MECHANISM OF SYNERGISM BETWEEN ANTIMICROBIAL PEPTIDES MAGAININ-2 AND PGLA

The antimicrobial peptides magainin 2 and PGLa, discovered in the skin of the African clawed frog, Xenopus laevis, exhibit marked synergism [Westerhoff, H. V., Zasloff, M., Rosner, J. L., Hendler, R. W., de Waa l, A., Vat Gomes, A., Jongsma, A. P. M., Riethorst, A., and Juretic, D ., fur. J. Biochem. 228, 257-264 (1995)], although the mechanism is no r yet clear. They are believed to kill bacteria by permeabilizing memb ranes. In this study, we examined the interactions of these peptides i n lipid bilayers. PGLa, like magainin 2, preferentially interacts with acidic lipids, forming an amphipathic helix. The peptide induces the release of a water-soluble dye, calcein, entrapped within liposomes. T he coexistence of magainin 2 enhances membrane permeabilization, which is maximal at a 1:1 molar ratio. Fluorescence experiments using L18W- PGLa revealed that both peptides form a stoichiometric 1:1 complex in the membrane phase with an association free energy of -15 kJ/mol. Sing le amino acid mutations in magainin 2 significantly altered the synerg istic activity, suggesting that precise molecular recognition is invol ved in complex formation. The complex as well. as each component pepti de form peptide-lipid supramolecular complex pores, which mediate the mutually coupled transbilayer transport of dye, lipid, and the peptide per se. The rate of pore formation rate is in the order complex great er than or equal to PGLa > magainin 2, whereas the pore lifetime is in the order magainin 2 > complex > PGLa. Therefore, the synergism is a consequence of the formation of a potent heterosupramolecular complex, which is characterized by fast pore formation and moderate pore stabi lity.