Characterizing the Effects of Membrane Fluidity and Lipid Chain Length on the Antimicrobial Activity of Protegrin-1

Abstract

Antimicrobial peptides are naturally occurring short amphipathic proteins, innate to the immune system and shown to induce selective lytic activity towards microbial pathogens. Protegrin-1 is an 18-residue, cationic, β-sheet antimicrobial peptide stabilized by two disulfide bonds. Concentration-dependent structural transformations of supported lipid bilayer patches as a result of peptide-membrane interactions have been visualized through the use of atomic force microscopy. A three-stage concentration-dependent transformation has been characterized, which begins with edge instability, followed by pore formation and worm-like micelle formation. This suggests that protegrin-1 acts to lower the line-energy at the edge of the bilayer. Membrane and lipid characteristics, including fluidity, charge and acyl chain length, can alter the activity of antimicrobial peptides. To identify the importance of both acyl-chain length and fluidity on the activity of protegrin-1, these two variables were decoupled. When the bilayers are examined at the same relative fluidity levels, they demonstrate the three-stage transformation observed on a fluid control bilayer, in contrast to the structural transformations that were observed in the gel phase bilayers. This suggests that fluidity exhibits a large influence on the transformations that occur as a result of protegrin-1. To examine the importance of acyl-chain length, the activity of antimicrobial peptides was studied using unsaturated bilayers. Our results indicate that the longer chain bilayers are less susceptible to disruption. This could be due to the hydrophobic mismatch between protegrin-1 and the thicker hydrophobic portion of longer chain lipid bilayers . These results highlight the importance of subtle membrane characteristics in the activity of antimicrobial peptides towards bacterial cells. Lipid bilayers with cholesterol are more accurate eukaryotic cell mimics and will allow examination of the selective preference of antimicrobial peptide activity.