Structure of tuberoinfundibular peptide of 39 residues.

The recently identified natural peptide ligand, tuberoinfundibular peptide of 39 residues (TIP39) for the parathyroid hormone-2 (PTH2) receptor has been structurally characterized by high resolution NMR, circular dichroism, and computer simulations. The structural features of TIP39, determined in the presence of a zwitterionic lipid to mimic the membrane environment of the G-protein-coupled PTH2 receptor, consist of two alpha-helices, Ala(5)-Arg(21) and Leu(26)-Val(35). Although TIP39 shares limited sequence homology with parathyroid hormone (PTH), a comparison of the structural features of TIP39 and PTH illustrates a similar topological display of residues of the N-terminal helix important for PTH2 receptor activation. The C-terminal helix of TIP39 differs from that of PTH with respect to size and amphipathicity, suggesting an altered mode of binding for TIP39, consistent with the receptor chimera and ligand truncation studies presented in the accompanying paper (Hoare, S. R. J., Clark, J. A., and Usdin, T. B. (2000) J. Biol. Chem. 275, 27274-27283). The structural characterization of TIP39 also provides some insight into the lack of affinity of this novel ligand for the PTH1 receptor.