Enzymatic Characterization of the Full-Length and C-Terminally Truncated Hepatitis C Virus RNA Polymerases: Function of the Last 21 Amino Acids of the C Terminus in Template Binding and RNA Synthesis

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Abstract
The nonstructural protein NS5B of hepatitis C virus (HCV) is an RNA-dependent RNA polymerase (RdRp), which plays a central role in viral replication. Most of the reported studies on HCV polymerase in vitro have used a truncated form of the enzyme lacking the C-terminal 21 amino acids (ΔC21-NS5B). In this study, we compared the enzymatic properties of the full-length NS5B (FL-NS5B) and this truncated form. Removal of the C21 domain enhanced the enzyme stability. Both enzymes are capable of performing de novo and primer-dependent RNA syntheses, but each possesses a unique set of biochemical requirements for optimal RdRp activity. Whereas RNA synthesis by FL-NS5B remained relatively constant at 12−100 mM KCl, synthesis by ΔC21-NS5B rapidly decreased at KCl concentrations greater than 12 mM. The different salt requirement for overall RNA synthesis by these two polymerases can in part be explained by the effect of monovalent ion concentration at the step of template binding, where binding by ΔC21-NS5B but not FL-NS5B decreased proportionally as the KCl concentration increased from 25 to 200 mM. Thus, the C21 domain appears to contribute to NS5B−RNA template binding, probably through the hydrophobic stacking interaction between its aromatic amino acids and the nucleotide bases of the RNA. This interpretation was supported by the observation that the C21 polypeptide by itself could also bind to RNA to form binary complexes that were resistant to changes in the KCl concentration. Though both enzymes exhibited similar Ks values for each of the four NTPs (1−5 μM), ΔC21-NS5B generally required lower NTP concentrations than FL-NS5B for optimal synthesis. Interestingly, ΔC21-NS5B became severely inhibited at elevated NTP concentrations, which most likely is due to competitive binding of the noncomplementary nucleotide to the polymerase catalytic center. Finally, the terminal transferase activity of ΔC21-NS5B was found to be distinct from that of FL-NS5B on several different RNA templates. Together, these findings indicated that the HCV NS5B C21 domain, in addition to being a membrane anchor, functions in template binding, NTP substrate selection, and modulation of terminal transferase activity.