Contribution of the hydrophobicity gradient to the secondary structure andactivity of fusogenic peptides

Fusogenic peptides belong to a class of helical amphipathic peptides charac terized by a hydrophobicity gradient along the long helical axis. According to the prevailing theory regarding the mechanism of action of fusogenic pe ptides, this hydrophobicity gradient causes the tilted insertion of the pep tides in membranes, thus destabilizing the lipid core and, thereby, enhanci ng membrane fusion. To assess the role of the hydrophobicity gradient upon the fusogenic activity, two of these fusogenic peptides and several variant s 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 remai ns predominantly helical in membranes. The variant peptides were designed t hrough amino acid permutations, to be either parallel, perpendicular, or to retain an oblique orientation relative to the lipid-water interface. Pepti de-induced vesicle fusion was monitored by lipid-mixing experiments, using fluorescent probes, the extent of peptide-lipid association, the conformati on of lipid-associated peptides and their orientation in lipids, were studi ed by Fourier Transformed Infrared Spectroscopy. A comparison of the proper ties of the wild-type and variant peptides shows that the hydrophobicity gr adient, which determines the orientation of helical peptides in lipids and their fusogenic activity, further influences the secondary structure and li pid binding capacity of these peptides.