PROTEIN-PROTEIN INTERACTION DOMAINS AND THE HETERODIMERIZATION OF THYROID-HORMONE RECEPTOR VARIANT ALPHA-2 WITH RETINOID-X-RECEPTORS

Heterodimerization between thyroid hormone receptors (TRs) and retinoi d X receptors (RXRs) is mediated by a weak dimerization interface with in the DNA-binding domains (DBDs) and a strong interface within the C- terminal ligand-binding domains of the receptors, Previous studies hav e shown that the conserved ninth heptad in the TR ligand-binding domai n appears to play a critical role in heterodimerization with RXR, Howe ver, despite lacking the full ninth heptad, TR variant alpha 2 (TRv al pha 2) can heterodimerize with RXR on specific direct repeat response elements, but not on palindromic elements or in solution. Two possibil ities may account for TRv alpha 2-RXR heterodimerization on direct rep eats. First, the DBD of TRv alpha 2 may play a critical role in hetero dimerization 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 DB D of RXR was replaced by either the TR DBD or an unrelated DBD from th e metalloregulatory transcription factor AMT1 to address the role of t he DBD dimerization interface in TRv alpha 2-RXR heterodimerization. G el mobility shift analyses showed that whereas TR alpha 1 formed heter odimers 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 het erodimerization with RXR, Mutations within the DBD dimerization interf ace abolished heterodimerization of full-length TRv alpha 2 with RXR b ut only marginally affected heterodimerization of full-length TR alpha 1 with RXR. These data support the hypothesis that the TR-RXR DBD dim erization interface plays a critical role in TRv alpha 2-RXR heterodim erization. Additional data show that the amino acid residues that make direct TR-RXR contacts within the DBDs also may play a role in recept or monomer binding to DNA, since mutations within these residues sever ely impair this interaction.