RESUMO
SUMOylation is a reversible posttranslational modification involved in the regulation of diverse biological processes. Growing evidence suggests that virus infection can interfere with the SUMOylation system. In the present study, we discovered that apoptosis inhibitor 5 (API5) is a SUMOylated protein. Amino acid substitution further identified that Lys404 of API5 was the critical residue for SUMO3 conjugation. Moreover, we found that Avibirnavirus infectious bursal disease virus (IBDV) infection significantly decreased SUMOylation of API5. In addition, our results further revealed that viral protein VP3 inhibited the SUMOylation of API5 by targeting API5 and promoting UBC9 proteasome-dependent degradation through binding to the ubiquitin E3 ligase TRAF3. Furthermore, we revealed that wild-type but not K404R mutant API5 inhibited IBDV replication by enhancing MDA5-dependent IFN-ß production. Taken together, our data demonstrate that API5 is a UBC9-dependent SUMOylated protein and deSUMOylation of API5 by viral protein VP3 aids in viral replication. IMPORTANCE Apoptosis inhibitor 5 (API5) is a nuclear protein initially identified for its antiapoptotic function. However, so far, posttranslational modification of API5 is unclear. In this study, we first identified that API5 K404 can be conjugated by SUMO3, and Avibirnavirus infectious bursal disease virus (IBDV) infection significantly decreased SUMOylation of API5. Mechanically, viral protein VP3 directly interacts with API5 and inhibits SUMOylation of API5. Additionally, the cellular E3 ligase TNF receptor-associated factor 3 (TRAF3) is employed by VP3 to facilitate UBC9 proteasome-dependent degradation, leading to the reduction of API5 SUMOylation. Moreover, our data reveal that SUMOylation of API5 K404 promotes MDA5-dependent beta interferon (IFN-ß) induction, and its deSUMOylation contributes to IBDV replication. This work highlights a critical role of conversion between SUMOylation and deSUMOylation of API5 in regulating viral replication.