Abstract
RNA interference (RNAi) is a major antiviral pathway that shapes evolution of RNA viruses. We show here that Nora virus, a natural Drosophila pathogen, is both a target and suppressor of RNAi. We detected viral small RNAs with a signature of Dicer-2 dependent small interfering RNAs in Nora virus infected Drosophila. Furthermore, we demonstrate that the Nora virus VP1 protein contains RNAi suppressive activity in vitro and in vivo that enhances pathogenicity of recombinant Sindbis virus in an RNAi dependent manner. Nora virus VP1 and the viral suppressor of RNAi of Cricket paralysis virus (1A) antagonized Argonaute-2 (AGO2) Slicer activity of RNA induced silencing complexes pre-loaded with a methylated single-stranded guide strand. The convergent evolution of AGO2 suppression in two unrelated insect RNA viruses highlights the importance of AGO2 in antiviral defense.
Highlights
An efficient antiviral immune response is essential for the control or elimination of virus infection and for survival of the infected host
Nora virus is a target of RNA interference (RNAi) in vivo Nora virus is an enteric (+) RNA virus that successfully establishes a persistent infection in flies [20]
To determine whether Nora virus is a target for Dcr-2, we analyzed the presence of Nora virus small RNAs in the w1118 Drosophila strain that is widely used as a recipient strain for transgenesis
Summary
An efficient antiviral immune response is essential for the control or elimination of virus infection and for survival of the infected host. Viruses evolved counter-defense mechanisms to evade, suppress or inactivate host immunity. Studying these mechanisms provides important insight in the critical steps of antiviral responses and may uncover novel components and regulators of immune pathways. The initial trigger of an antiviral RNAi response is the recognition and cleavage of viral double-stranded RNA (dsRNA) into viral small interfering RNAs (vsiRNAs), in insects by the ribonuclease Dicer-2 (Dcr-2). These vsiRNAs act as specificity determinants of the Argonaute-2 (AGO2) containing effector nuclease complex RISC (RNA induced silencing complex). Direct evidence supporting this model, for example by the detection of viral Slicer products, is lacking
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