Effect of membrane potential on pore formation by the antimicrobial peptide magainin 2 in lipid bilayers

The effect of membrane potential on plasma membrane damage generated by antimicrobial peptides (AMPs) is an important, yet poorly characterized, process. Here, we studied the effect of membrane potential (φm) on pore formation by magainin 2 (Mag) in single giant unilamellar vesicles (GUVs) composed of dioleoylphosphatidylglycerol (DOPG)/dioleoylphosphatidylcholine (DOPC) membranes. Various membrane potentials in GUVs containing gramicidin A were generated as a result of K+ concentration gradients. First, we examined Mag-generated membrane permeation of the water-soluble fluorescent probe calcein in single DOPG/DOPC-GUVs in the presence of membrane potential. The results indicate that the rate constant (kp) of Mag-induced pore formation increased with increasing negative membrane potentials. Analysis of the rim intensity of single GUVs interacting with low concentrations of a fluorescent probe, carboxyfluorescein-labeled Mag (CF-Mag), using confocal laser scanning microscopy (CLSM) shows that the concentration of CF-Mag in the membrane greatly increased with negative membrane potentials. This indicates that the binding constant of CF-Mag to the membrane increased with more negative membrane potentials. To elucidate the location of Mag in a GUV with φm during Mag-induced pore formation, we examined the interaction of Mag and a low concentration of a CF-Mag mixture with single GUVs containing the water-soluble fluorescent probe AF647 using CLSM. The data indicate that CF-Mag locates in the external leaflet of single GUVs until just before pore formation. Based on these data, we conclude that the increase in the surface concentration of Mag is one of the primary causes of the increase in kp with negative membrane potential.