Modulation of Antimicrobial Peptide Potency in Stressed Lipid Bilayers

It is shown that the tendency of an archetypal antimicrobial peptide to insert into and perforate a simple lipid bilayer is strongly modulated by tensile stress in the membrane. The results, obtained through molecular dynamics simulations, have been demonstrated with several lipid compositions and appear to be general, although quantitative details differ. The findings imply that the potency of antimicrobial peptides may not be a purely intrinsic chemical property and, instead, depends on the mechanical state of the target membrane.

Figure 1

Snapshots from MD simulations (top and side views) of a $3:1$ DLPC:DLPG membrane without peptides (top) and with CecB peptides (bottom): (a) after 500 ns, no tensile stress applied; (b) after 50 ns, $\lambda =20\mathrm{dyn}/\mathrm{cm}$; and (c) after 150 ns of increasing stress, $20\text{−}50\text{−}60\mathrm{dyn}/\mathrm{cm}$. Water is shown as red spheres, peptides in yellow, and lipids as gray sticks with head groups as spheres. (d) Side view of the pore formed without peptides. (e) Side view of the pore formed with peptides.

Figure 2

(a)–(c): Area per lipid over time (left to right: DLPC:DLPG, POPC, and POPC:POPG). (d)–(f): Membrane thickness over time (left to right: DLPC:DLPG, POPC, and POPC:POPG). For each lipid composition, these quantities are plotted with or without peptide present, and with/without applied stress. The colored bars above each graph show the time profile of the applied stress.

Figure 3

The free energy of peptide insertion is changed qualitatively by the application of stress. PMF for CecB insertion into a tension-free DLPC:DLPG membrane (red) and one with $\lambda =50\mathrm{dyn}/\mathrm{cm}$ (green). The origin of both lines is the membrane surface, calculated as the average of the upper leaflet phosphates position along the membrane normal. (a) Top and side views of the initial equilibrated system, which is the same for both curves, with the peptide $N$ terminal 6 Å above the membrane surface ($d=6$). (b) Peptide $N$ terminal at $d=2$. Top and side views with no stress applied are shown on the left, with applied stress on the right. (c) As for Fig. 3, with $d=0$. CecB peptide is represented as a ribbon, with residues color coded as follows: red, negatively charged; blue, positively charged; green, polar; and white, hydrophobic.

Figure 4

Distribution of lipidic phosphate groups characterizing poration of a $3:1$ DLPC:DLPG membrane. (a)–(d): Map of z coordinates for headgroup phosphates at $t=0$ and at the end of the simulation for top and bottom leaflets. (a) Membrane only, no stress; (b) membrane only, $\lambda =60\mathrm{dyn}/\mathrm{cm}$; (c) $\text{membrane}+\mathrm{CecB}$, no stress; and (d) $\text{membrane}+\mathrm{CecB}$, $\lambda =20\mathrm{dyn}/\mathrm{cm}$. Circles indicate the location of a well-defined pore. (e) Snapshot of pore 4 (pink circle) formed by CecB with applied stress. Waters are represented as red dots, phosphorus atoms are colored by the $z$ coordinate (blue to $\mathrm{red}=\text{top to bottom}$). (f) Time course of the average number of close neighbors ($P$ pairs closer than 5 Å) for all the phosphorus atoms within 50 Å from the pore nucleation site, over the last 50 ns of simulation. Asterisks [same colors as in Figs. 4 and 4] indicate the point in time when the pore first appears.