To block this antiviral response, many insect viruses encode suppressors that inhibit host RNAi pathways ( Galiana-Arnoux et al., 2006 Li et al., 2002 Nayak et al., 2010 van Cleef et al., 2014 van Mierlo et al., 2012 van Rij et al., 2006). The insect antiviral RNAi response requires the Dicer-2 (Dcr2) and the Argonaute 2 (Ago-2) enzymes, which cleaves double-stranded (ds) viral RNA replication intermediates and single-stranded viral RNA genomes, respectively, to limit virus replication ( Aliyari et al., 2008 Marques et al., 2013). The importance of RNAi-mediated immunity and viral evasion strategies is exemplified by insects that transmit a wide variety of viral infectious agents to humans including sandfly fever, Dengue, Chikungunya, and Zika virus ( Blair, 2011 Nayak et al., 2013 Varjak et al., 2017).

In many species, including insects, RNA interference (RNAi) serves as a system of adaptive antiviral immunity. The conflict between viral pathogens and their hosts drives the coevolution of both host defenses and viral suppression mechanisms that inhibit them ( tenOever, 2016). While the direct inhibition of Ago-2 activity provides an efficient mechanism to establish infection, the recruitment of a ubiquitin ligase complex, enables CrPV-1A to amplify Ago-2 inactivation to restrict further antiviral RNAi immunity.

Our study uncovers a viral-based dual regulatory program that restricts antiviral immunity by direct interaction with and modulation of host proteins. CrPV-1A also contains a BC-box motif that enables the virus to hijack a host Cul2-Rbx1-EloBC ubiquitin ligase complex, which promotes Ago-2 degradation and virus replication.

Mutations disrupting Ago-2-binding attenuates viral pathogenesis in wild-type but not Ago-2-deficient flies. The atomic resolution structure of CrPV-1A uncovered a flexible loop that interacts with Argonaute 2 (Ago-2), thereby inhibiting Ago-2 endonuclease-dependent immunity. Using the Drosophila model, we combined structural, biochemical, and virological approaches to elucidate the strategies by which CrPV-1A restricts RNAi immunity. The dicistrovirus, Cricket paralysis virus (CrPV) encodes an RNA interference (RNAi) suppressor, 1A, which modulates viral virulence.

L 3 the talos principle background