19F NMR screening of unrelated antimicrobial peptides shows that membrane interactions are largely governed by lipids

Many amphiphilic antimicrobial peptides permeabilize bacterial membranes via successive steps of binding, realignment and/or oligomerization. Here, we have systematically compared the lipid interactions of two structurally unrelated peptides: the cyclic I3-pleated gramicidin S (GS), and the or-helical PGLa.19F NMR was used to screen their molecular alignment in various model membranes over a wide range of temperatures. Both peptides were found to respond to the phase state and composition of these different samples in a similar way. In phosphatidylcholines, both peptides first bind to the bilayer surface. Above a certain threshold concentration they can re-align and immerse more deeply into the hydrophobic core, which presumably involves oligomerization. Re-alignment is most favorable around the lipid chain melting temperature, and also promoted by decreasing bilayer thickness. The presence of anionic lipids has no influence in fluid membranes, but in the gel phase the alignment states are more complex. Unsaturated acyl chains and other lipids with intrinsic negative curvature prevent re-alignment hence GS and PGLa do not insert into mixtures resembling bacterial membranes, nor into bacterial lipid extracts. Cholesterol, which is present in high concentrations in animal membranes, even leads to an expulsion of the peptides from the bilayer and prevents their binding altogether. However, a very low cholesterol content of 10% was found to promote binding and re-alignment of both peptides. Overall, these findings show that the ability of amphiphilic peptides to re-align and immerse into a membrane is determined by the physico-chemical properties of the lipids, such as spontaneous curvature. This idea is reinforced by the remarkably similar behavior observed here for two structurally unrelated molecules (with different conformation, size, shape, charge), which further suggests that their activity at the membrane level is largely governed by the properties of the constituent lipids, while the selectivity towards different cell types is additionally ruled by electrostatic attraction between peptide and cell surface. This article is part of a Special Issue entitled: Interfacially Active Peptides and Proteins. Guest Editors: William C. Wimley and Kalina Hristova. (C) 2014 Elsevier B.V. All rights reserved.