GRIP1, A NOVEL MOUSE PROTEIN THAT SERVES AS A TRANSCRIPTIONAL COACTIVATOR IN YEAST FOR THE HORMONE-BINDING DOMAINS OF STEROID-RECEPTORS

The yeast two-hybrid system was used to isolate a clone from a 17-day- old mouse embryo cDNA library that codes for a novel 812-aa long prote in fragment, glucocorticoid receptor-interacting protein 1 (GRIP1), th at can interact with the hormone binding domain (HBD) of the glucocort icoid receptor. In the yeast two-hybrid system and in vitro, GRIP1 int eracted with the HBDs of the glucocorticoid, estrogen, and androgen re ceptors in a hormone-regulated manner. When fused to the DNA binding d omain of a heterologous protein, the GRIP1 fragment activated a report er gene containing a suitable enhancer site in yeast cells and in mamm alian cells, indicating that GRIP1 contains a transcriptional activati on domain. Overexpression of the GRIP1 fragment in mammalian cells int erfered with hormone-regulated expression of mouse mammary tumor virus -chloramphenicol acyltransferase gene and constitutive expression of c ytomegalovirus-beta-galactosidase reporter gene, but not constitutive expression from a tRNA gene promoter. This selective squelching activi ty suggests that GRIP1 can interact with an essential component of the RNA polymerase II transcription machinery. Finally, while a steroid r eceptor HBD fused with a GAL4 DNA binding domain did not, by itself, a ctivate transcription of a reporter gene in yeast, coexpression of thi s fusion protein with GRIP1 strongly activated the reporter gene. Thus , in yeast, GRIP1 can serve as a coactivator, potentiating the transac tivation functions in steroid receptor HBDs, possibly by acting as a b ridge between HBDs of the receptors and the basal transcription machin ery.