Interaction of wheat alpha-thionin with large unilamellar vesicles

The interaction of the wheat antibacterial peptide alpha-thionin with large unilamellar vesicles has been investigated by means of fluorescence spectr oscopy. Binding of the peptide to the vesicles is followed by the release o f vesicle contents, vesicle aggregation, and lipid mixing. Vesicle fusion, i.e., mixing of the aqueous contents, was not observed. Peptide binding is governed by electrostatic interactions and shows no cooperativity. The amph ipatic nature of wheat alpha-thionin seems to destabilize the membrane bila yer and trigger the aggregation of the vesicles and lipid mixing. The prese nce of distearoylphosphatidylethanolamine-poly(ethylene glycol 2000) (PEG-P E) within the membrane provides a steric barrier that inhibits vesicle aggr egation and Lipid mixing but does not prevent leakage. Vesicle leakage thro ugh discrete membrane channels is unlikely, because the release of encapsul ated large fluorescent dextrans is very similar to that of 8-aminonaphthale ne-1,3,6,trisulfonic acid (ANTS). A minimum number of 700 peptide molecules must bind to each vesicle to produce complete leakage, which suggests a me chanism in which the overall destabilization of the membrane is due to the formation of transient pores rather than discrete channels.