Contribution of the hydrophobicity gradient to the secondary structure and activity of fusogenic peptides

Fusogenic peptides belong to a class of helical amphipathic peptides characterized by a hydrophobicity gradient along the long helical axis. According to the prevailing theory regarding the mechanism of action of fusogenic peptides, this hydrophobicity gradient causes the tilted insertion of the peptides in membranes, thus destabilizing the lipid core and, thereby, enhancing membrane fusion. To assess the role of the hydrophobicity gradient upon the fusogenic activity, two of these fusogenic peptides and several variants were synthesized. The LCAT-(57-70) peptide, which is part of the sequence of the lipolytic enzyme lecithin cholesterol acyltransferase, forms stable beta-sheets in lipids, while the apolipoprotein A-II (53-70) peptide remains predominantly helical in membranes. The variant peptides were designed through amino acid permutations, to be either parallel, perpendicular, or to retain an oblique orientation relative to the lipid-water interface. Peptide-induced vesicle fusion was monitored by lipid-mixing experiments, using fluorescent probes, the extent of peptide-lipid association, the conformation of lipid-associated peptides and their orientation in lipids, were studied by Fourier Transformed Infrared Spectroscopy. A comparison of the properties of the wild-type and variant peptides shows that the hydrophobicity gradient, which determines the orientation of helical peptides in lipids and their fusogenic activity, further influences the secondary structure and lipid binding capacity of these peptides.