THE CONSERVED 9TH C-TERMINAL HEPTAD IN THYROID-HORMONE AND RETINOIC ACID RECEPTORS MEDIATES DIVERSE RESPONSES BY AFFECTING HETERODIMER BUT NOT HOMODIMER FORMATION

The receptors for thyroid hormone (T3R), all-trans-retinoic acid (RAR) , and 9-cis-retinoic acid (RXR) bind DNA response elements as homo- an d heterodimers. The ligand-binding domains of these receptors contain nine conserved heptads proposed to play a role in dimerization. Mutant receptors with changes in the first or last hydrophobic amino acids i n the highly conserved ninth heptad of chick T3Ralpha (cT3Ralpha) [cT3 R(L365R) and cT3R(L372R)] and human RARalpha (hRARalpha) [hRAR(M377R) and hRAR(L384R)] reveal that this heptad is essential for certain hete rodimeric interactions and for diverse functional activities. Without ligands, wild-type receptors form both homodimers and heterodimers, wh ile these mutants form only homodimers. Surprisingly, the cognate liga nd for each mutant enables heterodimer formation between cT3R(L365R) a nd RAR or RXR and between hRAR(M377R) and T3R or RXR. Both cT3R(L365R) and hRAR(M377R) mediate ligand-dependent transcriptional regulation. However, unlike the wild-type receptor, non-ligand-associated cT3R(L36 5R) does not suppress the basal activity of certain promoters containi ng thyroid hormone response elements, suggesting that this silencing e ffect of T3R is mediated by unliganded heterodimers of T3R and endogen ous RXR or related factors. Heterodimerization is also necessary for t he strong ligand-independent inhibition between T3R and RAR on a commo n response element, since the ninth-heptad mutants function as poor in hibitors. However, with a T3R-specific response element, hRAR(M377R) a cts as a retinoic acid-dependent inhibitor of cT3R, indicating the imp ortance of heterodimerization for this inhibition. Our studies also su ggest that the ninth heptad is necessary for the dominant inhibition o f wild-type T3Rs by mutant T3Rs, as has been found for the thyroid hor mone-resistant syndrome in humans. Thus, the ninth heptad repeat is re quired for heterodimerization, suppression of basal promoter activity, and dominant negative effects of T3R and RAR. Lastly, the finding tha t cT3R(L365R) and hRAR(M377R) require ligands for heterodimer formatio n also raises the possibility that heterodimeric interactions are medi ated by the ninth heptad without ligands but by a second region of the se receptors with ligands.