FUNCTIONAL EVIDENCE FOR LIGAND-DEPENDENT DISSOCIATION OF THYROID-HORMONE AND RETINOIC ACID RECEPTORS FROM AN INHIBITORY CELLULAR FACTOR

The ligand-binding domains of thyroid hormone (L-triiodothyronine [T3] ) receptors (T3Rs), all-trans retinoic acid (RA) receptors (RARs), and 9-cis RA receptors (RARs and RXRs) contain a series of heptad motifs thought to be important for dimeric interactions. Using a chimera cont aining amino acids 120 to 392 of chicken T3R alpha (cT3R alpha) positi oned between the DNA-binding domain of the yeast GAL4 protein and the potent 90-amino-acid transactivating domain of the herpes simplex,viru s VP16 protein (GAL-T3R-VP16), we provide functional evidence that bin ding of ligand releases T3Rs and RARs from an inhibitory cellular fact or. GAL4-T3R-VP16 does not bind T3 and does not activate transcription from a GAL4 reporter when expressed alone but is able to activate tra nscription when coexpressed with unliganded T3R or RAR. This activatio n is reversed by T3 or RA, suggesting that these receptors compete wit h GAL4-T3R-VP16 for a cellular inhibitor and that ligand reverses this effect by dissociating T3R or R;UI from the inhibitor. A chimera cont aining the entire ligand-binding domain of cT3R alpha (amino acids 120 to 408) linked to VP16 [G;AL4-T3R(JOS)-VP16] is activated by unligand ed receptor as well as by T3. In contrast, GAL4-T3R containing the ami no acid 120 to 408 ligand-binding region without the VP16 domain is ac tivated only by T3. The highly conserved ninth heptad, which is involv ed in heterodimerization, appears to participate in the receptor-inhib itor interaction, suggesting that the inhibitor is a related member of the receptor gene family. In striking contrast to T3R and RAR, RXR ac tivates GAL4-T3R-VP16 only with its ligand, 9-cis Ri, but unliganded R XR does not appear to be the inhibitor suggested by these studies. Fur ther evidence that an orphan receptor may be the inhibitor comes from our finding that COUP-TF inhibits activation of GAL4-T3R-VP16 by unlig anded T3R and the activation of GAL4-T3R by T3. These and other result s suggest that an inhibitory factor suppresses transactivation by the T3Rs and RARs while these receptors are bound to DNA and that ligands act, in part, by inactivating or promoting dissociation of a receptor- inhibitor complex.