Characterization of parathyroid hormone/receptor interactions: structure of the first extracellular loop.

The structural features of the first extracellular loop (ECL1) of the parathyroid hormone receptor (PTH1R) in the presence of dodecylphosphocholine micelles have been determined using high-resolution NMR techniques. The structure of the receptor fragment, PTH1R(241-285), includes three alpha-helices for residues 241-244, 256-264, and 275-284. The first and third correspond to the end and the beginning of transmembrane helices 2 and 3, respectively. Centrally located in the second helix is L(261), found to cross-link to Lys(27) of parathyroid hormone, PTH(1-34) [Greenberg, Z., Bisello, A., Mierke, D. F., Rosenblatt, M., and Chorev, M. (2000) Biochemistry 39, 8142-8152]. On the basis of nitroxide radical-induced relaxation studies, the central helix is found to associate with the surface of the membrane mimetic. These data, in conjunction with previous results indicating a preference of PTH for the lipid surface, suggest a membrane-associated pathway for the initial recognition and binding of PTH to its G-protein-coupled receptor. Using the structural features of ECL1 as determined here, along with the structure of the PTH(1-34), the intermolecular interactions consistent with the contact point between L(261)(receptor)-Lys(27)(ligand) are identified.