Interactions of the Novel Antimicrobial Peptide Buforin 2 with Lipid Bilayers: Proline as a Translocation Promoting Factor

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Abstract
Buforin 2 is an antimicrobial peptide discovered in the stomach tissue of the Asian toad Bufo bufo gargarizans. The 21-residue peptide with +6 net charge shows antimicrobial activity an order of magnitude higher than that of magainin 2, a membrane-permeabilizing antimicrobial peptide from Xenopus laevis [Park, C. B., Kim, M. S., and Kim, S. C. (1996) Biochem. Biophys. Res. Commun. 218, 408−413]. In this study, we investigated the interactions of buforin 2 with phospholipid bilayers in comparison with magainin 2 to obtain insight into the mechanism of action of buforin 2. Equipotent Trp-substituted peptides were used to fluorometrically monitor peptide−lipid interactions. Circular dichroism measurements showed that buforin 2 selectively bound to liposomes composed of acidic phospholipids, assuming a secondary structure similar to that in trifluoroethanol/water, which is an amphipathic helix distorted around Pro11 with a flexible N-terminal region [Yi, G. S., Park, C. B., Kim, S. C., and Cheong, C. (1996) FEBS Lett. 398, 87−90]. Magainin 2 induced the leakage of a fluorescent dye entrapped within lipid vesicles coupled to lipid flip-flop. These results have been interpreted as the formation of a peptide−lipid supramolecular complex pore [Matsuzaki, K. (1998) Biochim. Biophys. Acta 1376, 391−400]. Buforin 2 exhibited much weaker membrane permeabilization activity despite its higher antimicrobial activity. In contrast, buforin 2 was more efficiently translocated across lipid bilayers than magainin 2. These results suggested that the ultimate target of buforin 2 is not the membrane but intracellular components. Furthermore, buforin 2 induced no lipid flip-flop, indicating that the mechanism of translocation of buforin 2 is different from that of magainin 2. The role of Pro was investigated by use of a P11A derivative of buforin 2. The derivation caused a change to magainin 2-like secondary structure and membrane behavior. Pro11 was found to be a very important structural factor for the unique properties of buforin 2.