Thyroxine-thyroid hormone receptor interactions.
Thyroid hormone (TH) actions are mediated by nuclear receptors (TRs alpha and beta) that bind triiodothyronine (T(3), 3,5,3'-triiodo-l-thyronine) with high affinity, and its precursor thyroxine (T(4), 3,5,3',5'-tetraiodo-l-thyronine) with lower affinity. T(4) contains a bulky 5' iodine group absent from T(3). 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 T(4) affects TR activity and conformation. We obtained several lines of evidence (ligand dissociation kinetics, migration on hydrophobic interaction columns, and non-denaturing gels) that TR-T(4) complexes adopt a conformation that differs from TR-T(3) complexes in solution. Nonetheless, T(4) behaves as an agonist in vitro (in effects on coregulator and DNA binding) and in cells, when conversion to T(3) does not contribute to agonist activity. We determined x-ray crystal structures of the TRbeta LBD in complex with T(3) and T(4) at 2.5-A and 3.1-A resolution. Comparison of the structures reveals that TRbeta accommodates T(4) 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 T(3) is the major active TH, our results suggest that T(4) could activate nuclear TRs at appropriate concentrations. The ability of TR to adapt to the 5' extension should be considered in TR ligand design.
Metabolic Research Unit and Diabetes Center, School of Medicine, University of California-San Francisco, 513 Parnassus Avenue, San Francisco, CA 94122-0540, USA.
Ben
B
Sandler
Paul
P
Webb
James W
JW
Apriletti
B Russell
BR
Huber
Marie
M
Togashi
Suzana T
ST
Cunha Lima
Sanja
S
Juric
Stefan
S
Nilsson
Richard
R
Wagner
Robert J
RJ
Fletterick
John D
JD
Baxter
D000818:Animals; D016466:CHO Cells; D002845:Chromatography; D006224:Cricetinae; D018360:Crystallography, X-Ray; D004247:DNA / Q000737:chemistry; D019281:Dimerization; D004305:Dose-Response Relationship, Drug; D004591:Electrophoresis, Polyacrylamide Gel; D005982:Glutathione Transferase / Q000378:metabolism; D007455:Iodine / Q000737:chemistry; D007700:Kinetics; D008024:Ligands; D008956:Models, Chemical; D008958:Models, Molecular; D011485:Protein Binding; D011487:Protein Conformation; D017434:Protein Structure, Tertiary; D011988:Receptors, Thyroid Hormone / Q000737:chemistry*; D013974:Thyroxine / Q000737:chemistry*; D013997:Time Factors; D014162:Transfection; D014284:Triiodothyronine / Q000737:chemistry
D008024:Ligands; D011988:Receptors, Thyroid Hormone; D014284:Triiodothyronine; D004247:DNA; D007455:Iodine; D005982:Glutathione Transferase; D013974:Thyroxine