A fish-specific E3 ubiquitin ligase NHRE3 suppresses antiviral immunity by degrading ISG15 via ubiquitination

Grass carp ( Ctenopharyngodon Idella ) hemorrhagic diseases (GCHD), caused by grass carp reovirus (GCRV), has posed a serious threat to the grass carp aquaculture industry in China. Ubiquitination plays a central role in viral evasion of the host interferon response; therefore, elucidating the mechanisms by which viruses exploit ubiquitination to achieve immune escape is of significant scientific importance and translational value. In a previous study, we identified an E3 ubiquitin ligase–like gene associated with susceptibility to hemorrhagic disease through random mutagenesis in rare minnow ( Gobiocypris rarus ). In this study, domain and amino acid sequence analyses confirmed that this gene encodes a fish-specific E3 ubiquitin ligase, designated NHRE3. nhre3 −/− rare minnows exhibited significantly enhanced resistance to GCRV. Interferon-stimulated gene 15 (ISG15) was identified as an NHRE3-interacting protein via Immunoprecipitation–Mass Spectrometry (IP–MS). Furthermore, NHRE3 possessed E3 ubiquitin ligase activity, catalyzing K48-linked polyubiquitination of ISG15 and leading to its degradation via the proteasome pathway. Given that the antiviral mechanism of ISG15 in fish remains poorly understood, we further investigated its function and found that ISG15 can interact with IRF3 and promote the expression of innate immune factors downstream of IRF3, thereby exerting its anti-GCRV effect. Taken together, NHRE3 promotes GCRV infection by mediating the ubiquitin–proteasome–dependent degradation of ISG15, may represent a mechanism unique to fish viruses. In mammals, viral immune evasion strategies targeting ISG15 primarily involve the inhibition of ISGylation or induction of deISGylation. The immune evasion mechanism identified in this study, in which a virus promotes degradation of ISG15 via a fish-specific ubiquitin ligase, may represent a more primitive or diverse form of innate immune regulation, revealing the ancient origins of the immunological arms race in vertebrates.