Role of the conserved C-terminal region of thyroid hormone receptor- α in ligand-dependent transcriptional activation

Abstract

The ligand binding domain (LBD) of thyroid hormone (T3) receptors contains subdomains that participate in transcriptional activation, hormone-relieved repression and dimerization. A sequence conserved within the nuclear receptor superfamily is found at positions 397–405 of the 408-amino acid chicken T3 receptor- α (cTR α) and is deleted in the related avian v-erbA. Since v-erbA exhibits compromised ligand binding and transcriptional activation, this conserved region may play a role in ligand-dependent transcriptional activation. Transfections reveal that cTR α(1–392) and site-directed mutants cTR α(L398R) and cTR α(F399E) are inactive, while cTR α(1–403) displays reduced ligand-dependent transcriptional activity. The loss of transcriptional activity in cTR α(1–392) is not caused by impaired DNA binding or receptor dimer formation. Proteolytic protection assays reveal that both transcriptionally active and inactive cTR α derivatives undergo T3-mediated conformational changes. Gal4 chimeras containing the final 16, 35 or 44 amino acids of cTR α indicate that the conserved C-terminal region does not function as an independent transactivation domain. Our results are consistent with a model in which ligand plays a structural role to position the conserved C-terminal regions of cTR α and related receptors in a transcriptionally active conformation.