Structure of the hormone binding domain of human beta 1 thyroid hormone nuclear receptor: is it an alpha/beta barrel?

To understand the structure of the hormone binding domain (HBD) of human beta 1 thyroid hormone nuclear receptor (h-TR beta 1), truncated h-TR beta 1 fragments, MD32 (M169-D456), KD29 (K201-D456), DD28 (D211-D456), KD25 (K235-D456), and KP28 (K201-P448), were analyzed by circular dichorism (CD). MD32 and KD29 show intense CD spectra with double minima at 222 and 208-210 nm, indicating the presence of extensive regions of alpha-helix. DD28 and KD25 have spectra which are reduced in intensity with minima around 215 nm, characteristic of a beta-sheet. The observed spectra are compatible with sequence analysis which predicts that HBD contains alternating stretches of alpha-helix and beta-strand. These extensive decreases in secondary structure in DD28 and KP28 in which the predicted first beta-strand or last alpha-helix was deleted, respectively, were accompanied by the loss of hormone binding activity. On the basis of these results, we suggest a new model for h-TR beta 1 consisting of the known DNA binding domain linked by an alpha-helical hinge to the HBD, with the tertiary structure of an alpha/beta barrel. The model is compatible with previous chemical and genetic studies on the structure of this protein.