Rearrangements in thyroid hormone receptor charge clusters that stabilize bound 3,5′,5-triiodo-L-thyronine and inhibit homodimer formation

In this study, we investigated how thyroid hormone (3,5′,5-triiodo-L- thyronine, T₃) inhibits binding of thyroid hormone receptor (TR) homodimers, but not TR-retinoid X receptor heterodimers, to thyroid hormone response elements. Specifically we asked why a small subset of TRβ mutations that arise in resistance to thyroid hormone syndrome inhibit both T₃ binding and formation of TRβ homodimers on thyroid hormone response elements. We reasoned that these mutations may affect structural elements involved in the coupling of T₃ binding to inhibition of TR DNA binding activity. Analysis of TR x-ray structures revealed that each of these resistance to thyroid hormone syndrome mutations affects a cluster of charged amino acids with potential for ionic bond formation between oppositely charged partners. Two clusters (1 and 2) are adjacent to the dimer surface at the junction of helices 10 and 11. Targeted mutagenesis of residues in Cluster 1 (Arg³³⁸, Lys³⁴², Asp³⁵¹, and Asp ³⁵⁵) and Cluster 2 (Arg⁴²⁹, Arg³⁸³, and Glu³¹¹) confirmed that the clusters are required for stable T ₃ binding and for optimal TR homodimer formation on DNA but also revealed that different arrangements of charged residues are needed for these effects. We propose that the charge clusters are homodimer-specific extensions of the dimer surface and further that T₃ binding promotes specific rearrangements of these surfaces that simultaneously block homodimer formation on DNA and stabilize the bound hormone. Our data yield insight into the way that T₃ regulates TR DNA binding activity and also highlight hitherto unsuspected T₃-dependent conformational changes in the receptor ligand binding domain.