DIFFERENTIAL SCANNING MICROCALORIMETRY INDICATES THAT HUMAN DEFENSIN,HNP-2, INTERACTS SPECIFICALLY WITH BIOMEMBRANE MIMETIC SYSTEMS

alpha-Defensins are antimicrobial peptides with 29-35 amino acid resid ues and cysteine-stabilized amphiphilic, triple-stranded beta-sheet st ructures. We used high-precision differential scanning microcalorimetr y to investigate the effects of a human neutrophil alpha-defensin, HNP -2, on the phase behavior of model membranes mimicking bacterial and e rythrocyte cell membranes. In the presence of this positively charged peptide, the phase behavior of liposomes containing negatively charged phosphatidylglycerol was markedly altered even at a high lipid-to-pep tide molar ratio of 500:1. Addition of HNP-2 to liposomes mimicking ba cterial membranes (mixtures of dipalmitoylphosphatidylglycerol and eth anolamine) resulted in phase separation owing to some domains being pe ptide-poor and others peptide-rich. The latter are characterized by an increase of the main transition temperature, most likely arising from electric shielding of the phospholipid headgroups by the peptide. On the other hand, HNP-2 did not affect the phase behavior of membranes m imicking erythrocyte membranes (equimolar mixtures of dipalmitoylphosp hatidylcholine and sphingomyelin) as well as the pure single component s. This is in contrast to melittin, which significantly affected the p hase behavior of choline phospholipids in accordance with its unspecif ic lytic activity. These results support the hypothesis of preferentia l interaction of defensins with negatively charged membrane cell surfa ces, a common feature of bacterial cell membranes, and demonstrate tha t HNP-2 discriminates between model membrane systems mimicking prokary otic and eukaryotic cell membranes.