Experimental evidence for predicted transmembrane peptide topography: incorporation of hydrophobic peptide alpha-helical rods with an N-terminal positive charge having a length comparable to the thickness of lipid bilayers into the membranes

Liposomes consisting of egg yolk phosphatidylcholine and hydrophobic peptides Nps- and Cl-.+H2-(Met-Met-Leu)n-OEt (n = 6-10) with various polypeptide chain lengths were prepared by the sonication method. The conformation of the peptides incorporated into the liposomes was examined by CD spectroscopy. All the peptides incorporated assumed alpha-helical conformation. Quantitative analyses of the peptides and lipids in the membranes showed that the concentration of the peptides with a positive charge at the N-terminus in the liposomes decreased markedly as the peptide chain length increased, reaching zero for the peptides over n = 8. The peptides without a positive charge were hardly incorporated into the liposomes. Infrared attenuated reflection spectroscopy of multilayered membranes containing the peptides suggests that the axis of the alpha-helical peptide rods is oriented in parallel with the molecular axis of lipids in the membranes. These results suggest that the hydrophobic peptides can be incorporated into the lipid bilayers of the liposomes in the alpha-helical conformation, the rods of which have a length comparable to the thickness of the lipid bilayers, and the N-terminal positive charge of the peptides is essential for the stable peptide incorporated into the membranes.