Translation initiation of the hepatitis C virus (HCV) RNA genome occurs through an internal ribosome entry site in a cap-independent manner. Here, we have examined the interaction between La antigen and the HCV 5′ noncoding region (5′NCR). In this analysis, competitor RNAs derived from HCV 5′NCR carrying deletions and a point mutation were used to identify the site(s) of La antigen binding during UV cross-linking assay. These studies suggest that La antigen recognizes the intact HCV 5′NCR structure. Further, these interactions occurred in the context of the initiator AUG. The latter view is supported by an analysis in which mutants of the HCV 5′NCR RNA with deletion or substitution in the initiator AUG codon failed to compete for La antigen binding to the wild-type 5′NCR. The evidence for the interaction between liver cell-derived La antigen and the HCV 5′NCR is provided by immunoprecipitation of a UV cross-linked species from the S100 fraction of Huh7 cell lysates. The functional relevance of this interaction was demonstrated by the stimulation of the HCV internal ribosome entry site-mediated translation in the presence of La protein. These results suggest an important functional role of La protein in the regulation of internal initiation of translation of the HCV RNA genome
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