Human La protein interaction with GCAC near the initiator AUG enhances hepatitis C Virus RNA replication by promoting linkage between 5' and 3' untranslated regions.

Human La protein has been implicated in facilitating the internal initiation of translation as well as replication of hepatitis C virus (HCV) RNA. Previously, we demonstrated that La interacts with the HCV internal ribosome entry site (IRES) around the GCAC motif near the initiator AUG within stem-loop IV by its RNA recognition motif (RRM) (residues 112 to 184) and influences HCV translation. In this study, we have deciphered the role of this interaction in HCV replication in a hepatocellular carcinoma cell culture system. We incorporated mutation of the GCAC motif in an HCV monocistronic subgenomic replicon and a pJFH1 construct which altered the binding of La and checked HCV RNA replication by reverse transcriptase PCR (RT-PCR). The mutation drastically affected HCV replication. Furthermore, to address whether the decrease in replication is a consequence of translation inhibition or not, we incorporated the same mutation into a bicistronic replicon and observed a substantial decrease in HCV RNA levels. Interestingly, La overexpression rescued this inhibition of replication. More importantly, we observed that the mutation reduced the association between La and NS5B. The effect of the GCAC mutation on the translation-to-replication switch, which is regulated by the interplay between NS3 and La, was further investigated. Additionally, our analyses of point mutations in the GCAC motif revealed distinct roles of each nucleotide in HCV replication and translation. Finally, we showed that a specific interaction of the GCAC motif with human La protein is crucial for linking 5' and 3' ends of the HCV genome. Taken together, our results demonstrate the mechanism of regulation of HCV replication by interaction of the cis-acting element GCAC within the HCV IRES with human La protein.