Modified Bacterial Lipids Which Alter Membrane Surface Charge Reduce Binding of Antimicrobial Peptides

Antimicrobial peptides (AMPs) initiate killing of bacteria by binding to and destabilizing their membranes. The multiple peptide resistance factor (MprF) provides a defence mechanism for bacteria against a broad range of AMPs. MprF reduces the negative charge of both Gram-positive and Gram--negative bacterial membranes through enzymatic conversion of the anionic lipid phosphatidyl glycerol (PG) to either zwitterionic alanyl-phosphatidyl glycerol (Ala-PG) or cationic lysylphosphatidyl glycerol (Lys-PG). The resulting change in membrane charge is suggested to reduce AMP-membrane binding and hinder downstream AMP activity. Using molecular dynamics to investigate the effects of these modified lipids on AMP-binding to model membranes, we show that AMPs have substantially reduced affinity for model membranes containing Ala-PG or Lys-PG. A total of ~7000 simulations are used to define the relationship between bilayer composition and binding for 5 different membrane active peptides. The reduction of degree of interaction of a peptide with the membrane is shown to correlate with the change in membrane surface charge density. Free energy profile (potential of mean force) calculations reveal that these lipid modifications alter the energy barrier to peptide helix penetration of the bilayer. These results will enable us to guide design of novel peptides which address the issue of resistance via MprF-mediated membrane modification.