244 : Homologous 2′,5′-phosphodiesterases from disparate RNA viruses antagonize antiviral innate immunity

Efficient and productive virus infection often requires viral countermeasures that block innate immunity. The interferon-inducible 2′,5-oligoadenylate (2-5A) synthetases (OAS) and ribonuclease L (RNase L) are components of a potent host antiviral pathway. We previously showed that murine coronavirus (MHV) accessory protein ns2, a 2H phosphoesterase superfamily member, is a phosphodiesterase (PDE) that cleaves 2′,5′-oligoadenylates (2-5A) thereby preventing activation of RNase L (Zhao et al., Cell Host and Microbe 11:607–16, 2012). The PDE activity of ns2 is required for viral replication in macrophages and for hepatitis. Here, we will demonstrate that group A rotavirus, the primary cause of severe, dehydrating gastroenteritis in children worldwide, encodes an homologous and similar PDE. The VP3 carboxy-terminal domain (VP3-CTD) shares sequence and predicted structural homology with ns2, including two catalytic His-x-Thr/Ser motifs. Recombinant VP3-CTD also demonstrated 2′,5′-PDE activity, which cleaved 2-5A in vitro. In addition, its expression in cells depleted 2-5A levels induced by OAS activation with poly (rI):poly (rC), preventing RNase L activation. In the context of recombinant chimeric MHV expressing inactive ns2, the VP3-CTD restored the ability to replicate efficiently in macrophages or in the livers of infected mice, while chimeras expressing inactive VP3-CTD (H718A or H798R) were attenuated. Also, chimeric viruses expressing active ns2 or VP3, but not mutant ns2 or VP3, were able to protect ribosomal RNA from RNase L-mediated degradation. Thus, VP3-CTD is a 2′,5′-PDE able to functionally substitute for ns2 in the context of MHV infection. Remarkably, therefore, two disparate RNA viruses encode proteins with homologous 2′,5′-PDEs that antagonize innate immunity. In addition, based on sequence and predicted structural homology with MHV ns2 and rotavirus VP3, several other viruses have been identified that encode proteins with predicted 2′,5′-PDE activities.