An Investigation on the Interactions of Alamethicin with Lecithin Bilayers

The interaction of alamethicin, an extracellular macrocyclic polypeptide found in the fungus Trichoderma viride, with lipid bilayer systems in both unsonicated multilayer and sonicated bilayer vesicle states was investigated with nuclear magnetic resonance spectroscopy and electron microscopy techniques. We found that alamethicin is a surface active agent, which interacts primarily with the polar choline head groups of the lecithin. In its interaction with small sonicated bilayer vesicles (~300Å in diameter), alamethicin greatly facilitates the aggregation and subsequent fusion of these vesicles. Using europium ion, a lanthanide shift reagent, to probe the detail of this fusion process we found that during the alamethicin-mediated fusion some of the antibiotic molecules became translocated from the extravesicular aqueous medium into the intravesicular space. Alamethicin entrapped in this manner affects the choline methyl proton signals only from the inner half of the bilayer. No evidence was obtained for transmembrane coupling of the two halves of the bilayer or for effects of the incorporation of alamethicin molecules into the hydrophobic core of the bilayer in the absence of a transmembrane potential. In the presence of a potential difference across the bilayer, however, alamethicin is capable of forming ion-conducting channels. Such channels were found to be unidirectional for europium ions at least. A model for the voltage-induced formation of alamethicin pores in lipid bilayers is proposed and this model is discussed in the light of previously reported results of electrical studies on alamethicin-modified black lipid membranes.