Asymmetrical ion-channel model inferred from two-dimensional crystallization of a peptide antibiotic.

The structural organization of ion channels formed in lipid membranes by amphiphilic alpha-helical peptides is deduced by applying direct structural methods to different lipid/alamethicin systems. Alamethicin represents a hydrophobic alpha-helical peptide antibiotic forming voltage-gated ion channels in lipid membranes. Here the first direct evidence for the existence of large-scale two-dimensional crystalline domains of alamethicin helices, oriented parallel to the air/water interface, is presented using synchrotron x-ray diffraction, fluorescence microscopy, and surface pressure/area isotherms. Proofs are obtained that the antibiotic peptide injected into the aqueous phase under phospholipid monolayers penetrates these monolayers, phase separates, and forms domains within the lipid environment, keeping the same, parallel orientation of the alpha-helices with respect to the phospholipid/water interface. A new asymmetrical, "lipid-covered ring" model of the voltage-gated ion channel of alamethicin is inferred from the structural results presented, and the mechanism of ion-channel formation is discussed.