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-transme mbrane receptor. Unlike most seven-transmembrane receptors, it is comp osed of two halves of equal size, the N-terminal extracellular exodoma in and the C-terminal membrane-associated endodomain. The exodomain is exclusively responsible for high affinity hormone binding, whereas re ceptor activation occurs only in the endodomain. This mutually exclusi ve physical separation of the two functional domains sets the lutropin receptor and its subfamily of receptors apart from all other seven-tr ansmembrane receptors, The mechanisms of hormone binding and receptor activation also appear to be different from those of other receptors i n that binding occurs in at least two steps. However, the precise horm one contact sites in the exodomain are unknown. To determine the hormo ne/receptor contact sites, we have examined the receptor using progres sive truncation from the C terminus, Ala scanning, immunofluorescence microscopy, and antibody binding. Progressive truncation from the C te rminus 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 t he exon 1-2 junction uncovered three alternating residues (Leu(20), Cy s(22),and Gly(24)) crucial for hormone binding. Ala substitution for a ny one of these residues abolished hormone binding, although the resul ting mutant receptors were successfully expressed on the cell surface. In contrast, Ala substitution for their flanking and intervening resi dues 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 th e hormone.