Regulation of nuclear receptor and coactivator functions by the carboxyl terminus of ubiquitin-conjugating enzyme 9

Small ubiquitin-related modifier (SUMO) is a protein moiety that is ligated to lysine residues in a variety of target proteins. The SUMO E2 enzyme ubiquitin-conjugating enzyme 9 (Ubc9) is sufficient for substrate recognition and lysine modification of known SUMO targets. Previous studies have demonstrated that mutated Ubc9 that has lost its SUMO-ligating activity retains its enhancement on transactivation mediated by androgen receptor (AR). In contrast to the binding ability to Ubc9, the sumoylation of AR via the association of SUMO-1 and PIAS1 is able to repress AR-dependent transcription. In the present study, we present several lines of evidence to explain the role of over-expressed Ubc9 as a cofactor in the nuclear receptor and coactivator functions, including (i) activity that is independent of its ability to catalyze SUMO-1 conjugation, (ii) an insight into the protein–protein interaction motif in its eight C-terminal residues, (iii) selective coactivator function in nuclear receptor-relevant transactivation activities, and (iv) a non-trichostatin A-sensitive autonomous transcription repression domain in its far C-terminal region. Taken together, our data suggest that the both the protein–protein interaction through the Ubc9 C-terminus and its sumoylation-modifying activity provide the mechanism for regulating nuclear receptor functions.