HYPELCIN A, AN ALPHA-AMINOISOBUTYRIC-ACID CONTAINING ANTIBIOTIC PEPTIDE, INDUCED FUSION OF EGG YOLK-L-ALPHA-PHOSPHATIDYLCHOLINE SMALL UNILAMELLAR VESICLES

Hypelcin A, an alpha-aminoisobutyric acid-containing antibiotic peptide inducing fusion of egg yolk-L-alpha-phosphatidylcholine (egg PC) small unilamellar vesicles (SUVs), was investigated by lipid-mixing assay based on resonance-energy transfer between fluorescent probes, electron microscopy, light scattering, and H-1-nuclear magnetic-resonance spectroscopy. At a high peptide-to-lipid ratio of approximately 1:5, the peptide fuses several SUVs of 20-30 nm in diameter into a 40-100 nm vesicle. Under mild conditions where the permeability enhancement (leakage of a trapped fluorescent dye, calcein) of lipid bilayers are observed (peptide to lipid ratios around 1/100), the fusion of the SUVs also occurs, although the fusion requires a somewhat larger amount of the peptide than the leakage does. Furthermore, at higher lipid concentrations, where the aggregation step is sufficiently rapid, the fusion rate is determined by the amount of the membrane bound peptide per lipid molecule, as is the leakage rate. In contrast, for egg PC large unilamellar vesicles (110 nm), hypelcin A induces the leakage, but not the fusion. We conclude that the leakage is not due to the fusion.