Conformational changes of human beta 1 thyroid hormone receptor induced by binding of 3,3',5-triiodo-L-thyronine.

To understand the structural basis in the hormone-dependent transcriptional regulation of human beta 1 thyroid hormone receptor (h-TR beta 1), we studied the conformational changes of h-TR beta 1 induced by binding of 3,3',5-triiodo-L-thyronine (T3). h-TR beta 1 was treated with trypsin alone or in the presence of T3, thyroid hormone response element (TRE) or T3 together with TREs. Without T3, h-TR beta 1 was completely digested by trypsin. Binding of TREs had no effect on the tryptic digestion pattern. However, T3-bound h-TR beta 1 became resistant to tryptic digestion and yielded trypsin-resistant peptide fragments with molecular weight of 28,000 and 24,000. Chymotryptic digestion also yielded a T3-protected 24 Kd peptide fragment. Using anti-h-TR beta 1 antibodies and amino acid sequencing, the 28 Kd fragment was identified to be Ser202-Asp456. The 24 Kd tryptic fragments were found to be Lys239-Asp456 and Phe240-Asp456. The 24 Kd chymotryptic fragment was identified to be Lys235-Asp456. The structural changes as a result of T3 binding could serve as a transducing signal to modulate the gene regulating activity of h-TR beta 1.