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Protein-Protein Interaction Domains and the Heterodimerization of Thyroid Hormone Receptor Variant 2 with Retinoid X Receptors

Division of Endocrinology and Metabolism University of Michigan Medical Center Ann Arbor, Michigan 48109-0678

Heterodimerization between thyroid hormone receptors (TRs) and retinoid X receptors (RXRs) is mediated by a weak dimerization interface within the DNA- binding domains (DBDs) and a strong interface within the C-terminal ligand- binding domains of the receptors. Previous studies have shown that the conserved ninth heptad in the TR ligand-binding domain appears to play a critical role in heterodimerization with RXR. However, despite lacking the full ninth heptad, TR variant 2 (TRv2) can heterodimerize with RXR on specific direct repeat response elements, but not on palindromic elements or in solution. Two possibilities may account for TRv2-RXR heterodimerization on direct repeats. First, the DBD of TRv2 may play a critical role in heterodimerization with RXR. Second, a specific sequence within the unique C terminus of TRv2 may promote the formation of TRv2-RXR heterodimers. In this study, we used receptor chimeras in which the DBD of RXR was replaced by either the TR DBD or an unrelated DBD from the metalloregulatory transcription factor AMT1 to address the role of the DBD dimerization interface in TRv2-RXR heterodimerization. Gel mobility shift analyses showed that whereas TR1 formed heterodimers with these chimeras, TRv2 failed to do so. Deletion of the unique C terminus of TRv2 had only a marginal effect on heterodimerization with RXR. Mutations within the DBD dimerization interface abolished heterodimerization of full-length TRv2 with RXR but only marginally affected heterodimerization of full-length TR1 with RXR. These data support the hypothesis that the TR-RXR DBD dimerization interface plays a critical role in TRv2-RXR heterodimerization. Additional data show that the amino acid residues that make direct TR-RXR contacts within the DBDs also may play a role in receptor monomer binding to DNA, since mutations within these residues severely impair this interaction.

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				P. M. Yen Physiological and Molecular Basis of Thyroid Hormone Action Physiol Rev, July 1, 2001; 81(3): 1097 - 1142. [Abstract] [Full Text] [PDF]