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.