Equine arteritis virus (EAV) and porcine reproductive and respiratory syndrome virus (PRRSV) represent two members of the family Arteriviridae and pose major threats for the horse- and swine-breeding industries worldwide. A previous study suggested that PRRSV nsp4, a 3C-like protease, antagonizes interferon (IFN)-β production by cleaving the NF-B essential modulator (NEMO) at a single site, glutamate 349 (E349). Herein, we demonstrated that EAV nsp4 also inhibited virus-induced IFN-β production by targeting NEMO for proteolytic cleavage, and that the scission occurred at four sites- E166, E171, glutamine 205 (Q205), and E349. Additionally, we found that, besides the previously reported cleavage site E349 in NEMO, the scission by PRRSV nsp4 took place at two additional sites, E166 and E171. These results imply that while cleaving NEMO is a common strategy utilized by EAV and PRRSV nsp4 to antagonize IFN induction, EAV nsp4 adopts a more complex substrate recognition mechanism to target NEMO. By analyzing the abilities of the eight different NEMO fragments resulting from EAV or PRRSV nsp4 scission to induce IFN-β production, we serendipitously found that a NEMO fragment (1–349) could activate IFN-β transcription more robustly than full-length NEMO whereas all other NEMO cleavage products were abrogated for the IFN-β-inducing capacity. Thus, NEMO cleavage at E349 alone may not be sufficient to completely inactivate IFN response via this signaling adaptor. Altogether, our findings suggest that EAV and PRRSV nsp4 cleave NEMO at multiple sites and that this strategy is critical for disarming the innate immune response for viral survival.
IMPORTANCE The arterivirus nsp4-encoded 3C-like protease (3CLpro) plays an important role in virus replication and immune evasion, making it an attractive target for antiviral therapeutics. Previous work suggested that PRRSV nsp4 suppresses type I IFN production by cleaving NEMO at a single site. In contrast, the present study demonstrated that both EAV and PRRSV nsp4 cleave NEMO at multiple sites, and that this strategy is essential for disruption of type I IFN production. Moreover, we revealed that EAV nsp4 also cleaves NEMO at glutamine 205 (Q205), which is not targeted by PRRSV nsp4. Notably, targeting a glutamine in NEMO for cleavage has only been observed with picornavirus 3C proteases (3Cpro) and coronavirus 3CLpro. In aggregate, our work expands knowledge of the innate immune evasion mechanisms associated with NEMO cleavage by arterivirus nsp4 and describes a novel substrate recognition characteristic of EAV nsp4.