High-resolution structures and orientations of antimicrobial peptides piscidin 1 and piscidin 3 in fluid bilayers reveal tilting, kinking, and bilayer immersion.

While antimicrobial peptides (AMPs) have been widely investigated as potential therapeutics, high-resolution structures obtained under biologically relevant conditions are lacking. Here, the high-resolution structures of the homologous 22-residue long AMPs piscidin 1 (p1) and piscidin 3 (p3) are determined in fluid-phase 3:1 phosphatidylcholine/phosphatidylglycerol (PC/PG) and 1:1 phosphatidylethanolamine/phosphatidylglycerol (PE/PG) bilayers to identify molecular features important for membrane destabilization in bacterial cell membrane mimics. Structural refinement of (1)H-(15)N dipolar couplings and (15)N chemical shifts measured by oriented sample solid-state NMR and all-atom molecular dynamics (MD) simulations provide structural and orientational information of high precision and accuracy about these interfacially bound α-helical peptides. The tilt of the helical axis, τ, is between 83° and 93° with respect to the bilayer normal for all systems and analysis methods. The average azimuthal rotation, ρ, is 235°, which results in burial of hydrophobic residues in the bilayer. The refined NMR and MD structures reveal a slight kink at G13 that delineates two helical segments characterized by a small difference in their τ angles (<10°) and significant difference in their ρ angles (~25°). Remarkably, the kink, at the end of a G(X)4G motif highly conserved among members of the piscidin family, allows p1 and p3 to adopt ρ angles that maximize their hydrophobic moments. Two structural features differentiate the more potent p1 from p3: p1 has a larger ρ angle and less N-terminal fraying. The peptides have comparable depths of insertion in PC/PG, but p3 is 1.2 Å more deeply inserted than p1 in PE/PG. In contrast to the ideal α-helical structures typically assumed in mechanistic models of AMPs, p1 and p3 adopt disrupted α-helical backbones that correct for differences in the amphipathicity of their N- and C-ends, and their centers of mass lie ~1.2-3.6 Å below the plane defined by the C2 atoms of the lipid acyl chains.

Complementary Effects of Host Defense Peptides Piscidin 1 and Piscidin 3 on DNA and Lipid Membranes: Biophysical Insights into Contrasting Biological Activities.

Piscidins were the first antimicrobial peptides discovered in the mast cells of vertebrates. While two family members, piscidin 1 (p1) and piscidin 3 (p3), have highly similar sequences and α-helical structures when bound to model membranes, p1 generally exhibits stronger antimicrobial and hemolytic activity than p3 for reasons that remain elusive. In this study, we combine activity assays and biophysical methods to investigate the mechanisms underlying the cellular function and differing biological potencies of these peptides, and report findings spanning three major facets. First, added to Gram-positive (Bacillus megaterium) and Gram-negative (Escherichia coli) bacteria at sublethal concentrations and imaged by confocal microscopy, both p1 and p3 translocate across cell membranes and colocalize with nucleoids. In E. coli, translocation is accompanied by nonlethal permeabilization that features more pronounced leakage for p1. Second, p1 is also more disruptive than p3 to bacterial model membranes, as quantified by a dye-leakage assay and (2)H solid-state NMR-monitored lipid acyl chain order parameters. Oriented CD studies in the same bilayers show that, beyond a critical peptide concentration, both peptides transition from a surface-bound state to a tilted orientation. Third, gel retardation experiments and CD-monitored titrations on isolated DNA demonstrate that both peptides bind DNA but p3 has stronger condensing effects. Notably, solid-state NMR reveals that the peptides are α-helical when bound to DNA. Overall, these studies identify two polyreactive piscidin isoforms that bind phosphate-containing targets in a poised amphipathic α-helical conformation, disrupt bacterial membranes, and access the intracellular constituents of target cells. Remarkably, the two isoforms have complementary effects; p1 is more membrane active, while p3 has stronger DNA-condensing effects. Subtle differences in their physicochemical properties are highlighted to help explain their contrasting activities.