Hepatitis C virus nonstructural protein NS3 binds to Sm-D1, a small nuclear ribonucleoprotein associated with autoimmune disease.

Hepatitis C virus (HCV) causes a persistent infection, chronic hepatitis, and leads to hepatocellular carcinoma. Nonstructural protein 3 (NS3) of HCV possesses protease, nucleoside triphosphatase, and helicase activities. Using the yeast two hybrid assay, we identified Sm-D1, a host protein that binds to NS3. Sm-D1 is a component of small nuclear ribonucleoprotein (snRNP) complexes which are associated with autoimmune disease. Sm-D1 has Gly-Arg (GR) repeats at the C terminus, which contains dimethylarginine modified by post-translational modification and may constitute an immunoreactive determinant. Deletion mutants revealed that the C-terminal region of Sm-D1 containing the GR repeats was the binding region for NS3, and the expression feature of Sm-D1 was affected by co-expression of NS3. Immunostaining assay demonstrated that NS3 was also present in the nucleus of cells overexpressing Sm-D1, although it was usually found in cytoplasm. The localization of NS3 could change following interaction with Sm-D1 and affect the function of Sm-D1.