Rotavirus NSP1 Subverts the Antiviral Oligoadenylate Synthetase-RNase L Pathway by Inducing RNase L Degradation

The interferon (IFN)-inducible 2′,5′-oligoadenylate synthetase (OAS)-RNase L pathway plays a critical role in antiviral immunity. Group A rotaviruses, including the simian SA11 strain, inhibit this pathway through two activities: an E3-ligase related activity of NSP1 that degrades proteins necessary for IFN signaling, and a phosphodiesterase (PDE) activity of VP3 that hydrolyzes the RNase L-activator 2′,5′-oligoadenylate. Unexpectedly, we found that a recombinant (r) SA11 double mutant virus deficient in both activities (rSA11-VP3H797R-NSP1ΔC17) retained the ability to prevent RNase L activation. Mass spectrometry led to the discovery that NSP1 interacts with RNase L in rSA11-infected HT29 cells. This interaction was confirmed through copulldown assay of cells transiently expressing NSP1 and RNase L. Immunoblot analysis showed that infection with wild-type rSA11 virus, rSA11-VP3H797R-NSP1ΔC17 double mutant virus, or single mutant forms of the latter virus all resulted in the depletion of endogenous RNase L. The loss of RNase L was reversed by addition of the neddylation inhibitor MLN4924, but not the proteasome inhibitor MG132. Analysis of additional mutant forms of rSA11 showed that RNase L degradation no longer occurred when either the N-terminal RING domain of NSP1 was mutated or the C-terminal 98 amino acids of NSP1 were deleted. The C-terminal RNase L degradation domain is positioned upstream and is functionally independent of the NSP1 domain necessary for inhibiting IFN expression. Our studies reveal a new role for NSP1 and its E3-ligase related activity as an antagonist of RNase L and uncover a novel virus-mediated strategy of inhibiting the OAS-RNase L pathway.