Protein-protein interaction domains and the heterodimerization of thyroid hormone receptor variant α2 with retinoid X receptors

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 alpha 2 (TRv alpha 2) can heterodimerize with RXR on specific direct repeat response elements, but not on palindromic elements or in solution. Two possibilities may account for TRv alpha 2-RXR heterodimerization on direct repeats. First, the DBD of TRv alpha 2 may play a critical role in heterodimerization with RXR, Second, a specific sequence within the unique C terminus of TRv alpha 2 may promote the formation of TRv alpha 2-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 TRv alpha 2-RXR heterodimerization. Gel mobility shift analyses showed that whereas TR alpha 1 formed heterodimers with these chimeras, TRv alpha 2 failed to do so. Deletion of the unique C terminus of TRv alpha 2 had only a marginal effect on heterodimerization with RXR, Mutations within the DBD dimerization interface abolished heterodimerization of full-length TRv alpha 2 with RXR but only marginally affected heterodimerization of full-length TR alpha 1 with RXR. These data support the hypothesis that the TR-RXR DBD dimerization interface plays a critical role in TRv alpha 2-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.