Examination of the role of the amphipathic .alpha.-helix in the interaction of neuropeptide Y and active cyclic analogs with cell membrane receptors and dimyristoylphosphatidylcholine

Abstract

To test the potential importance of the putative C-terminal amphipathic .alpha.-helical region of neuropeptide Y (NPY) in receptor binding, the interactions of porcine NPY and several peptide analogues with lipid and cell membrane receptors were compared. Cyclic analogues were designed to constrain the N- and C-terminal regions of the peptide and to retain the folded conformation of NPY predicted from its sequence analogy with pancreatic polypeptide and its similar spectral behavior. The three cyclic peptides were [Cys2, 8-aminooctanoic acid5-24, D-Cys27]-NPY (C2-NPY), [Cys5, 8-aminooctanoic acid7-20, D-Cys24]-NPY (C5-NPY), and [D-Cys7, 8-aminooctanoic acid8-17, Cys20]-NPY (C7-NPY). All of the peptides bind with high affinity to pig spleen membranes, but only NPY and [Glu16, Ser18, Ala22, Leu28,31]-NPY (ESALL-NPY) bind quantitatively to dimyristoylphosphatidylcholine (DPMPC) liposomes. C7-NPY and NPY20-36 bind with moderate affinity to liposomes, but only NPY and C7-NPY bind with high affinity to mouse brain receptors. Thus, lipid binding and receptor binding are not correlated in this series of peptides, and binding to the pig spleen receptor appears to require only the C-terminal region of the peptide. Simple lipid binding, as in NPY20-36, is sufficient for binding to the mouse brain receptor, suggesting that the N-terminal region of the peptide is required for high-affinity binding to this receptor. Data from fluorescence, differential scanning calorimetry, and liposome cleaning experiments suggest that, although the interaction of NPY with lipid is consistent with formation of an amphipathic .alpha.-helix, a simple amphipathic .alpha.-helical model for the interaction with the high-affinity NPY receptor is insufficient to explain the data. Rather, the data suggest that the amphipathic .alpha.-helix in NPY plays a significant role in stabilizing the spatial relationship between the N- and C-terminal residues of the peptide when bound to its receptor.