Abstract

Cullin–RING ligases (CRLs) constitute the most diverse E3 ligase family in the ubiquitin–proteasome system, yet their antiviral roles in bony fish remain poorly understood. Here, we identify the catalytic cores of CRLs, the RING-box proteins RBX1 and RBX2, as key modulators of antiviral signaling in cyprinid fish. Phylogenetic and structural analyses revealed that Rbx1 and Rbx2 are highly conserved and widely expressed, with preferential enrichment in primordial germ and immune-related cells. Upon infection with grass carp reovirus (GCRV) or spring viremia of carp virus (SVCV) in cyprinid fish, Rbxs were rapidly induced in vivo and in vitro . RBX1 and RBX2 synergistically promote viral replication by directly interacting with interferon regulatory factors (IRF) 3 and IRF7 and facilitating their ubiquitin-mediated regulation. Together, our findings uncover RBX1 and RBX2 as evolutionarily conserved negative regulators of fish innate immunity and provide mechanistic insight into the ubiquitin-mediated control of interferon homeostasis across vertebrates.