MOLECULAR AND STRUCTURAL BIOLOGY OF THYROID-HORMONE RECEPTORS

1. Thyroid hormone receptors (TR) are expressed from two separate gene s (alpha and beta) and belong to the nuclear receptor superfamily, whi ch also contains receptors for steroids, vitamins and prostaglandins. 2. Unliganded TR are bound to DNA thyroid hormone response elements (T RE) predominantly as homodimers, or as heterodimers with retinoid X-re ceptors (RXR), and are associated with a complex of proteins containin g corepressor proteins, Ligand binding promotes corepressor dissociati on and binding of a coactivator.3. Recent studies from our group have focused on the acquisition and use of X-ray crystallographic structure s of ligand-binding domains (LBD) of both the rat (r) TR alpha and the human (h) TR beta bound to several different ligands. We have also de veloped ligands that bind selectively to the TR beta, which may provid e ways to explore the differential functions of TR alpha compared with TR beta isoforms, 4. The LED is comprised mostly of alpha-helices, Th e ligand is completely buried in the receptor and forms part of its hy drophobic core. Kinetic studies suggest that the limiting step in form ation of high-affinity Ligand-receptor complexes is the rate of foldin g of the receptor around the ligand, Ligands can be fitted tightly in the ligand-binding pocket and small differences in this fitting may ex plain many structure-activity relationships. Interestingly, analysis o f the structures of antagonists suggests that they have chemical group s, 'extensions', that could impair receptor folding around them and, t hus, prevent the agonist-induced conformation changes in the receptor. 5. The TR structures allowed us to see that the mutations that occur in the syndrome of generalized resistance to thyroid hormone are locat ed in the vicinity of the ligand-binding pocket. 6. X-ray structure of the TR has also been used to guide construction of mutations in the T R surface that block binding of various proteins important for recepto r function. Studies with these TR mutants reveal that the interfaces f or homo- and heterodimerization map to similar residues in helix 10 an d 11 and also allow the definition of the surface for binding of coact ivators, which appears to be general for nuclear receptors, Formation of this surface, which involves packing of helix 12 of the TR into a s caffold formed by helices 3 and 5, appears to be the major change in t he receptor structure induced by hormone occupancy.