TRAM-1, A NOVEL 160-KDA THYROID-HORMONE RECEPTOR ACTIVATOR MOLECULE, EXHIBITS DISTINCT PROPERTIES FROM STEROID-RECEPTOR COACTIVATOR-1

Nuclear hormone receptors (NRs) are ligand-dependent transcription fac tors that regulate target gene transcription, We report the molecular cloning and characterization of a novel human cDNA encoding TRAM-1, a thyroid hormone receptor activator molecule, a similar to 160-kDa prot ein homologous with SRC-1/TIF2, by far-Western-based expression screen ing. TRAM-1 binds to thyroid hormone receptor (TR) and other NRs in a ligand-dependent manner and enhances ligand-induced transcriptional ac tivity of TR. The AF-2 region in NRs has been thought to play a critic al role in mediating ligand-dependent transactivation by the interacti on with coactivators. Surprisingly, TRAM-1 retains strong ligand-depen dent interaction with an AF-2 mutant of TR (E457A), while SRC-1 fails to interact with this mutant. Furthermore, we identified a critical TR AM-1 binding site in rat TR beta 1 outside of AF-2, as TRAM-1 shows we ak ligand-dependent interaction with a helix 3 ligand binding domain T R mutant (K288A), compared with SRC-1. These results suggest that TRAM -1 is a coactivator that may exhibit its activity by interacting with subdomains of NRs other than the AF-2 region, in contrast to SRC-1/TIF 2.