Further Studies of Amino Acids (268–304) in Thyrotropin (TSH)-Lutropin/Chorionic Gonadotropin (LH/CG) Receptor Chimeras: Cysteine-301 Is Important in TSH Binding and Receptor Tertiary Structure

Our previous study of chimeric TSH-LH/CG receptors showed that substituting amino acid residues 268–304 of the TSH receptor with homologous residues from the LH/CG receptor markedly decreased high affinity TSH binding as evidenced by ligand displacement assays [Akamizu et al. Endocr J 40:363–372, 1993]. Despite this change in TSH binding, there was a minimal change in TSH-stimulated activity in cAMP assays. To explain this dissociation of TSH binding and function, further characterization of individual residues within the 268–304 segment was performed. Five additional chimeric TSH-LH/CG receptors within this region were constructed by substituting corresponding residues of rat LH/CG receptor. After transfection in Cos-7 cells, TSH receptor activities of these chimeras were evaluated. A single point mutation of cysteine-301 to glutamate resulted in a mutant receptor that exhibited the same receptor bioactivity as the chimeric receptor with amino acids 268–304 substituted by LH/CG receptor residues: apparent low affinity TSH binding in ligand displacement assays but significant retention of the cAMP response to TSH. The four other chimeric substitutions yielded cDNAs coding for receptors that behaved like wild-type receptors. Western blot analyses using a TSH receptor-specific antibody showed that all five of the new mutants were synthesized and integrated in the membrane. These results suggest that cysteine-301 is the critical residue whose mutation accounts for the original chimeric results and the dissociation of binding and functional activity. The possibility must be considered that cysteine-301 is involved in disulfide bond formation and is important in tertiary structure and that conformational changes in the receptor result from its mutation.