Differential Recognition of Liganded and Unliganded Thyroid Hormone Receptor by Retinoid X Receptor Regulates Transcriptional Repression

Abstract

Thyroid hormone receptor (TR) functions as part of multiprotein complexes that also include retinoid X receptor (RXR) and transcriptional coregulators. We have found that both the TR CoR box and ninth heptad are required for RXR interaction and in turn for interaction with corepressor proteins N-CoR and SMRT. Remarkably, the recruitment of RXR to repression-defective CoR box and ninth-heptad mutants via a heterologous dimerization interface restores both corepressor interaction and repression. The addition of thyroid hormone obviates the CoR box requirement for RXR interaction, provided that the AF2 activation helix at the C terminus of TR is intact. These results indicate that RXR differentially recognizes the unliganded and liganded conformations of TR and that these differences appear to play a major role in the recruitment of corepressors to TR-RXR heterodimers.Thyroid hormone receptor (TR) is a multifunctional protein. It is a transcriptional repressor in the absence of ligand and a transcriptional activator in the presence of thyroid hormone (T3). Repression is mediated by interaction with a family of corepressor proteins, including N-CoR and SMRT (10,17,34,45). A ligand-induced conformational change causes dissociation of the corepressor and recruitment of a transcriptional coactivator to the DNA-bound TR. TR binds DNA most effectively as a heterodimer with retinoid X receptor (RXR) (6,19,22,23,25,44,47). Although RXR itself is a retinoid receptor (24), its main function in TR action does not appear to require retinoid binding (14). The TR-RXR interaction is stable in solution in both the presence and absence of ligand, and it has previously been suggested that the primary role of RXR is to increase the affinity and specificity of TR for T3 response elements, which often consist of two TR half-sites separated by 4 bp (39).The primary region of importance for the TR-RXR interaction in solution is the region which is generally known as the ninth heptad, corresponding to helices 10 and 11 of the crystal structure of the TR ligand binding domain (LBD) (40). The importance of this domain explains why C-terminal deletions of TR and the C-terminal variant TR␣2 do not interact with RXR in solution (31, 42). Interestingly, although the interaction between TR and RXR is ligand independent, two groups have previously described TR ninth-heptad point mutants which interact with RXR only in the presence of T3 (1, 28). Although the TR LBD structure has been solved only in the presence of ligand, a comparison with the unliganded RXR suggests that one of the effects of ligand binding is conformational change in the region between helices 10 and 11 in addition to the turning back of the amphipathic AF2 helix (helix 12) towards the core of the TR (5, 40).In contrast to the importance of C-terminal domains for RXR interaction, previous studies of TR interaction with NCoR and SMRT have focused on the CoR box within the hinge region of TR (10, 17). Like the ninth heptad, the CoR box is highly conserved among receptors that int...

Functional Evidence for Retinoid X Receptor (RXR) as a Nonsilent Partner in the Thyroid Hormone Receptor/RXR Heterodimer

“…Interestingly, a study by Zhang et al showed that the Gal4 fusion with a TR hinge mutant fails to repress (54), a result also consistent with the notion that the hinge region mutation leads to a structural alteration in the LBD (42). However, cotransfection of Gal4-RXR restores the repression function of the mutant TR, although Gal4-RXR alone does not lead to repression (54). This finding was interpreted in a model in which efficient corepressor binding engages surfaces contributed by both TR and RXR (54).…”

“…1C). The notion that the apo-RXR LBD does not associate tightly with the inhibitor/corepressor is supported by the findings that a Gal4-RXR LBD fusion mediates little repression (43,54) and that a Gal4-RXR (LBD)-VP16 chimera is constitutively active (43).…”

Vitamin D-Dependent Recruitment of Corepressors to Vitamin D/Retinoid X Receptor Heterodimers

“…Many nuclear receptors require heterodimerization with RXR for high-affinity DNA binding, and this receptor appears to play an active role in corepressor interaction (49). It has been proposed that sequence-specific interactions between H12 of RXR and a hydrophobic cleft of unliganded TR formed by residues in H3, H4, and H5 reposition H12 and unmask the corepressor interaction surface of RXR, allowing the unoccupied TR/RXR heterodimer to repress transcription.…”

Differences between the silencing-related properties of the extreme carboxyl-terminal regions of thyroid hormone receptors alpha 1 and beta 1

“…The stronger silencing potency of F451X was believed to be, at least in part, due to its increased ability to interact with RXR and SMRT. It has been reported that RXR potentiates and stabilizes TR-co-repressor interaction in solution (Zhang et al 1997). However, F451X did not increase the ability of TR to associate with N-CoR, despite the enhanced heterodimerization with RXR ( Fig.…”