LIGAND-DEPENDENT AND NUCLEAR FACTOR-DEPENDENT CHANGE IN HYDROPHOBICITY OF THYROID-HORMONE BETA(1) RECEPTOR

An aqueous two-phase partitioning assay was performed using in vitro t ranslated human thyroid hormone beta(1) receptor (TR beta(1)). Wild-ty pe TR beta(1) was less hydrophobic in the presence of both triiodothyr onine (T-3) and nuclear extract. This reflects a conformational change , or change in electrostatic properties, of the TR beta(1)-nuclear fac tor complex as a result of T-3 binding. Mutant TR beta(1)s with reduce d T-3 binding affinity required a higher concentration of T-3 for the shift of hydrophobicity, anti a mutant without T-3 binding activity di d not show any shift, even in the presence of 1 mM T-3. The unique mut ant receptor, R243Q, has impaired transcriptional function despite vir tually normal binding affinity for T-3. When this mutant was examined in this assay, the shift of hydrophobicity was significantly impaired even in the presence of both nuclear extract and a high concentration of T-3. Nuclear extact of COS1 cells did not affect the T-3-binding af finity of R243Q. These results indicate that the R243Q mutant has impa ired a ligand-dependent conformational change and interaction with nuc lear factor(s). Inability of R243Q to interact normally with nuclear f actor(s) may explain, in part, the molecular mechanism of discordance between ligand binding and transactivation function of this mutant.