ISOLATION AND CHARACTERIZATION OF A NOVEL LIGAND-DEPENDENT THYROID-HORMONE RECEPTOR-COACTIVATING PROTEIN

The thyroid hormone receptor (TR) regulates the expression of target g enes upon binding to triiodothyro nine (T-3) response elements, In the presence of T-3, the TR recruits coactivating proteins that both modu late and integrate the ligand response, We report here the cloning of a novel protein using the TR ligand-binding domain as bait in the yeas t two-hybrid system. Analysis of a putative full length clone demonstr ates a cDNA sequence that encodes a protein of 920 amino acids with a size of 120 kDa (p120), Alignment with known sequences shows homology to a previously identified protein of unknown function, termed skeleta l muscle abundant protein, Interaction studies demonstrate that p120 i nteracts with the TR AF-2 domain in the presence of ligand through a 1 11-amino acid region. Northern analysis demonstrates widespread expres sion in human tissues. Cotransfection assays in CV-1 cells demonstrate that p120 enhances TR-mediated transactivation on multiple T-3 respon se elements in the presence of T-3. In addition, CREB-binding protein synergizes with p120 to enhance this effect. When linked to the GAL4 D NA-bind ing domain, p120 is an activator of transcription alone. Thus, p120 satisfies a number of important criteria as a nuclear receptor c oactivator.