Inclusion of Menaquinone in Lipid Membranes Decreases Susceptibility to Antimicrobial Peptides

Abstract

Most studies on the interaction of antimicrobial peptides with lipid bilayers have used unsaturated, fluid-state phospholipids to model bacterial membranes. However, unsaturated lipids are rarely found in cell membranes of gram-(+) bacteria, including Staphylococcus aureus. To maintain cell membrane function, S. aureus uses fully saturated but terminally branched lipid acyl chains, which have a significantly lower phase transition temperature than their linear counterparts. We found that lipid vesicles composed of synthetic branched phospholipids are much more susceptible to attack by an amphipathic peptide than lipid vesicles composed of unsaturated phospholipids with the same headgroup composition. By contrast, natural membrane lipid extracts from S. aureus produce lipid vesicles that are as resistant to membrane-active peptides as those produced using synthetic, unsaturated lipids. We postulated that an unidentified component present in the S. aureus lipid extracts serves to stabilize the bacterial cytoplasmic membrane. A possible candidate is menaquinone, which participates in the bacterial electron transport chain but could, in addition, have a structural effect on the lipid bilayer. In the work presented here, we investigated the kinetics of dye efflux from lipid vesicles containing between 1 and 5 mol% menaquinone, induced by the antimicrobial peptide PMAP-23. PMAP-23 is a 23 amino acid, linear peptide of the cathelicidin family with the sequence RIIDLLWRVRRPQKPKFVTVWVR. We found that the presence of physiological concentrations of menaquinone had a notable effect on peptide-induce dye efflux.