Membrane Pores Induced by Magainin

Abstract

Magainin, found in the skin of Xenopus laevis, belongs to a broad class of antimicrobial peptides which kill bacteria by permeabilizing the cytoplasmic membrane but do not lyse eukaryotic cells. The 23-residue peptide has been shown to form an amphiphilic helix when associated with membranes. However, its molecular mechanism of action has been controversial. Oriented circular dichroism has detected helical magainin oriented perpendicular to the plane of the membrane at high peptide concentrations, but Raman, fluorescence, differential scanning calorimetry, and NMR all indicate that the peptide is associated with the head groups of the lipid bilayer. Here we show that neutron in-plane scattering detects pores formed by magainin 2 in membranes only when a substantial fraction of the peptide is oriented perpendicular to the membrane. The pores are almost twice as large as the alamethicin pores. On the basis of the in-plane scattering data, we propose a toroidal (or wormhole) model, which differs from the barrel-stave model of alamethicin in that the lipid bends back on itself like the inside of a torus. The bending requires a lateral expansion in the head group region of the bilayer. Magainin monomers play the role of fillers in the expansion region thereby stabilizing the pore. This molecular configuration is consistent with all published magainin data.

Mechanism of Lipid Bilayer Disruption by the Human Antimicrobial Peptide, LL-37

“…Since the helical conformation remains unchanged over a wide range of conditions and is stabilized by aggregation, the same helical structure is expected to remain in the solid state, and this is consistent with the 13 C CPMAS experiments (Figure 2). The extent of the helical region has been predicted to range from residues 11-32 based on the percent of helical residues indicated by CD spectra and analysis of the peptide sequence, and the 13 C isotropic chemical shifts of 13 15 N spectra show that the peptide helix is aligned on the surface of the bilayer in a variety of lipid environments, as was predicted for such an amphipathic R-helix (Figures 5-7). The 15 N spectra of LL-37 in MLVs indicate that LL-37 does not rotate rapidly about the membrane normal as has been shown to occur for another surface-oriented antimicrobial peptide, ovispirin (51).…”

“…The two 13 C labels are placed near either end of the helical region and used to confirm the extent of the helix. To enable easy resolution and assignment in the doubly 13 C-labeled peptide, one 13 C R and one 13 CdO label were used. Alanine was substituted for isoleucine at residue 13 due to the limited availability of 13 C R -labeled Fmoc amino acids.…”

“…The raw data were converted to molar heat capacity using the CPCalc program provided with the calorimeter and the lipid concentration and molecular weight of each sample along with a partial specific volume of 0.988 mL/g (30). 31 P, 13 C, and 15 N NMR. 31 P, 13 C, and 15 N NMR spectra were acquired using a Varian/Chemagnetics 400 MHz spectrometer with 1 H, 31 P, 13 C, and 15 N frequencies of 400.139, 161.978, 100.618, and 40.551 MHz, respectively.…”

“…31 P, 13 C, and 15 N NMR. 31 P, 13 C, and 15 N NMR spectra were acquired using a Varian/Chemagnetics 400 MHz spectrometer with 1 H, 31 P, 13 C, and 15 N frequencies of 400.139, 161.978, 100.618, and 40.551 MHz, respectively. Home-built, double-resonance, flat-coil probes were used for the mechanically aligned samples with a coil just large enough to hold the samples.…”

Membrane interactions of designed cationic antimicrobial peptides: The two thresholds

“…14 Biophysical studies have demonstrated that there are some general nonreceptor-mediated mechanisms responsible for peptide antimicrobial activity, which appear to involve permeabilization of phospholipid bilayer membranes via ''barrel-stave,'' ''toroidal pore,'' or ''carpet detergent-like'' arrangements. 3,8,11,15 Since the majority of antimicrobial peptides are positively charged at physiological pH, while outer leaflets of mammalian and bacterial plasma membranes are totally neutral and anionic, respectively, the mechanisms of their antimicrobial and hemolytic activities might differ in detail.…”