Mode of Action of an Antimicrobial Peptide, Tachyplesin I, on Biomembranes

Abstract

An antimicrobial peptide, tachyplesin I, isolated from hemocytes of the horseshoe crab, Tachypleus tridentatus, increased the K⁺ permeability of Staphylococcus aureus and Escherichia coli cells, concomitantly reducing cell viability. At a higher concentration range, this peptide also enhanced the permeability of human erythrocytes. Tachyplesin decreased the phase transition temperature of an artificial membrane composed of dipalmitoylphosphatidylglycerol and, further, broadened it extensively, while it did not affect that of dipalmitoylphosphatidylcholine membrane. The latter result related closely to the fact that this peptide acted weakly on erythrocytes in which acidic lipids constitute a minor portion. Tachyplesin altered the normal discoid shape of human erythrocytes to a crenated form, suggesting that the peptide accumulated predominantly in the outer half of the membrane bilayer and destabilized the membrane structure, thus causing the change in permeability. The mode of action of tachyplesin was compared with that of gramicidin S, a peptide forming an amphiphilic structure analogous to tachyplesin.