Negative modulation of RXRα transcriptional activity by small ubiquitin-related modifier (SUMO) modification and its reversal by SUMO-specific protease SUSP1

Retinoid X receptor alpha(RXR alpha) belongs to a family of ligand-activated transcription factors that regulate many aspects of metazoan life. Here we demonstrate that RXR alpha is a target substrate of a small ubiquitin-related modifier ( SUMO)-specific protease, SUSP1, which is capable of controlling the transcriptional activity of RXR alpha. RXR alpha was modified by SUMO-1 in vivo as well as in vitro, and the Lys-108 residue within the IKPP sequence of RXR alpha AF-1 domain was identified as the major SUMO-1 acceptor site. Prevention of SUMO modification by Lys-to-Arg mutation led to an increase not only in the transcriptional activity of RXR alpha but also in the activity of its heterodimeric complex with retinoic acid receptor-alpha or peroxisome proliferator-activated receptor-gamma ( PPAR gamma). SUSP1 co-localized with RXR alpha in the nucleus and removed SUMO-1 from RXR alpha but not from androgen receptor or PPAR gamma. Moreover, overexpression of SUSP1 caused an increase in the transcriptional activity of RXR alpha, whereas small hairpin RNA-mediated knockdown of endogenous SUSP1 led to a decrease in RXR alpha activity. These results suggest that SUSP1 plays an important role in the control of the transcriptional activity of RXR alpha and thus in the RXR alpha-mediated cellular processes.