Molecular basis for membrane selectivity of NK-2, a potent peptide antibiotic derived from NK-lysin.

Increasing resistance of pathogenic bacteria against antibiotics is a severe problem in health care. Natural antimicrobial peptides and derivatives thereof have emerged as promising candidates for "new antibiotics". In contrast to classical antibiotics, these peptides act by direct physical destabilization of the target cell membrane. Nevertheless, they exhibit a high specificity for bacteria over mammalian cells. However, the precise mechanism of action and the molecular basis for membrane selectivity are still a matter of debate. We have designed a new peptide antibiotic (NK-2) with enhanced antimicrobial activity based on an effector protein of mammalian immune cells (NK-lysin). Here we describe the interaction of this alpha-helical synthetic peptide with membrane mimetic systems, designed to mimic the lipid compositions of mammalian and bacterial cytoplasmic membranes. Utilizing fluorescence and biosensor assays, we could show that on one hand, NK-2 strongly interacts with negatively charged membranes; on the other hand, NK-2 is able to discriminate, without the necessity of negative charges, between the zwitterionic phospholipids phosphatidylethanolamine (PE) and phosphatidylcholine (PC), the major constituents of the outer leaflet of the cytoplasmic membranes of bacteria and mammalian cells, respectively.