Influence of truncation of avian β-defensin-4 on biological activity and peptide-membrane interaction

Defensins are important components in host defense systems. The therapeutic use of β-defensins is limited by their innate toxicity and high cost due to the size and complex disulfide pairing. In this study, we used linear avian β-defensin-4 (RL38) without disulfide bonds as model peptide to derive two peptides by the truncation. GL23 is the Cterminal truncated sequence of RL38, and GLI23 is the derivative of GL23 by the replacement of cysteines with isoleucines. Results showed that these peptides exhibited strong antibacterial activity against gram-negative and gram-positive bacteria. An exception was that GL23 showed weak antimicrobial activity against gallinaceous pathogenic bacteria Salmonella Pullorum C79-13. Two truncated peptides GL23 and GLI23 displayed no or weak hemolysis, which was in accordance with little blue shifts of the peptides in the presence of synthetic eukaryotic membranes. CD spectroscopy demonstrated that these peptides appeared to be unfolded in aqueous solution but acquire structure in the presence of membrane-mimicking phospholipids. GLI23 kept the antibacterial activity at high concentrations of NaCl or low concentration of divalent cations (Mg²⁺ and Ca ²⁺). The peptides preferentially bound to negatively charged phospholipids over zwitterionic phospholipids, which led to greater cell selectivity. The outer and inner membranes assay displayed that GLI23 killed bacteria by targeting the cell membrane. These results suggest the peptides derived by truncation of linear β-defensins may be a promising candidate for future antibacterial agent.