Residues in the α subunit of human choriotropin that are important for interaction with the lutropin receptor

Synthetic peptides were used to probe the structure-function relationships between human choriotropin (hCG) and the lutropin (LH) receptor. Previously, a peptide region of the α subunit of hCG, residues 26-46, had been shown to inhibit binding of ¹²⁵I-hCG to the LH receptor in rat ovarian membranes (Charlesworth, M. C., McCormick, D. J., Madden, B., and Ryan, R. J. (1987) J. Biol. Chem. 262, 13409-13416). To determine which residues are important for this inhibitory activity, peptides were truncated from either the amino or carboxyl terminus, or individual residues were substituted with alanine. The amino-terminal boundary was determined to be Gly-30 and the carboxyl-terminal boundary, Lys-44. This core peptide contained all the residues needed for full activity of the parent peptide 26-46. Arg-35 and Phe-33 were particularly important residues; when they were substituted with alanine, the peptide inhibitory potencies were decreased. Ser-43, Arg-42, Cys-32, and Cys-31 were also important but to a lesser degree. These results are consistent with predictions based on chemical and enzymatic modification studies and provide insight into which residues are important for interaction between hCG and the LH receptor.