A NATURAL TRANSACTIVATION MUTATION IN THE THYROID-HORMONE BETA-RECEPTOR - IMPAIRED INTERACTION WITH PUTATIVE TRANSCRIPTIONAL MEDIATORS

The syndrome of resistance to thyroid hormone is characterized by elev ated serum free thyroid hormones, failure to suppress pituitary thyrot ropin secretion, and variable peripheral refractoriness to hormone act ion. Here we describe a novel leucine to valine mutation in codon 454 (L454V) of the thyroid hormone beta receptor (TR beta) in this disorde r, resulting in a mutant receptor with unusual functional properties, Although the mutant protein binds ligand comparably to wild-type recep tor and forms homo- and heterodimers on direct repeat, everted repeat, or palindromic thyroid response elements, its ability to activate tra nscription via these elements is markedly impaired, The hydrophobic le ucine residue lies within an amphipathic alpha-helix at the carboxyl t erminus of TR beta and the position of the homologous residue in the c rystal structure of TR alpha indicates that its side chain is solvent- exposed and might interact with other proteins, We find that two putat ive transcriptional mediators (RIP140 and SRC-1) exhibit hormone-depen dent association with wild-type TR. In comparison, the interaction of this natural mutant (L454V) and artificial mutants (L454A, E457A) with RIP140 and SRC-1 is markedly reduced, Furthermore, coexpression of SR C-1 is able to restore the transcriptional activity of the L454V mutan t receptor, indicating that the interaction of this residue with acces sory proteins is critical for transcriptional activation, Finally, the occurrence of the L454V mutation in resistance to thyroid hormone, to gether with impaired negative regulation of the thyroid-stimulating ho rmone a promoter by this mutant, suggests that the amphipathic alpha-h elix also mediates hormone-dependent transcriptional inhibition, perha ps via interaction with these or other accessory factors.