Solid-state NMR and simulation studies of equinatoxin II N-terminus interaction with lipid bilayers

The interaction with model membranes of a peptide, EqtII(1-32), corresponding to the N-terminal region of the pore-forming toxin equinatoxin II (EqtII) has been studied using solid-state NMR and molecular dynamics (MID) simulations. The distances between specifically labeled nuclei in [19F-para]Phe16-[1-13C]Leu19 and [19F-para]Phe16-[15N]Leu23 analogs of EqtII1¿32 measured by REDOR in lyophilized peptide were in agreement with published crystal and solution structures. However, in both DMPC and mixed DMPC:SM membrane environments, significant changes in the distances between the labeled amino acid pairs were observed, suggesting changes in helical content around the experimentally studied region, 16¿23, in the presence of bilayers. 19F-31P REDOR experiments indicated that the aromatic ring of Phe16 is in contact with lipid headgroups in both membrane environments. For the DMPC:SM mixed bilayers, a closer interaction between Phe16 side chains and lipid headgroups was observed, but an increase in distances was observed for both labeled amino acid pairs compared with those measured for EqtII1¿32 in pure DMPC bilayers. The observed differences between DMPC and DMPC:SM bilayers may be due to the greater affinity of EqtII for the latter. MD simulations of EqtII1¿32 in water, on a pure DMPC bilayer, and on a mixed DMPC:SM bilayer indicate significant peptide secondary structural differences in the different environments, with the DMPC-bound peptide adopting helical formations at residues 16¿24, whereas the DMPC:SM-bound peptide exhibits a longer helical stretch, which may contribute to its enhanced activity against PC:SM compared with pure PC bilayers.