Enhancing the membrane activity of Piscidin 1 through peptide metallation and the presence of oxidized lipid species: Implications for the unification of host defense mechanisms at lipid membranes.

Piscidins are host-defense peptides (HDPs) from fish that exhibit antimicrobial, antiviral, anti-cancer, anti-inflammatory, and wound-healing properties. They are distinctively rich in histidine and contain an amino terminal copper and nickel (ATCUN) binding motif due to the presence of a conserved histidine at position 3. Metallation lowers their total charge and provides a redox center for the formation of radicals that can convert unsaturated fatty acids (UFAs) into membrane-destabilizing oxidized phospholipids (OxPLs). Here, we focus on P1, a particularly membrane-active isoform, and investigate how metallating it and making OxPL available influence its membrane activity. First, we quantify through dye leakage experiments the permeabilization of the apo- and holo-forms of P1 on model membranes containing a fixed ratio of anionic phosphatidylglycerol (PG) and zwitterionic phosphatidylcholine (PC) but varying amounts of Aldo-PC, an OxPL derived from the degradation of several UFAs. Remarkably, metallating P1 increases membranolysis by a factor of five in each lipid system. Conversely, making Aldo-PC available improves permeabilization by a factor of two for each peptide form. Second, we demonstrate through CD-monitored titrations that the strength of the peptide-membrane interactions is similar in PC/PG and PC/PG/Aldo-PC. Thus, peptide-induced membrane activity is boosted by properties intrinsic to the peptide (e.g., charge and structural changes associated with metallation) and bilayer (e.g., reversal of sn-2 chain due to oxidation). Third, we show using oriented-sample 15N solid-state NMR that the helical portion of P1 lies parallel to the bilayer surface in both lipid systems. 31P NMR experiments show that both the apo- and holo-states interact more readily with PC in PC/PG. However, the presence of Aldo-PC renders the holo-, but not the apo-state, more specific to PG. Hence, the membrane disruptive effects of P1 and its specificity for the anionic lipids found on pathogenic cell membrane surfaces are simultaneously optimized when it is metallated and the OxPL is present. Overall, this study deepens our insights into how OxPLs affect peptide-lipid interactions and how host defense metallopeptides could help integrate the effects of antimicrobial agents.