Mutational analysis of Aedes aegypti Dicer 2 helicase and RNase III domains gives insights into the hierarchy of antiviral activity and small interfering RNA biogenesis in the exogenous siRNA pathway response to Semliki Forest virus

The exogenous small interfering RNA pathway is a key antiviral control mechanism in the mosquito Aedes aegypti, a widely distributed vector of human pathogenic arboviruses. It is induced by viral-derived double-stranded RNA that are cleaved by the nuclease Dicer 2 into predominantly 21 nucleotide virus-derived small interfering RNAs which are used by the effector protein Argonaute 2 to cleave viral target RNA, thus reducing replication. The Dcr2 protein contains several domains which are relevant for its activities, including helicase and RNAse III domains. In Drosophila melanogaster Dcr2, the helicase domain has been associated with binding termini of blunt dsRNA and a processive siRNA production mechanism, where Platform-PAZ domains bind 3 overhang dsRNA with subsequent distributive siRNA production. Here we analysed the contributions of the helicase and RNase III domains in Dcr2 antiviral activity and silencing. Conserved amino acids in helicase and RNase III domains were identified to investigate Dcr2 antiviral activities in an Ae. aegypti derived Dcr2 knockout cell line by reporter assays and following infection with the positive-stranded RNA arbovirus, Semliki Forest virus.