Dysregulation of RIG-I activation by picornavirus 3A protein

Retinoic acid-inducible gene I (RIG-I)-like receptors, including RIG-I and MDA5, are key cytoplasmic pattern-recognition receptors that detect viral RNA in the cytoplasm and trigger the production of interferon (IFN). Among them, MDA5, rather than RIG-I, is considered the primary sensor during picornavirus infection due to the unique features of picornaviral RNA. However, previous studies have indicated that RIG-I also possesses antiviral activity against several picornaviruses, suggesting its potential importance during viral replication. Here, we found that 3A proteins from various picornaviruses, including Senecavirus A (SVA), Enterovirus 71 (EV71), Encephalomyocarditis virus (EMCV), foot-and-mouth disease virus (FMDV), and Coxsackievirus A16 (CA16), directly target RIG-I to suppress RIG-I-like receptor-mediated IFN-β production. Mechanistically, these picornaviral 3A proteins partially share the following similar strategies to dysregulate RIG-I activation: (i) all interact with RIG-I, (ii) EMCV and FMDV 3A reduce RIG-I expression, and (iii) SVA, EV71, EMCV, and FMDV 3A impair the interaction between RIG-I and MAVS by diminishing K63-linked ubiquitination of RIG-I. These findings broaden our understanding of how picornaviruses employ nonstructural proteins to evade innate immune responses during early infection.IMPORTANCEPicornaviruses cause a broad spectrum of human and animal diseases; however, the mechanisms by which they counteract host antiviral defenses remain incompletely understood. Owing to the distinct structural features of picornaviral RNA, MDA5 is widely regarded as the primary sensor mediating antiviral responses during picornavirus infection. However, accumulating evidence suggests that RIG-I also contributes to antiviral defense. Picornaviruses have evolved various means to suppress RIG-I and MDA5 activity, thereby facilitating evasion of the innate immune response and underscoring the importance of RIG-I during picornavirus infection. This study identifies RIG-I as a conserved target of the nonstructural protein 3A from multiple picornaviruses, including Senecavirus A, Enterovirus 71, Encephalomyocarditis virus, foot-and-mouth disease virus, and Coxsackievirus A16, and uncovers both shared and virus-specific strategies that dysregulate RIG-I-mediated interferon production. Collectively, these findings expand our understanding of the antagonistic mechanisms of how picornaviruses manipulate the host innate immune system.