AbstractThe function of transcription factors can be critically regulated by SUMOylation. c‐Maf, the cellular counterpart of v‐maf oncogene, is a potent transactivator of the IL‐4 gene in Th2 cells. We found in a yeast two‐hybrid screen that c‐Maf can interact with Ubc9 and PIAS1, two key enzymes of the SUMOylation pathway. In this study, we report that c‐Maf co‐localized with these two SUMO (small ubiquitin‐like modifier) ligases in the nucleus and that c‐Maf can be SUMOylated in vitro and also in primary Th2 cells. We also demonstrated that lysine‐33 is the dominant, if not the only, SUMO acceptor site of c‐Maf. SUMOylation of c‐Maf attenuated its transcriptional activity. Reciprocally, a SUMOylation resistant c‐Maf was more potent than WT‐c‐Maf in driving IL‐4 production in c‐Maf‐deficient Th2 cells. Furthermore, we showed that ablation of the SUMO site did not alter the subcellular localization or the stability of c‐Maf protein but instead enhanced its recruitment to the Il4‐promoter. We conclude that SUMOylation at lysine‐33 is a functionally critical post‐translational modification event of c‐Maf in Th cells.