SUMO modification regulates the transcriptional repressor function of aryl hydrocarbon receptor repressor

The aryl hydrocarbon receptor (AhR) repressor (AhRR) inhibits the AhR activity. AhRR acts by competing with AhR for heterodimer formation with the AhR nuclear translocator (Arnt) and preventing the AhR·Arnt complex from binding the xenobiotic-responsive elements. Here, we report that AhRR has three evolutionarily conserved SUMOylation consensus sequences within its C-terminal repression domain and that Lys-542, Lys-583, and Lys-660 at the SUMOylation sites are modified by SUMO-1 in vivo. Arginine mutation of the three lysines results in a significant reduction of transcriptional repression activity. SUMOylation of the three lysine residues is important for the interaction between AhRR and ANKRA2, HDAC4, and HDAC5, which are important corepressors for AhRR. Arnt, a heterodimer partner for AhRR, markedly enhanced the SUMOylation of AhRR. AhRR, but not AhR, also significantly enhanced the SUMOylation of Arnt. The SUMOylation of both AhRR and Arnt is important for the efficient transcriptional repression activity of the AhRR/Arnt heterodimer.