Viral Determinants in the NS3 Helicase and 2K Peptide That Promote West Nile Virus Resistance to Antiviral Action of 2',5'-oligoadenylate Synthetase 1b

The interferon-inducible 2',5'-oligoadenylate synthetase 1b (Oas1b) protein inhibits West Nile virus (WNV) infection by preventing viral RNA (vRNA) accumulation in infected cells. Serial passage of WNV in Oas1b-expressing mouse cells selected a virus variant with improved growth capacity. Two major amino acid substitutions were identified in this Oas1b-resistant WNV variant: NS3-S365G in the ATPase/helicase domain of NS3 and 2K-V9M in the C-terminal segment of NS4A. To assess their effect on antiviral activity of Oas1b, the NS3 and 2K mutations were engineered into an infectious WNV cDNA clone. The NS3 mutation alters requirement of ATP for ATPase activity and attenuates Oas1b-mediated suppression of vRNA accumulation. However, growth of NS3-mutant virus remains impaired in Oas1b-expressing cells. Only the 2K-V9M mutation efficiently rescued viral growth by promoting vRNA replication. Thus, WNV resistance to Oas1b antiviral action could be attributed to the 2K-V9M substitution with a potential role of NS3-S365G through rescue of vRNA accumulation.