INTERACTION OF THE UBC9 HUMAN HOMOLOG WITH C-JUN AND WITH THE GLUCOCORTICOID RECEPTOR

Glucocorticoid hormones convert the glucocorticoid receptor (GR) from an inactive cytosolic complex to a nuclear form that regulates transcr iption. Binding of GR to palindromic DNA-recognition sites (hormone re sponse elements) leads to activated target gene transcription. GR also exerts negative actions on transcription, e.g., by interfering with t he function of several other transcription factors such as AP-1, NK-ka ppa B, CREB, and Oct-1. Physical interactions of GR with AP-1 subunits are readily detectable but do not seem sufficient since non-repressin g GR mutants still interact in vitro, so that specific conformational changes and/or interactions with additional partner proteins may be re quired for negative action. In an attempt to find such partner protein s, we defined regions of c-Jun and GR essential for mutual interferenc e and used in those in a yeast two-hybrid screen for interacting prote ins. Repeatedly we isolated overlapping cDNA sequences of one protein interaction with both c-Jun and GR. This protein does not interact wit h c-Fos or a non-repressing GR mutant and expressed in mammalian cells does not substantially affect AP-1 or GR activity. Interestingly, how ever, the protein rescues yeast cells from the toxic effects of the GR fragment used for screening. The protein represents the human homolog ue of the yeast E2 ubiquitin-conjugating enzyme, Ubc9; its specific in teractions with both GR and c-Jun, but not mutant GR, suggest that it may exert physiologic regulatory functions.