Molecular characterization of a ligand-tethered parathyroid hormone receptor.

It was recently shown that the covalent tethering of the N-terminus of parathyroid hormone (PTH) to the seventh helical bundle of the G-protein coupled PTH-receptor (PTH1R) leads to autoactivation [Shimizu et al., J. Biol. Chem. 275 (2000) 19456-19460]. Here, we have developed molecular models for the interaction of PTH(1-11) tethered to PTH1R and refined them with molecular dynamics simulations. The starting structure of the ligand/receptor complex is based on experimental data from a series of spectroscopic structural studies of PTH(1-34) and the extracellular domains of PTH1R and intermolecular contact points derived from photoaffinity labeling. The resulting PTH1R/[Arg(11)]PTH(1-11) complex has the N-terminus of PTH interacting with residues of the third extracellular loop of PTH1R, as a possible mode for receptor activation. The hydrophobic residues leucine-5 and methionine-8, centrally located in the N-terminal alpha-helix of PTH(1-11), are located in deep, well-defined hydrophobic pockets in the central core of the seventh helical bundle, consistent with the requirement of these amino acids for autoactivation. We postulate that the improved signaling properties of [Arg(11)]PTH(1-11) over wild type PTH(1-11) is due to a stable hydrogen bond between Arg(11) and E444, at the beginning of TM7. The model provides atomic insight into currently available biochemical data as well as numerous putative ligand/receptor interactions, and thereby may further the rational design of reduced-size PTH agonists at the PTH1 receptor.