Lower concentration of La protein required for internal ribosome entry on hepatitis C virus RNA than on poliovirus RNA

Translation initiation of poliovirus and hepatitis C virus (HCV) RNA occurs by entry of ribosomes to the internal RNA sequence, called the internal ri bosomal entry site (IRES), Both IRES bind to the La protein and are thought to require the protein for their translation initiation activity, although they are greatly different in both the primary and predicted secondary str uctures. To compare the La protein requirement for these IRES, we took adva ntage of I-RNA from the yeast Saccharomyces cerevisiae, which has been repo rted to bind to La protein and block poliovirus IRES-mediated translation i nitiation, In a cell-free translation system prepared from HeLa cells, yeas t I-RNA inhibited translation initiation on poliovirus RNA as expected, but did not significantly inhibit translation initiation on HCV RNA, However, the translation initiation directed by either IRES was apparently inhibited by I-RNA in rabbit reticulocyte lysates, in which La protein is limiting. I-RNA-mediated inhibition of HCV IRES-dependent translation in rabbit retic ulocyte lysates was reversed by exogenous addition of purified recombinant La protein of smaller amounts than necessary to reverse poliovirus IRES-dep endent translation, These results suggest that HCV IRES requires lower conc entrations of La protein for its function than does poliovirus IRES. Immuno fluorescence studies showed that HCV infection appeared not to affect the s ubcellular localization of La protein, which exists mainly in the nucleus, although La protein redistributed to the cytoplasm after poliovirus infecti on. The data are compatible with the low requirement of La protein for HCV IRES activity.