases 342-344. Structural elements of this and other IRES domains substitute for a 5' terminal cap structure in protein synthesis. Recent work (Nadal, A., Martell, M., Lytle, J. R., Lyons, A. J., Robertson, H. D., Cabot, B., Esteban, J. I., Esteban, R., Guardia, J., and Gomez, J. (2002) J. Biol. Chem. 277, 30606-30613) has demonstrated that the host pre-tRNA processing enzyme, RNase P, can cleave the HCV RNA genome at a site in the IRES near the AUG initiator triplet. Although this step is unlikely to be part of the HCV life cycle, such a reaction could indicate the presence of a tRNA-like structure in this IRES. Because susceptibility to cleavage by mammalian RNase P is a strong indicator of tRNA-like structure, we have conducted the studies reported here to test whether such tRNA mimicry is unique to HCV or is a general property of IRES structure. We have assayed IRES domains of several viral RNA genomes: two pestiviruses related to HCV, classical swine fever virus and bovine viral diarrhea virus; and two unrelated viruses, encephalomyocarditis virus and cricket paralysis virus. We have found similarly placed RNase P cleavage sites in these IRESs. Thus a tRNA-like domain could be a general structural feature of IRESs, the first IRES structure to be identified with a functional correlate. Such tRNA-like features could be recognized by pre-existing ribosomal tRNA-binding sites as part of the IRES initiation cycle.

Structural determinants of insert retention of poliovirus expression vectors with recombinant IRES elements

Although picornaviruses provide attractive vectors for expression of foreign genes, poor genetic stability restricts their use for immunization purposes. A new prototype vector was generated to increase foreign insert retention, by shifting of the initiation codon to a cryptic AUG within the internal ribosomal entry site (IRES) and replacement of IRES domain VI with foreign ORFs. Using our strategy to replace regulatory noncoding sequences with unrelated foreign genetic material, we generated stable poliovirus-based expression vectors with robust long-term expression of foreign ORFs. Our studies revealed that size and predicted secondary structure formed by the heterologous sequences govern long-term retention and efficiency of expression of foreign inserts replacing IRES structures. These observations indicate that, with certain limitations imposed by structural preferences, foreign sequences can functionally replace IRES substructures in stable picornavirus immunization vectors.

Impaired binding of standard initiation factors mediates poliovirus translation attenuation.

In the oral poliovirus vaccine, three attenuated virus strains generated by Albert Sabin are used. However, insufficient genetic stability of these strains causes major problems in poliovirus eradication. In infected cells, translation of the plus-strand poliovirus RNA genome is directed by the internal ribosome entry site (IRES), a cis-acting RNA element that facilitates the cap-independent binding of ribosomes to an internal site of the viral RNA. In each Sabin vaccine strain, a single point mutation in the IRES secondary-structure domain V is a major determinant of neurovirulence attenuation. Here we report how these decisive mutations in the IRES confer a reduction in poliovirus translation efficiency. These single-nucleotide exchanges impair the interaction of the standard translation initiation factor eIF4G with the IRES domain V. Moreover, binding of eIF4B and the polypyrimidine tract-binding protein and the association of ribosomes with the viral RNA are affected by these mutations. However, the negative effects of the IRES mutations are completely relieved by addition of purified eIF4F. This indicates that eIF4G is the crucial factor that initially binds to the poliovirus IRES and recruits the IRES to the other components of the translational apparatus, while impaired binding of eIF4G plays a key role in attenuation of poliovirus neurovirulence.

In vitro selection of RNA aptamers that bind to domain II of HCV IRES.

The translation of HCV starts at the internal ribosomal entry site (IRES) within the 5' untranslated region and domain II of IRES is essential for translation activity. However, the information of function is limited. We attempted to obtain RNA that bind HCV IRES domain II. Selected aptamers will give us much information, such as RNA-RNA interaction between IRES and other cellular RNAs as well as target sites for anti-HCV drugs. We developed a novel selection method using biotinylated DNA probe. This method can be applied to many RNAs and would identify the most suitable target position experimentally and simply. The binding kinetics of aptamers were analyzed by using Biacore. Furthermore, inhibition of translation by aptamers was analyzed.

Domains on the hepatitis C virus internal ribosome entry site for 40s subunit binding.

The internal ribosome entry site (IRES) of the hepatitis C virus (HCV) RNA is known to interact with the 40S ribosomal subunit alone, in the absence of any additional initiation factors or Met-tRNAi. Previous work from this laboratory on the 80S and 48S ribosomal initiation complexes involving the HCV IRES showed that stem-loop III, the pseudoknot domain, and some coding sequence were protected from pancreatic RNase digestion. Stem-loop II is never protected by these complexes. Furthermore, there is no prior evidence reported showing extensive direct binding of stem-loop II to ribosomes or subunits. Using direct analysis of RNase-protected HCV IRES domains bound to 40S ribosomal subunits, we have determined that stem-loops II and III and the pseudoknot of the HCV IRES are involved in this initial binding step. The start AUG codon is only minimally protected. The HCV-40S subunit binary complex thus involves recognition and binding of stem-loop II, revealing its role in the first step of a multistep initiation process that may also involve rearrangement of the bound IRES RNA as it progresses.