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Determination of nuclear receptor corepressor interactions with the thyroid hormone receptor
Anita Makowski1, Sabrina Brzostek, Ronald N Cohen
1Thyroid Unit, Division of Endocrinology, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, Massachusetts 02215, USA.
Abstract:
The thyroid hormone receptor (TR) recruits the nuclear corepressors, nuclear receptor corepressor (NCoR) and silencing mediator of retinoid and thyroid hormone receptors (SMRT), to target DNA elements in the absence of ligand. While the TR preferentially recruits NCoR, the mechanism remains unclear. The corepressors interact with the TR via interacting domains (IDs) present in their C terminus which contain a conserved motif termed a CoRNR box. Despite their similarity, the corepressor IDs allow for nuclear receptor specificity. Here we demonstrate that NCoR stabilizes the TR homodimer when bound to DNA by preventing its dissociation from thyroid hormone response elements. This suggests that NCoR acts to hold the repression complex in place on target elements. The TR homodimer recruits NCoR through two of its three IDs, one of which is not present in SMRT. This unique ID, N3, contains a CoRNR box but lacks the extended helical motif present in each of the other IDs. Instead, N3 contains an isoleucine just proximal to this motif. This isoleucine is also conserved in N2 but not in the corresponding S2 domain in SMRT. On thyroid hormone response elements and in mammalian cells this residue is critical in both N3 and N2 for high-affinity TR binding. In addition, this residue also controls specificity for the interactions of TR with NCoR. Together these data suggest that the specific recruitment of NCoR by the TR through a unique motif allows for stabilization of the repression complex on target elements.
Insights
Nuclear receptor corepressor (NCoR) stabilizes thyroid hormone receptor (TR) binding to DNA, preventing complex dissociation. A unique NCoR motif ensures specific TR recruitment, maintaining transcriptional repression.
Area of Science:
- Molecular Endocrinology
- Nuclear Receptor Signaling
Background:
- Thyroid hormone receptor (TR) interacts with nuclear corepressors, nuclear receptor corepressor (NCoR) and silencing mediator of retinoid and thyroid hormone receptors (SMRT), to regulate gene expression.
- The mechanism by which TR preferentially recruits NCoR over SMRT is not fully understood, despite both corepressors interacting with TR via conserved motifs (CoRNR boxes) within their interacting domains (IDs).
Purpose of the Study:
- To elucidate the mechanism underlying the preferential recruitment of NCoR by TR.
- To investigate the role of specific interacting domains and residues in TR-corepressor complex stabilization and specificity.
Main Methods:
- Demonstrated NCoR's role in stabilizing the TR homodimer on DNA using thyroid hormone response elements (TREs).
- Identified a unique NCoR interacting domain (N3) and a critical isoleucine residue within N3 and N2.
- Assessed the impact of this isoleucine on TR binding affinity and specificity in mammalian cells.
Main Results:
- NCoR binding stabilizes the TR homodimer on TREs, preventing its dissociation and anchoring the repression complex.
- TR preferentially recruits NCoR via two interacting domains, including a unique N3 domain absent in SMRT.
- A specific isoleucine residue within N3 and N2 is crucial for high-affinity TR binding and NCoR-TR interaction specificity.
Conclusions:
- NCoR actively stabilizes the TR-DNA interaction, ensuring the integrity of the transcriptional repression complex.
- A unique motif within NCoR (N3) and a specific isoleucine residue mediate high-affinity and specific recruitment of NCoR by TR.
- These findings provide mechanistic insight into the selective recruitment of corepressors by nuclear receptors, impacting gene regulation.