Antagonist recognition of a G protein-coupled receptor

The neuropetide TIP 39 (Tuberoinfundibular peptide comprising 39 residues) is a potent and specific agonist for human parathyroid hormone 2 receptor (PTH2R). The PTH2R shares 70% amino acid sequence homology with the well characterized parathyroid hormone 1 receptor (PTH1R) [1;2]. Both PTH1R and PTH2R belong to class B G-protein coupled receptors (GPCR) mediating cellular signaling. It has been shown that TIP 39 interacts to the PTH1R with moderate affinity as an antagonist [3]. However, these studies did not reveal the structural mechanism of interaction. To explore the structural features, we used fluorescent spectroscopy, circular dichroism (CD), isothermal titration calorimetry (ITC), and nuclear magnetic resonance spectroscopy (NMR). We could purify unlabeled and NMR isotopic labeled TIP 39 as SUMO-fusion protein [4;5] and assigned the resonances by standard NMR experiments. Our preliminary results show that the TIP 39 interacts to the extracellular domain of PTH1R in the lower micro molar range. Understanding these interactions on a molecular level has therapeutic implications for the current treatment of diseases such as osteoporosis and certain forms of hypercalcemia which accrue by disorder in calcium metabolism [6].