Antimicrobial Peptides Piscidin 1 and Piscidin 3 Kink at a Central Glycine to Maximize their Hydrophobic Moments

Abstract

Piscidin 1 (p1) and the less active piscidin 3 (p3) are α-helical, amphipathic, and cationic antimicrobial peptides. They adsorb onto the outer membrane of pathogenic species and induce leakage beyond a threshold peptide concentration. While different mechanisms of membrane disruption have been proposed, an atomic-level description of the events leading to cell death is lacking. Elucidating these mechanisms is critical in the design of new antibiotics. Here, the structures, orientations, and positions of p1 and p3 in two different bacterial membrane mimics (3:1 DMPC/DMPG and 1:1 POPE/POPG) are studied by solid-state NMR spectroscopy, all-atom molecular dynamics (MD) simulations, and neutron diffraction to better understand molecular determinants that contribute to antimicrobial activity. The tilts of the helical axis, τ, are perpendicular to the bilayer normal (83-93°) and the average azimuthal rotation, ρ, corresponds to burial of hydrophobic residues in the bilayer. The refined NMR and MD structures reveal a slight kink at Gly13 of the α-helix. This kink is characterized by a small difference in τ (<10°) and significant difference in ρ (∼20°) and maximizes the hydrophobic moment for the α-helix. The depths of insertion are similar in DMPC/DMPG. In contrast, p1 and p3 are 1.0 and 3.0 A less inserted in POPE/POPG. Insertion of the peptides corresponds to greater thinning of the bilayer and a decrease in the lipid order parameter. Deformation of the DMPC/DMPG bilayer is characterized by uniform membrane thinning around the peptide, while deformation in the POPE/POPG bilayer is characterized by localized thinning near the N-terminus of the peptide. The increase in thinning in DMPC/DMPG relates to a greater activity than in POPE/POPG in dye-leakage assays and may be a precursor to a more disruptive state.