Mutations in NS5B Polymerase of Hepatitis C Virus: Impacts on in Vitro Enzymatic Activity and Viral RNA Replication in the Subgenomic Replicon Cell Culture

Abstract

Hepatitis C virus (HCV) nonstructural protein 5B (NS5B) is an RNA-dependent RNA polymerase (RdRp) essential for virus replication. Several consensus sequence motifs have been identified in NS5B, some of which have been shown to be critical for its enzymatic activity. A unique β-hairpin structure located between amino acids 443 and 454 in the thumb subdomain has also been shown to play an important role in ensuring terminal initiation of RNA synthesis in vitro. However, the importance of these sequence and structural elements in viral RNA replication in infected cells has not been established, mainly due to the lack of a reliable cell culture system for HCV. In this study, we investigated the effect of several single amino acid substitutions and β-hairpin truncations in NS5B on viral RNA replication by using the subgenomic replicon cell culture system. A strong correlation between in vitro polymerase activity and viral RNA replication was observed with most of the substitutions. Interestingly, truncations of the β-hairpin (by four and eight amino acid residues, respectively), which did not reduce the in vitro enzymatic activity, completely abolished the ability of the replicon RNA to replicate in Huh-7 cells, demonstrating its essential role in viral RNA replication. Furthermore, a conservative substitution in motif D, from an arginine residue (AMTR345), which is conserved among all HCV isolates, to a lysine residue, resulted in significant improvements in both transient RNA replication and colony formation efficiencies. This result also correlates with a previous observation that the enzymatic activity of NS5B increased by about 50% when the same NS5B substitution was introduced (V. Lohmann, F. Korner, U. Herian, and R. Bartenschlager, J. Virol. 1997, 71, 8416–8428).