GRIP1, A TRANSCRIPTIONAL COACTIVATOR FOR THE AF-2 TRANSACTIVATION DOMAIN OF STEROID, THYROID, RETINOID, AND VITAMIN-D RECEPTORS

After binding to enhancer elements, transcription factors require tran scriptional coactivator proteins to mediate their stimulation of trans cription initiation, A search for possible coactivators for steroid ho rmone receptors resulted in identification of glucocorticoid receptor interacting protein 1 (GRIP1), The complete coding sequence for GRIP1, isolated from a mouse brain cDNA library, contains an open reading fr ame of 1,462 codons, GRIP1 is the probable ortholog of the subsequentl y identified human protein transcription intermediary factor 2 (TIF2) and is also partially homologous to steroid receptor coactivator 1 (SR C-1), The full-length GRIP1 interacted with the hormone binding domain s (HBDs) of all five steroid receptors in a hormone-dependent manner a nd also with HBDs of class II nuclear receptors, including thyroid rec eptor alpha, vitamin D receptor, retinoic acid receptor alpha, and ret inoid X receptor alpha. In contrast to agonists, glucocorticoid antago nists did not promote interaction between the glucocorticoid receptor and GRIP1. In yeast cells, GRIP1 dramatically enhanced the transcripti onal activation function of proteins containing the HBDs of any of the above-named receptors fused to the GAL4 DNA binding domain and thus s erved as a transcriptional coactivator for them. This finding contrast s with previous reports of TIF2 and SRC-I, which in mammalian cells en hanced the transactivation activities of only a subset of the steroid and nuclear receptors that they physically interacted with. GRIP1 also enhanced the hormone-dependent transactivation activity of intact glu cocorticoid receptor, estrogen receptor, and mineralocorticoid recepto r, Experiments with glucocorticoid receptor truncation and point mutan ts indicated that GRIP1 interacted with and enhanced the activity of t he C-terminal AF-2 but not the N-terminal AF-I transactivation domain of the glucocorticoid receptor, These results demonstrate directly tha t AF-1 and AF-2 domains accomplish their transactivation activities th rough different mechanisms: AF-2 requires GRIP1 as a coactivator, but AF-I does not.