Impact of Aminoacylated Phospholipids on the Interactions of Antimicrobial Peptides with Phospholipid Vesicles

Aminoacylated phosphatidylglycerols (PG) are common lipids in the cytoplasmic membranes of Gram-positive bacteria. Their presence in staphylococcal membranes has been linked to increased resistance to a number of antibacterial agents, including antimicrobial peptides. Most commonly, the PG headgroup is esterified to lysine, which converts anionic PG into a cationic lipid with a considerably increased headgroup size. In the present work, we investigated the interactions of two well-studied antimicrobial peptides, cecropin A and mastoparan X, with lipid vesicles composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG), containing varying fractions of an aminoacylated phosphatidylethanolamine (PE), a stable analog of the corresponding PG-derivative. To differentiate between the effects of headgroup charge and size on peptide-lipid interactions, we synthesized two different derivatives. In one, the headgroup was modified by the addition of lysine, and in the other, by glutamine. The modification by glutamine results in a phospholipid with a headgroup size comparable to that of lysylated version. However, whereas lysyl-PE is cationic, glutamyl-PE is zwitterionic. We found that as long as the concentration of aminoacylated PEs did not exceed ∼30 mol%, binding of either peptide was not significantly altered. These observations are understood through the interplay between lipid charge and headgroup size and their effect on membrane binding and peptide-induced release of content from lipid vesicles.