A Hyperactive Kunjin Virus NS3 Helicase Mutant Demonstrates Increased Dissemination and Mortality in Mosquitoes

The unwinding of double-stranded RNA intermediates is critical for replication and packaging of flavivirus RNA genomes. This unwinding activity is achieved by the ATP-dependent nonstructural protein 3 (NS3) helicase. In previous studies, we investigated the mechanism of energy transduction between the ATP and RNA binding pockets using molecular dynamics simulations and enzymatic characterization. Our data corroborated the hypothesis that Motif V is a communication hub for this energy transduction. More specifically, mutations T407A and S411A in Motif V exhibit a hyperactive helicase phenotype leading to the regulation of translocation and unwinding during replication. However, the effect of these mutations on viral infection in cell culture and in vivo is not well understood. Here, we investigated the role of Motif V in viral replication using T407A and S411A West Nile virus (Kunjin subtype) mutants in cell culture and in vivo. We were able to recover S411A Kunjin but unable to recover T407A Kunjin. Our results indicated that S411A Kunjin decreased viral infection, and increased cytopathogenicity in cell culture as compared to WT Kunjin. Similarly, decreased infection rates in surviving S411A-infected Culex quinquefasciatus mosquitoes were observed, but S411A Kunjin infection resulted in increased mortality compared to WT Kunjin. Additionally, S411A Kunjin increased viral dissemination and saliva positivity rates in surviving mosquitoes compared to WT Kunjin. These data suggest that S411A Kunjin increases pathogenesis in mosquitoes. Overall, these data indicate that NS3 Motif V may play a role in the pathogenesis, dissemination, and transmission efficiency of Kunjin virus.