Delineation of amino acid residues within hTSHr 256-275 that participate in hormone binding

The amino acid sequence 256-275 of the human thyrotropin (TSH) receptor extracellular domain has previously been shown to participate in a high affinity TSH binding site by a synthetic peptide approach as well as by site- directed mutagenesis. To further investigate this binding site, we synthesized a series of peptides with alanine substitutions for each residue in the native sequence. Peptides were also synthesized containing truncations or deletions of the native sequence. Each peptide was tested for its ability to inhibit ¹²⁵I-bTSH binding to porcine thyroid membrane preparations, and the concentration at which 50% inhibition of binding occurred was determined (EC₅₀). Alanine substitution at residues Tyr³⁵⁸, Cys²⁶², Cys²⁶³, Phe²⁶⁵, Lys²⁶⁶, Asn²⁶⁷, Lys²⁶⁹, Lys²⁷⁰, and Arg²⁷² all resulted in statistically significant decreases in activity when compared to the native sequence (p < 0.05). Alanine substitution of the remaining residues did not alter their activity. Comparison of this sequence with the corresponding sequences of the remaining glycoprotein hormone receptors (human lutropin and human follitropin receptors) reveals that these residues lie within one of the most highly conserved regions of the extracellular domain. We conclude that 9 specific amino acids within the sequence 256-275 of hTSHr (-Y-CC-FKN-KK-R-) participate in the interaction of the hTSHr- extracellular domain with TSH. This may represent a site in which the nonconserved residues are involved in the binding of the β-subunit and the conserved residues are involved in the binding of the common α-subunit or a region of the β-subunit that is common to all glycoprotein hormones.