The autophagic and nonautophagic functions of CcATG5 jointly promote SVCV replication

Autophagy-related gene 5 (Atg5) encodes a core protein for autophagosome formation and plays a critical role in autophagy, interferon (IFN) responses, and inflammatory regulation in mammals. In this study, we identified and characterized an Atg5 homolog (CcAtg5) from Cyprinus carpio. The CcAtg5 gene encodes a 275-amino acid polypeptide that exhibits 93.82 % and 81.09 % sequence similarity to its homologs in Danio rerio and Homo sapiens, respectively. Structural analysis reveals that CcATG5 contains a conserved APG5 domain, along with two ubiquitin-like domains, and a helical domain. Subcellular localization analysis shows that CcATG5 is distributed in both the cytoplasm and nucleus of EPC cells. Functional studies demonstrate that CcATG5 overexpression promotes spring viremia of carp virus (SVCV) replication, as evidenced by facilitating viral attachment, entry, gene transcription, protein synthesis, and viral yield. Additionally, CcATG5 downregulates the expression of key immune regulators (IRF3, IRF7, IL-6, IL-1β) as well as the promoter activities of IFN, IFN-stimulated response element (ISRE), and nuclear factor κB (NF-κB). The siRNA-mediated knockdown of CcAtg5 reverses these aforementioned effects. Taken together, our findings indicate that CcATG5 promotes SVCV replication through enhancing autophagy, suppressing innate antiviral immunity, and facilitating the early stage of virus entry. This study demonstrates that the proviral role of CcATG5 arises from the combined effects of its autophagic and nonautophagic functions.