Position-dependent hydrophobicity of the antimicrobial magainin peptide affects the mode of peptide-lipid interactions and selective toxicity

Cationic antimicrobial peptides are promising candidates as novel antibiotics of clinical usefulness. Magainin 2, a representative antimicrobial peptide isolated from the skin of the African clawed frog Xenopus leavis, electrostatically recognizes anionic lipids that are abundant in bacterial membranes, forming a peptide-lipid supramolecular complex pore, whereas the peptide does not effectively bind to zwitterionic phospholipids constituting the outer leaflets of mammalian cell membranes because of the low hydrophobicity of the peptide [Matsuzaki, K. (1999) Biochim. Biophys. Acta 1462, 1-10]. In this study, two magainin analogues with enhanced hydrophobicity, MG-H1 (GIKKFLHIIWKFIKAFVGEIMNS) and MG-H2 (IIKKFLHSIWKFGKAFVGEIMNI), with identical amino acid compositions were designed and interactions with lipid bilayers and biological activities were examined in comparison with those of MG (GIGKWLHSAKKFGKAFVGEIMNS = F5W-magainin 2). The apparent hydrophobicities and hydrophobic moments of MG-H1 and MG-H2, conventionally calculated assuming that all residues are involved in helix formation, were almost the same. MG-H2 behaved like MG except for greatly enhanced activity against zwitterionic membranes and erythrocytes. In contrast, despite a very similar calculated hydrophobicity, the observed hydrophobicity of MG-H1 was larger than that of MG-H2 because of a tendency toward helix fraying near the termini. Therefore, the physicochemical parameters of only the helical portion should be considered in characterizing peptide-lipid interactions, although this point was overlooked in most studies. Moreover, MG-H1 induced aggregation and/or fusion of negatively charged membranes. Furthermore, the peptide hydrophobicity was found to affect pore formation rate, pore size, and pore stability. These observations demonstrate that the hydrophobicity of the peptide also controls the mode of action and is dependent on the position of the hydrophobic amino acids in the peptide sequence.