Unconjugated interferon‐stimulated gene 15 specifically interacts with the hepatitis C virus NS5A protein via domain I

Interferon‐stimulated gene 15 (ISG15), a ubiquitin‐like protein, is induced by type I INF. Although several groups have reported ISGylation of the HCV NS5A protein, it is still unclear whether ISGylation of NS5A has anti‐ or pro‐viral effects in hepatitis C virus (HCV) infection. In the present study, the role of ISGylation‐independent, unconjugated ISG15 in HCV infection was examined. Immunoprecipitation analyses revealed that ISG15 interacts specifically with NS5A domain I. ISG15 mutants lacking the C‐terminal glycine residue that is essential for ISGylation still interacted with NS5A protein. Taken together, these results suggest that unconjugated ISG15 affects the functions of HCV NS5A through protein–protein interaction.