MOLECULAR-CLONING AND PROPERTIES OF A FULL-LENGTH PUTATIVE THYROID-HORMONE RECEPTOR COACTIVATOR

Thyroid hormone receptors (TRs) are ligand-dependent transcription fac tors that regulate target gene transcription( The conserved carboxy-te rminal region of the ligand-binding domain (AF-2) has been thought to play a critical role in mediating ligand-dependent transactivation by the interaction with coactivator(s). Using bacterially-expressed TR as a probe, far-Western-based expression cDNA library screening identifi ed cDNAs that encode, in part, the recently reported partial steroid r eceptor coactivator-1 (SRC-1) sequence. Additional work, including 5' RACE, has characterized a full-length cDNA that encodes a similar to 1 60 kD protein asa putative thyroid hormone receptor coactivator (F-SRC -1), In vitro binding studies show that F-SRC-1 binds to a variety of nuclear hormone receptors in a ligand-dependent manner, along with TBP and TFIIB, suggesting that F-SRC-1 may play a role as a bridging mole cule between nuclear hormone receptors and general transcription facto rs. Interestingly AF-2 mutants also retain ligand-dependent interactio n with F-SRC-1. Although F-SRC-1 recognizes the ligand-induced conform ational changes of nuclear hormone receptors, our observations suggest that F-SRC-1 may bind directly with subregion(s) in nuclear hormone r eceptors other than the AF-2 region.