Probing membrane topology of the antimicrobial peptide distinctin by solid-state NMR spectroscopy in zwitterionic and charged lipid bilayers.

Distinctin is a 47-residue antimicrobial peptide, which interacts with negatively charged membranes and is active against Gram-positive and Gram-negative bacteria. Its primary sequence comprises two linear chains of 22 (chain 1) and 25 (chain 2) residues, linked by a disulfide bridge between Cys19 of chain 1 and Cys23 of chain 2. Unlike other antimicrobial peptides, distinctin in the absence of the lipid membrane has a well-defined three-dimensional structure, which protects it from protease degradation. Here, we used static solid-state NMR spectroscopy to study the topology of distinctin in lipid bilayers. We found that, in mechanically aligned lipid bilayers (charged or zwitterionic), this heterodimeric peptide adopts an ordered conformation absorbed on the surface of the membrane, with the long helix (chain 2), approximately parallel to the lipid bilayer (~5 degrees from the membrane plane) and the short helix (chain 1) forming a ~24 degrees angle. Since at lipid-to-protein molar ratio of 50:1, the peptide does not disrupt the macroscopic alignment of either charged or zwitterionic lipid bilayers, it is possible that higher concentrations might be needed for the hypothesized pore formation, or alternatively, distinctin elicits its cell disruption action by other mechanisms.