The Effect of Aminoacylated Phospholipids on Membrane Binding of the Antimicrobial Peptides Cecropin a and Mastoparan X

Abstract

Aminoacylated phosphatidylglycerols are common lipids in the cytoplasmic membranes of Gram-positive bacteria. Their presence in staphylococcal cytoplasmic membranes has been linked to increased resistance to antimicrobial peptides. We showed previously that physiological lysyl-phosphatidylglycerol (lysyl-PG) concentrations did not reduce membrane binding of a synthetic peptide modeled on the C-terminal microbicidal domain of the mammalian platelet factor-4. Here, we addressed two additional questions. First, we investigated if that behavior is observed only for 6W-RP-1 or also for other antimicrobial peptides. Second, we explored the importance of the charge of the aminoacylated-phospholipid on peptide binding to lipid membranes. Binding of cecropin A and mastoparan X to unilamellar lipid vesicles composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG) was measured as a function of the concentration of an aminoacylated phosphatidylethanolamine (PE), a stable analog of the corresponding PG-derivative. Cecropin A and mastoparan X are well characterized antimicrobial peptides that form amphipathic α-helices when bound at the membrane-water interface. Peptide binding was measured through fluorescence energy transfer from the intrinsic tryptophan residue in the peptides to an acceptor fluorophore embedded in the membrane at low concentrations. Two aminoacylated PEs were used to explore the effect of lipid charge on peptide binding: lysyl-PE, a cationic phospholipid, and glutamyl-PE, a zwitterionic derivative. We found that as long as the concentration of aminoacylated PEs did not exceed 30 mol%, binding of both peptides was not significantly altered. In an attempt to understand these observations, we discuss the interplay between lipid charge and headgroup size on the activity of antimicrobial peptides.