Equine lentivirus Gag protein degrades mitochondrial antiviral signaling protein via the E3 ubiquitin ligase Smurf1.

Equine infectious anemia virus (EIAV) and HIV-1 are both members of the Lentivirus genus and are similar in virological characters. EIAV is of great concern in the equine industry. Lentiviruses establish a complex interaction with the host cell to counteract the antiviral responses. There are various pattern recognition receptors in the host, for instance, the cytosolic RNA helicases interact with viral RNA to activate the mitochondrial antiviral signaling protein (MAVS) and subsequent interferon (IFN) response. However, viruses also exploit multiple strategies to resist host immunity by targeting MAVS, but the mechanism by which lentiviruses are able to target MAVS has remained unclear. In this study, we found that EIAV infection induced MAVS degradation, and that EIAV Gag protein recruited the E3 ubiquitin ligase Smurf1 to polyubiquitinate and degrade MAVS. The CARD domain of MAVS and the WW domain of Smurf1 are responsible for the interaction with Gag. EIAV Gag is a precursor polyprotein of the membrane-interacting matrix p15, the capsid p26, and the RNA-binding nucleocapsid proteins p11 and p9. Therefore, we analyzed which protein domain of Gag could interact with MAVS and Smurf1. We found that p15 and p26, but not p11 or p9, target MAVS for degradation. Moreover, we identified the key amino acid residues that support the interactions between p15 or p26 and MAVS or Smurf1. The present study describes a novel role of the EIAV structural protein Gag in targeting MAVS to counteract innate immunity, and reveals the mechanism by which the equine lentivirus can antagonize against MAVS.IMPORTANCEHost anti-RNA virus innate immunity relies mainly on the recognition by retinoic acid-inducible gene I (RIG-I) and melanoma differentiation-associated protein 5 (MDA5), and subsequently initiates downstream signaling through interaction with mitochondrial antiviral signaling protein (MAVS). However, viruses have developed various strategies to counteract MAVS-mediated signaling, although the method of antagonism of MAVS by lentiviruses is still unknown. In this article, we demonstrate that the precursor (Pr55gag) polyprotein of EIAV and its protein domains p15 and p26 target MAVS for ubiquitin-mediated degradation through E3 ubiquitin ligase Smurf1. MAVS degradation leads to the inhibition of the downstream IFN-β pathway. This is the first time that lentiviral structural protein has been found to have antagonistic effects on MAVS pathway. Overall, our study reveals a novel mechanism by which equine lentiviruses can evade host innate immunity, and provides insight into potential therapeutic strategies for the control of lentivirus infection.