A biophysical study of the interaction of the lipopeptide antibiotic iturin A with aqueous phospholipid bilayers

Iturin A is a lipopeptide extracted from the culture media of Bacillus subt ilis which shows a strong antifungal action. The interaction of iturin A wi th multilamellar vesicles of dimyristoylphosphatidylcholine (DMPC) induced structures which did not sediment during centrifugation. Electron microscop y after negative staining showed that, at 30 mol%, iturin A/DMPC vesicles w ere visible but smaller than those formed by pure DMPC. Thermograms of DMPC /iturinA obtained after differential scanning calorimetry, at low concentra tions of iturin A, were interpreted as indicating the presence of two later ally separated phases, one formed by pure phospholipid and the other by lip opeptide-phospholipid complexes, these two separated phases being already d etected even at low concentrations such as 2 mol%. Fluorescence quenching e xperiments showed that the D-Tyr residue of the lipopeptide was fully acces sible to the aqueous medium, indicating that the polar part of iturin A is located outside of the membrane hydrophobic palisade. It was concluded that the membrane barrier properties are Likely to be damaged in the area where the lipid complexes are accumulated, due to structural fluctuations, and t his may be one of the bases of its biological activity. Iturin-A was also a ble to greatly destabilize dielaidoylphosphatidylethanolamine (DEPE) membra nes in the fluid form, producing a new structure which had a poor correlati on in X-ray diffraction, and in P-31 NMR spectroscopy gave rise to a spectr um containing a double isotropic signal. Iturin A was shown to induce DEPE to adopt phases other than Hn inverted hexagonal, underlining that this lip opeptide is capable of modifying the curvature of the membrane, which may a lso be important in explaining the tendency of iturin A to create small ves icles and which may be another of the bases of its biological activity. (C) 2000 Academic Press.