Equine arteritis virus (EAV) is a positive-sense, single-stranded RNA virus that belongs to the family Arteriviridae, which also includes porcine reproductive and respiratory syndrome virus (PRRSV). EAV is the causative agent of equine viral arteritis (EVA), an economically important systemic, reproductive, and respiratory disease of equids. EAV infection triggers host innate immunity, yet the precise strategies employed by the virus to evade immune defenses and achieve productive infection are poorly characterized. In this study, it was observed that EAV infection can induce a cellular IFN response; however, EAV simultaneously significantly suppresses the expression of IFN-β in host cells. Our findings indicate that host cells sense EAV and elicit an IFN response via mitochondrial antiviral-signaling protein (MAVS)-mediated signal, but the virus can dampen this signaling to enhance infection. Further investigation showed that EAV nsp10 protein could interact with MAVS and promote its degradation. By screening, we found that the E3 ubiquitin ligases Smurf1 and MARCH5 are recruited by nsp10 to polyubiquitinate and degrade MAVS. Interestingly, the degradation of MAVS promoted by nsp10 depends on the dimerization of nsp10, which occurs through the interactions of zinc finger motifs. The CARD or PRR domain of MAVS and the 1A domain of nsp10 are responsible for the interaction between MAVS and nsp10. Moreover, we have identified the key amino acid residues that mediate the interactions between nsp10 and its binding partners. Specifically, D249, S287, and the S1/F39/N41 sites are critical for its binding to MAVS, Smurf1, and MARCH5, respectively. This study demonstrated a novel role for the arteriviral RNA helicase nsp10 in targeting MAVS to counteract innate immunity and reveals the mechanism by which EAV antagonizes MAVS.IMPORTANCEDue to the MAVS functions as a "switch" in the immune signal transduction against RNA viruses, MAVS has emerged as the central regulatory target by viruses. Recently, researchers show increasing interest in viral evasion strategies targeting MAVS. The method of antagonism of MAVS by EAV is still unknown. To date, the roles of arteriviral RNA helicases, such as the EAV helicase nsp10, in regulating host cellular responses have received little research attention. In this study, we found that EAV nsp10 could mediate MAVS degradation through the proteasome via the E3 ubiquitin ligases Smurf1 and MARCH5. This is the first time that an arteriviral RNA helicase has been found to have an antagonistic effect on the innate immunity signaling pathway. Overall, our study reveals a novel mechanism by which EAV can evade host innate immunity and provides insight into potential therapeutic strategies for the control of arterivirus infection. 

Keywords: E3 ubiquitin ligase; EAV; MARCH5; MAVS; Nidovirales; Nsp10; RIG-I; Smurf1; arterivirus; helicases.