Antimicrobial peptides derived from the SPLUNC1 protein perturb bacterial and eukaryotic lipid model membranes

SPLUNC1 is a host defense protein found in the human respiratory. Five novel antimicrobial peptides (AMPs) were rationally designed from SPLUNC1 with different lengths, charges, hydrophobicities (H) and hydrophobic moments ($\mu $H). The goal of this study was to compare the biological activities of these AMPs by means of testing against paired clinical isolates of the Gram-negative (G(-)) bacteria. In addition, mechanistic studies were carried out to study interactions between the AMPs and bacterial lipid model membranes utilizing x-ray diffuse scattering (XDS), circular dichroism (CD) and neutron reflectivity (NR). One of the SPLUNC1-derived AMPs, A4-112, displayed superior antibacterial activity and the lowest toxicity to mammalian cells at low peptide concentration. CD indicated A4-112 has the highest $\alpha $-helical content and the lowest $\mu $H/H ratio. NR and XDS revealed A4-112 is located primarily in the headgroup region in a G(-) model membrane with only a shallow hydrocarbon penetration. XDS revealed that A4-112's mechanism of bacterial killing could involve domain formation with leakage of ions and water along the domain walls. A4-198, with the same amino acid composition but minimal $\mu $H, displayed the least helicity but with almost no bacterial killing activity, suggesting that helicity and effectiveness are correlated in these AMPs.