Functional Characterization Of The Internal Ribosome Entry Site Of Coxsackievirus B3 RNA

Abstract

CoxsackievirusB3 (CVB3), a member of the Picornaviridae family is the causative agent of Virus-induced Myocarditis and Dilated Cardiomyopathy. The 5’UTR contains an Internal Ribosome Entry Site or IRES element that recruits ribosomes in a cap-independent manner. The ribosomes are recruited upstream of the AUG triplet at 591 (AUG591), also called as the cryptic AUG, after which they scan downstream for about 150 nucleotide, before initiating at the initiator AUG or AUG741. The 3’UTR of CVB3 is 99 nts long, highly structured RNA containing conserved domains, and is followed by a poly (A) tail of variable lengths. We have investigated possible involvement of host proteins which may interact with CVB3 IRES and influence its activity. We have demonstrated the role of Poly-pyrimidine tract binding protein (PTB) and established PTB as a bona-fide ITAF for CVB3, by characterizing the effect of partial silencing of PTB ex-vivo in HeLa cells. The IRES activity in BSC-1 cells, reported to have very low level of endogenous PTB, is found to be significantly low compared to that in HeLa cells. PTB is observed to interact with both the 5’ and 3’ UTR of CVB3, although with different affinities. Finer mapping of the interaction between PTB and the UTRs showed that the protein interacts with multiple regions of both UTRs. We have also shown the cis-acting effect of the CVB3-3’UTR on IRES mediated translation. The PTB contact points on the 3’UTRwas found to map to conserved regions, the deletion of which abrogates the 3’UTR mediated enhancement of the IRES activity. The possible role played by PTB in enhancing IRES activity by CVB3 3’UTR suggests that PTB protein might help in circularization of the CVB3 RNA by bridging the ends necessary for efficient translation of the viral RNA. In the second part, we have investigated possible role of some of the cis-acting element present in the CVB3 5’UTR RNA particularly the cryptic AUG. We have shown that mutation in cryptic AUG reduces the efficiency of translation mediated by the CVB3 IRES. Mutation in cryptic AUG moiety also reduces the interaction of mutant RNA with La protein. We have demonstrated that binding of 48S ribosomal complex with mutant IRES RNA was weaker compared to wt IRES RNA. We have investigated the possible alteration in secondary structure in the mutant RNA by chemical and enzymatic modification, which suggests that there is marginal alteration in the local structure due to mutation. It appears that integrity of cryptic AUG is important for efficient translation initiation by the CVB3 IRES. Results suggest that cryptic AUG plays a significant role in mediating internal initiation of translation of CVB3 RNA by mediating precise La binding and correct positioning of the 48S ribosomal complex. Finally, we have investigated the importance of a conserved hexa-nucleotide stretch in the apical loop within stem loop C (SLC, nt104-180), upstream of the ribosome landing site, on CVB3 IRES function. It has been already shown from our laboratory that the deletion at this apical loop resulted in significant decrease in IRES activity. This deletion mutant was shown to alter the secondary structure of the CVB3 5’UTR RNA. Here we have investigated the effect of point mutation in the apical loop SLC/c on CVB3 IRES activity by generating substitution mutation in the apical loop SLC/c in order to avoid possible alteration in secondary structure. Both the deletion or substitution mutation at this apical loop resulted in significant decrease in IRES activity. Both the mutant IRES RNAs (deletion and substitution mutant) failed to interact with certain trans-acting factors. Furthermore, expression of CVB3 2A protease significantly enhanced IRES activity of the wild type, but the effect was not so pronounced on the mutant IRESs. It is possible that the mutant RNAs were unable to interact with some trans-acting factors critical for enhanced IRES function. We have short-listed three proteins of approximate molecular mass of 56, 64 and 90 kDa, which showed reduced binding with mutant IRESs. By using RNA affinity column with biotinylated UTP labeled RNA we have purified couple of proteins and identified p64 as Cyto Keratin 1 protein by performing in-gel trypsin digestion followed by MALDI analysis. Overall, the results characterize the CVB3 IRES structurally and functionally, which could be useful in targeting critical RNA-protein interactions to develop candidate antiviral agent against Coxsackievirus infection.