Small ubiquitin-related modifier-1 (SUMO-1) is covalently attached to many cellular targets to regulate protein—protein and protein—DNA interactions, as well as localization and stability of the target protein. The SUMO-1-conjugating E2 enzyme Ubc9 is known to interact with the glucocorticoid receptor (GR), a ligand-dependent transcription factor. In the present study, we show that GR is post-translationally modified by SUMO-1 (sumoylated) in a ligand-enhanced fashion. We identify experimentally three consensus SUMO attachment sites, two in the N-terminal transactivation region and one in the ligand-binding domain of GR. The two N-terminal sites are the major acceptor sites for SUMO-1 attachment. Mutation of these sites enhances transcriptional activity of GR on minimal promoters, but has no clear effect on the more complex mouse mammary tumour virus promoter. Thus SUMO-1 modification of GR influences receptor function in a promoter context-dependent fashion.

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