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. IMPORTANCE For productive infection, rotavirus and other RNA viruses must suppress interferon (IFN) signaling and the expression of IFN-stimulated antiviral gene products. Particularly important is inhibiting the interferon (IFN)-inducible 2',5'-oligoadenylate synthetase (OAS)-RNase L pathway, as activated RNase L can direct the nonspecific degradation of viral and cellular RNAs, thereby blocking viral replication and triggering cell death pathways. In this study, we have discovered that the simian SA11 strain of rotavirus employs a novel strategy of inhibiting the OAS-RNase L pathway. This strategy is mediated by SA11 NSP1, a nonstructural protein that hijacks E3 cullin-RING ligases, causing the ubiquitination and degradation of host proteins essential for IFN induction. Our analysis shows that SA11 NSP1 also recognizes and causes the ubiquitination of RNase L, an activity resulting in depletion of endogenous RNase L. These data raise the possibility of using therapeutics targeting cellular E3 ligases to control rotavirus infections.
Keywords: OAS; OAS-RNase L; RNase L; innate immunity; interferon; interferons; nonstructural protein 1; phosphodiesterase; rotavirus.