Formation of ion channels in planar lipid bilayer membranes by synthetic basic peptides

We made use of a planar lipid bilayer system to examine the action of synthetic basic peptides which model the prepiece moiety of mitochondrial protein precursors and have antibacterial activity against Gram-positive bacteria. The sequences of the peptides used were as follows: Ac-(Ala-Arg-Leu)3-NHCH3 (3(3], Ac-(Leu-Ala-Arg-Leu)2-NHCH3 (4(2], Ac-(Leu-Ala-Arg-Leu)3-NHCH3 (4(3], Ac-(Leu-Leu-Ala-Arg-Leu)2-NHCH3 (5(2]. These peptides interacted differently with planar lipid bilayer membranes and membrane conductance increased by the formation of ion channels. The effects of the peptides on the macroscopic current-increase and on the probability of channel formation, at the single channel level were in the order of 4(3) greater than 4(2) approximately 5(2) much greater than 3(3), a finding which correlates with the antibacterial activity of these peptides. The micromolar (microM) order concentration at which the channel was formed resembles that causing antibacterial activity. Thus, the peptide antibacterial activity may occur through an increase in ion permeability of the bacterial membrane. The single-channel properties were investigated in detail using 4(3), the peptide with the highest ion channel-forming activity. Many types of channels were observed with respect to conductance (2-750 pS) and voltage dependency of gating. However, the channels were all cation-selective. These results suggest that the ion channels formed by peptide 4(3) may be able to take on a variety of conformations and/or assembly.