Characterization of Membrane Pores Formed by Cationic Amphipathic α-Helical Antimicrobial Peptides

Abstract

Antimicrobial peptides can kill bacteria by permeabilizing their membranes. One proposed mechanism is by forming a membrane-spanning toroidal pore, where peptides in a transmembrane orientation are lining the pore. It is hard to study such transient pores unless conditions to stabilize the pore are found. We have recently shown that pore formation is promoted and pores stabilized by lipids with a positive spontaneous curvature, like lyso-lipids [1,2]. Using KIA peptides with a repetitive amino acid sequence KIAGKIA of different length, we observed that the amphipathic α-helices can insert into DMPC membranes in the presence of lyso-MPC, and that the tilt angle of the peptides depend on hydrophobic matching between the peptide length and the membrane thickness, i.e., that longer peptides are more tilted for a given bilayer thickness [3,4]. We used solid-state 31P-, 15N- and 2H-NMR on the cationic antimicrobial peptide MSI-103 (also called KIA21) to characterize the pore in detail. The minimum concentration of peptide and lyso-lipid for pore formation was determined, and the effect of charges on the normal lipid or the lyso-lipid was studied. Our results show that pore formation is enhanced in the presence of anionic lyso-MPG compared to neutral lyso-MPC, indicating that a toroidal wormhole pore, enriched in lyso-lipids, is indeed formed. We also found that pore formation is affected by terminal charges of the peptides: charged residues are improving activity when located at the N-terminus but reducing it at the C-terminus. This is discussed in terms of the 3D hydrophobic moment [5] of the peptides.