Structure of rat transthyretin (rTTR) complex with thyroxine at 2.5 angstrom resolution: first non-biased insight into thyroxine binding reveals different hormone orientation in two binding sites

The first observation of the unique environment for thyroxine (T-4) binding in tetrameric rat transthyretin (rTTR) is reported as determined by X-ray diffraction. These data revealed different modes of hormone binding in the two unique hormone-binding sites in the rat TTR tetramer channel. Differenc es in the orientation of thyroxine and the position of water molecules in t he two binding sites further suggest a mechanism for the docking pathway of the hormone into the channel of TTR. Crystals of the rat transthyretin-thy roxine complex are isomorphous with those reported for apo rTTR and crystal lized in the tetragonal space group P4(3)2(1)2 with four independent TTR mo nomeric subunits in the asymmetric part of the crystal lattice. Data were c ollected to 2.5 Angstrom resolution and the structure was refined to R = 20 .9% for 15 384 data in the resolution range 12-2.5 Angstrom. Similar to hum an TTR, the rat protein is also a 54 000 Da tetramer with four identical po lypeptide chains of 127 amino-acid residues. Of the 22 amino-acid residues which differ between the human and rat sequences, none are in the thyroxine -binding domains. Analysis of these structural data reveals that the tertia ry structure is similar to that of hTTR, with only small differences in the flexible loop regions on the surface of the structure. Conformational chan ges of the amino acids in the channel result in a hydrogen-bonded network t hat connects the two binding domains, in contrast to the hydrogen bonds for med along the tetramer interface in the apo transthyretin structure. These changes suggest a mechanism for the signal transmission between thyroxine-b inding domains.