Thyroxine-thyroid hormone receptor interactions

Thyroid hormone (TH) actions are mediated by nuclear receptors (TRs α and β) that bind triiodothyronine (T₃, 3,5,3′-triiodo-L- thyronine) with high affinity, and its precursor thyroxine (T₄, 3,5,3′,5′-tetraiodo-L-thyronine) with lower affinity. T₄ contains a bulky 5′ iodine group absent from T₃. Because T ₃ is buried in the core of the ligand binding domain (LBD), we have predicted that TH analogues with 5′ substituents should fit poorly into the ligand binding pocket and perhaps behave as antagonists. We therefore examined how T₄ affects TR activity and conformation. We obtained several lines of evidence (ligand dissociation kinetics, migration on hydrophobia interaction columns, and non-denaturing gels) that TR-T₄ complexes adopt a conformation that differs from TR-T₃ complexes in solution. Nonetheless, T₄ behaves as an agonist in vitro (in effects on coregulator and DNA binding) and in cells, when conversion to T₃ does not contribute to agonist activity. We determined x-ray crystal structures of the TRβ LBD in complex with T₃ and T₄ at 2.5-Å and 3.1-Aø resolution. Comparison of the structures reveals that TRβ accommodates T₄ through subtle alterations in the loop connecting helices 11 and 12 and amino acid side chains in the pocket, which, together, enlarge a niche that permits helix 12 to pack over the 5′ iodine and complete the coactivator binding surface. While T₃ is the major active TH, our results suggest that T4 could activate nuclear TRs at appropriate concentrations. The ability of TR to adapt to the 5′ extension should be considered in TR ligand design.