RIP-140 interacts with multiple nuclear receptors by means of two distinct sites

We have characterized two distinct binding sites, called site 1 and site 2, in the nuclear protein RIP-140 which interact with the ligand binding domain of the estrogen receptor both in solution and when the receptor is bound to DNA. Both sites are capable of independently interacting with other nuclear receptors, including the thyroid hormone and retinoic acid receptors, but they are not identical since the interaction with retinoid X receptor is mediated primarily by site 1. The interaction is enhanced by agonists but not by antagonists, and the in vitro binding activities to a number of mutant receptors correlate with their abilities to stimulate transcription in vivo. When RIP-140 is fused to heterologous DNA binding domains, it is able to stimulate the transcription of reporter genes in both yeast and mammalian cells. Thus, RIP-140 is likely to function as a bridging protein between receptors and the basal transcription machinery and thereby stimulate the transcription of target genes.