The amino-terminal region of the luteinizing hormone choriogonadotropin receptor contacts both subunits of human choriogonadotropin I. Mutational analysis

The luteinizing hormone/choriogonadotropin receptor is a seven-transmembrane receptor. Unlike most seven-transmembrane receptors, it is composed of two halves of equal size, the N-terminal extracellular exodomain and the C-terminal membrane-associated endodomain. The exodomain is exclusively responsible for high affinity hormone binding, whereas receptor activation occurs only in the endodomain. This mutually exclusive physical separation of the two functional domains sets the lutropin receptor and its subfamily of receptors apart from all other seven-transmembrane receptors, The mechanisms of hormone binding and receptor activation also appear to be different from those of other receptors in that binding occurs in at least two steps. However, the precise hormone contact sites in the exodomain are unknown. To determine the hormone/receptor contact sites, we have examined the receptor using progressive truncation from the C terminus, Ala scanning, immunofluorescence microscopy, and antibody binding. Progressive truncation from the C terminus of the receptor indicates several discrete regions that impact hormone binding. These regions are around the boundaries of exons 1-2, 4-5, 6-7, and 9-10, Ala scanning of the Asp(17)-Arg(26) region near the exon 1-2 junction uncovered three alternating residues (Leu(20), Cys(22),and Gly(24)) crucial for hormone binding. Ala substitution for any one of these residues abolished hormone binding, although the resulting mutant receptors were successfully expressed on the cell surface. In contrast, Ala substitution for their flanking and intervening residues did not impair hormone binding. These results and the data in the accompanying article (Phang, T,, Kundu, G,, Hong, S,, Ji, I., and Ji, T, (1998) J, Biol, Chem, 273, 13841-13847) indicate that this region directly contacts the hormone and suggest a novel mode of embracing the hormone.