ARGININE-186 IN THE EXTRACELLULAR N-TERMINAL REGION OF THE HUMAN PARATHYROID-HORMONE-1 RECEPTOR IS ESSENTIAL FOR CONTACT WITH POSITION-13 OF THE HORMONE

PTH maintains blood calcium concentrations within the physiological ra nge by acting on a G protein-coupled heptahelical receptor (PTH1 Re) l ocated primarily in cells in bone and kidney. We have undertaken a pho toaffinity cross-linking approach to elucidate the nature of the bimol ecular interaction of PTH with the human (h) PTH1 Re. Specifically, we have studied the region of the receptor that interacts with the midre gion of PTH-(1-34), position 13, using a benzophenone-containing photo affinity ligand, I-125-[Nle(8,18), Lys(13)(epsilon-pBz(2)), L-2-Nal(23 ),Arg(26,27),Tyr(34)]bPTH-(1-34)NH2 (I-125-K13), Using site-directed m utagenesis in combination with biochemical analysis, we have reduced o ur previously identified contact domain, 17 residues in the extracellu lar region of the receptor (173-189), to an 8-amino acid domain (182-1 89). Furthermore, we have found arginine 186 to be of critical importa nce to the interaction of the hPTH1 Re with (125I)-K13: modification o f Arg(186) to either lysine or alanine does not modify receptor avidit y or signal transduction by the receptor, but eliminates cross-linking to I-125-K13.