SUMO-1 (small ubiquitin-like modifier) conjugation regulates the subcellular localization, stability, and activity of a variety of proteins. We show here that SUMO-1 overexpression markedly enhances progesterone receptor (PR)-mediated gene transcription. PR undergoes a sumoylation at lysine 388 located in its N-terminal domain. However, sumoylation of the receptor is not responsible for enhanced transcription because substitution of its target lysine did not abolish the effect of SUMO-1 and even converted the receptor into a slightly more active transactivator. Furthermore estrogen receptor ␣ (ER␣)-driven transcription is also enhanced by SUMO-1 overexpression contrasting with the absence of sumoylation of this receptor. We thus analyzed SUMO-1 conjugation to the steroid receptor coactivator SRC-1. We showed that this protein contains two major sites of conjugation at Lys-732 and Lys- The progesterone receptor (PR) 1 is a transcription factor belonging to the superfamily of nuclear receptors. It contains two domains involved in transcription activation: the constitutive activation function 1 (AF-1) localized in the N-terminal region of the protein and the ligand-regulated activation function 2 (AF-2) corresponding to the ligand binding domain. X-ray diffraction analysis of receptor crystals have allowed a detailed study of the molecular mechanisms involved in ligand-induced conformational changes of the AF-2 region (1-9).To activate transcription, steroid hormone receptors recruit coactivators among which the p160 family seems to be of special importance. This family comprises SRC-1/NCoA-1 (steroid receptor coactivactor-1), SRC-2/TIF2/GRIP1 and SRC-3/ TRAM-1/ACTR/AIB1/RAC3/pCIP (for reviews, see Refs. 10 -11). CREB-binding protein (CBP) and p300 interact with both the nuclear receptors and the coactivators. Recruitment of histone acetylases allows local decondensation of chromatin thus facilitating transcription (for reviews, see .Aside from their regulation by specific ligands, nuclear receptors and especially steroid hormone receptors are known to undergo covalent modifications. Phosphorylation (for review, see have been shown to regulate their functions and stability. Recently a new covalent modification of proteins and especially of transcriptional regulators have been described: SUMO-1 has been found to be conjugated to a growing list of proteins: the Ran GTPaseactivating protein RanGAP1 (21-24), the NFB inhibitor IB␣ (25), the promyelocytic leukemia protein PML (26 -29), the nuclear dot protein Sp100 (27), the homeodomain-interacting protein kinase 2 HIPK2 (30), the topoisomerases I and II (31-32), the tumor suppressor protein p53, and the protooncogene c-Jun (33-35).SUMO-1 is a protein of 101 amino acids that has a low (18%) but significant homology to ubiquitin. Modification by SUMO-1 (sumoylation) has a marked similarity with ubiquitin conjugation reactions (for reviews, see Refs. 36 -37). It requires an E1-activating enzyme (SAE1/SAE2 in mammals, Aos1/Uba2 in yeast) and the E2-conjugating enzyme Ubc...

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We have identified 7-ketocholesterol (7-KC) as an endogenous modulator that inhibits transactivation by the arylhydrocarbon receptor (