Alanine scan and 2H NMR analysis of the membrane-active peptide BP100 point to a distinct carpet mechanism of action

• BP100 is a short, multi-functional, membrane-active peptide.
• An Ala-scan was performed and improved therapeutic peptides identified.
• The peptide is highly helical in membranes.
• The orientation in the membrane could be determined from solid-state 2H NMR.
• BP100 membrane function is most likely based on a carpet mechanism.

The short membrane-active peptide BP100 [KKLFKKILKYL-NH2] is known as an effective antimicrobial and cell penetrating agent. For a functional alanine scan each of the 11 amino acids was replaced with deuterated Ala-d3, one at a time. MIC assays showed that a substitution of Lys did not affect the antimicrobial activity, but it decreased when a hydrophobic residue was replaced. In most cases, a reduction in hydrophobicity led to a decrease in hemolysis, and some peptide analogues had an improved therapeutic index. Circular dichroism showed that BP100 folds as an amphiphilic α-helix in a bilayer. Its alignment was determined from 2H NMR in oriented membranes of different composition. The azimuthal rotation angle was the same under all conditions, but the average helix tilt angle and the dynamical behavior of the peptide varied in a systematic manner. In POPC/POPG bilayers, with a negative spontaneous curvature, the peptide was found to lie flat on the bilayer surface, and with little wobble. In DMPC/DMPG, with a positive spontaneous curvature, BP100 at higher concentrations became tilted obliquely into the membrane, with the uncharged C-terminus inserted more deeply into the lipid bilayer, experiencing significant fluctuations in tilt angle. In DMPC/DMPG/lyso-MPC, with a pronounced positive spontaneous curvature, the helix tilted even further and became even more mobile. The 11-mer BP100 is obviously too short to form transmembrane pores. We conclude that BP100 operates via a carpet mechanism, whereby the C-terminus gets inserted into the hydrophobic core of the bilayer, which leads to membrane perturbation and induces transient permeability.

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