The Effects of Membrane Curvature on the High-Resolution Structures of Membrane-Bound Antimicrobial Piscidin 1 and Piscidin 3

First discovered in the mast cells of hybrid striped sea bass, piscidin 1 and piscidin 3 (p1 and p3) are two isoforms of the piscidin family of antimicrobial, cationic, and amphipathic peptides (ACAPs) that have demonstrated broad spectrum activity against both Gram-positive and Gram-negative bacteria. Although recently identified as being active against HIV-1, p3 has lower antimicrobial and hemolytic activity than p1 in general. In the presence of membrane-mimetic lipid bilayers, both p1 and p3 adopt cationic, amphipathic, alpha-helical structures, which are critical for the specificity of membrane binding and subsequent cell lysis. However, the precise mechanism of action and the structural factors that lead to differences in activities of these peptides remain unclear.Previously, the structure of p1 and p3 was elucidated in 3:1 DMPC: DMPG bilayers that mimic the charge of the E. coli cell membrane while reducing membrane curvature. Here, 1:1 POPE: POPG phospholipid bilayers were used since this composition closely mimics the negative charge and curvature of the phospholipid membranes of Bacillus cereus. Oriented bilayer samples of 1:20 peptide:lipid were analyzed using high-definition solid state NMR, which yields 15N-1H dipolar coupling as well as 15N and 1H chemical shifts for each labeled residue. This data provided structural constraints for the computational determination of a high definition atomic structure of each isoform at the water-bilayer interface.By comparing the structural differences of p1 and p3 in various membrane curvatures to the activities of the peptides against bacteria with the same membrane composition, specific structural moieties that are critical to the antimicrobial activity can be determined. Understanding the particular interactions between the peptide with the membrane that lead to cell death is an important step for designing novel antibiotic pharmaceuticals.