SPECIFIC MUTATIONS IN THE LIGAND-BINDING DOMAIN SELECTIVELY ABOLISH THE SILENCING FUNCTION OF HUMAN THYROID-HORMONE RECEPTOR-BETA

Although most nuclear hormone receptors are ligand-dependent transcrip tional activators, certain members of this superfamily, such as thyroi d hormone receptor (TR) and retinoic acid receptor (RAR), are involved in transcriptional repression. The silencing function of these recept ors has been localized to the ligand binding domain (LBD). Previously, we demonstrated that overexpression of either the entire LBD or only the N-terminal region of the LBD (amino acids 168-259) is able to inhi bit the silencing activity of TR. From this result we postulated the e xistence of a limiting factor (corepressor) that is necessary for TR s ilencing activity. To support this hypothesis, we identified amino aci ds in the N-terminal region of the LBD of TR that are important for th e corepressor interaction and for the silencing function of TR. The si lencing activity of TR was unaffected by overexpression of the LED of mutant TR (V174A/D177A), suggesting that valine at position 174 and/or aspartic acid at position 177 are important for corepressor interacti on. This mutant receptor protein, V174/D177, also lost the ability to silence target genes, suggesting that these amino acids are important for silencing function. Control experiments indicate that this mutant TR maintains its wild-type hormone binding and transactivation functio ns. These findings further strengthen the idea that the N-terminal reg ion of the LBD of TR interacts with a putative corepressor protein(s) to achieve silencing of basal gene transcription.