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.