SUMO1 modification stabilizes polymerase protein VP1 of Blotched Snakehead Virus

SUMOylation, the covalent attachment of SUMO molecules to target proteins, is a post-translational modification (PTM) that plays essential roles in the replication of many viruses. In this study, we demonstrate that SUMO1 effectively conjugates to VP1, the RNA-dependent RNA polymerase (RdRp) of Blotched Snakehead Virus (BSNV). SUMO1 modification of BSNV VP1 requires the active sites of both SUMO1 and Ubc9. Site-directed mutagenesis revealed that the SIM motif (³⁹³IYIY³⁹⁶) of VP1 is essential for its SUMOylation. Notably, the SIM mutant VP1 exhibited significantly reduced protein stability compared to wild-type (WT) VP1, and this degradation was markedly suppressed by the proteasome inhibitor MG132. Mechanistically, the SIM mutant VP1 showed increased levels of both total and K48-linked ubiquitination, explaining its instability. Our findings reveal that SUMOylation of BSNV VP1 stabilizes the polymerase by antagonizing ubiquitin-proteasome-mediated degradation, providing novel insights into the regulation of birnavirus replication.