Bilayer lipid composition modulates the activity of dermaseptins, polycationic antimicrobial peptides

(IF : 2,409)International audienceThe primary targets of defense peptides are plasma membranes, and the induced irreversible depolarization is sufficient to exert antimicrobial activity although secondary modes of action might be at work. Channels or pores underlying membrane permeabilization are usually quite large with single-channel conductances two orders of magnitude higher than those exhibited by physiological channels involved, e.g., in excitability. Accordingly, the ion specificity and selectivity are quite low. Whereas, e.g., peptaibols favor cation transport, polycationic or basic peptides tend to form anion-specific pores. With dermaseptin B2, a 33 residue long and mostly a-helical peptide isolated from the skin of the South American frog Phyllomedusa bicolor, we found that the ion specificity of its pores induced in bilayers is modulated by phospholipidcharged headgroups. This suggests mixed lipid–peptide pore lining instead of the more classical barrel–stave model. Macroscopic conductance is nearly voltage independent, and concentration dependence suggests that the pores are mainly formed by dermaseptin tetramers. The two most probable single-channel events are well resolved at 200 and 500 pS (in 150 mM NaCl) with occasional other equally spaced higher or lower levels. In contrast to previous molecular dynamics previsions, this study demonstrates that dermaseptins are able to form pores, although a related analog (B6) failed to induce any significant conductance. Finally, the model of the pore we present accounts for phospholipid headgroups intercalated between peptide helices lining the pore and for one of the most probable single-channel conductance

Conformational Changes in Alamethicin Associated with Substitution of Its α-Methylalanines with Leucines: A FTIR Spectroscopic Analysis and Correlation with Channel Kinetics

Alamethicin, a 20 residue-long peptaibol remains a favorite high voltage-dependent channel-forming peptide. However, the structural significance of its abundant noncoded residues (α-methylalanine or Aib) for its ion channel activity remains unknown, although a previous study showed that replacement of all Aib residues with leucines preserved the essential channel behavior except for much faster single-channel events. To correlate these functional properties with structural data, here we compare the secondary structures of an alamethicin derivative where all the eight Aibs were replaced by leucines and the native alamethicin. Fourier transform infrared (FTIR) spectra of these peptides were recorded in methanol and in aqueous phospholipid membranes. Results obtained show a significant conformational change in alamethicin upon substitution of its Aib residues with Leu. The amide I band occurs at a lower frequency for the Leu-derivative indicating that its α-helices are involved in stronger hydrogen-bonding. In addition, the structure of the Leu-derivative is quite sensitive to membrane fluidity changes. The amide I band shifts to higher frequencies when the lipids are in the fluid phase. This indicates either a decreased solvation due to a more complete peptide insertion or a peptide stretching to match the full thickness of the bilayer. These results contribute to explain the fast single-channel kinetics displayed by the Leu-derivative

Interaction of the 14-residue peptaibols, harzianins HC, with lipid bilayers: permeability modifications and conductance properties

AbstractHarzianins HC are a series of 14-residue peptaibols containing three Aib-Pro motives separated by sequences of two usual amino acids (Aib-Pro-Xaa-Xaa)n. They are organized in a subtype of the 310-helix, which results in an approximate length of about 27–30 Å for the helical rods, allowing them to span a bilayer. Permeabilization of small unilamellar vesicles composed of zwitterionic lipids (egg phosphatidylcholine/cholesterol 7/3 and 8/2) by harzianins HC was observed, as well as voltage-gated macroscopic conductance and single-channel formation in planar lipid bilayers (DOPE/POPC 7/3). The permeabilization process was shown to increase with increasing the helix global hydrophobicity. The ion channel-forming properties appeared rather favoured by an increase in the peptide amphipathicity. The set of conductance levels increasing in geometrical progression, reflecting the sequential uptake and release of monomers which is characteristic of the barrel-stave model for ion-channels described for alamethicin was not observed. The passage of ions through the bilayer would rather be the result of a set of aggregates with fixed numbers of monomers formed in the bilayer. The permeability process and the voltage-gated properties could thus result from different mechanisms showing that harzianins HC can permeabilize membranes via bilayer destabilization or channels, depending on the membrane system, composition and application of voltage