Permeabilization and morphological changes in phosphatidylglycerol bilayers induced by an antimicrobial peptide, tachyplesin I

Abstract

Tachyplesin I, a broad-spectrum antimicrobial peptide fromTachypleus tridentatus has a basic (+7), amphiphilic, and cyclic β-sheet structure. We reported (Matsuzaki K. et al. (1991) Biochim. Biophys. Acta 1070:259–264) that 1) the action mechanism of tachyplesin I may be the permeabilization of bacterial membranes, 2) the peptide specifically permeabilizes acidic phospholipid bilayers, and 3) its Trp² residue is located in the hydrophobic region near the surface of the bilayers. In this paper, we found that tachyplesin I dose-dependently induces not only the permeabilization but also aggregation/fusion and micellization of the phosphatidylglycerol large unilamellar vesicles (100 nm in diameter) either in the gel (L-α-dipalmitoylphosphatidyl-DL-glycerol (DPPG)) or liquid-crystalline (egg yolk L-α-phosphatidyl-DL-glycerol (egg PG)) phase, as revealed by light scattering and electron micrograph techniques. The solid DPPG vesicles were more susceptible to the peptide. At peptide to lipid molar ratios (P/L) of 1/500 to 1/200, interpeptide interactions formed a pore through which calcein, a fluorescent dye, can leak out of the vesicles. The pore lifetime was longer in the DPPG vesicles. Further addition of the peptide caused aggregation and/or fusion of the vesicles. At a charge-neutralizingP/L ratio of 1/7, the enlarged vesicles disintegrated into small spherical particles (10–20 nm in diameter). The mechanism for these morphological changes will be discussed.