We studied the synergistic mechanism of equimolar mixtures of magainin 2 (MG2a) and PGLa in phosphatidylethanolamine/phosphatidylglycerol mimics of Gram-negative cytoplasmic membranes. In a preceding article of this series, we reported on the early onset of parallel heterodimer formation of the two antimicrobial peptides already at low concentrations and the resulting defect formation in the membranes. Here, we focus on the structures of the peptide-lipid aggregates occurring in the synergistic regime at elevated peptide concentrations. Using a combination of calorimetric, scattering, electron microscopic, and in silico techniques, we demonstrate that the two peptides, even if applied individually, transform originally large unilamellar vesicles into multilamellar vesicles with a collapsed interbilayer spacing resulting from peptide-induced adhesion. Interestingly, the adhesion does not lead to a peptide-induced lipid separation of charged and charge-neutral species. In addition to this behavior, equimolar mixtures of MG2a and PGLa formed surface-aligned fibril-like structures, which induced adhesion zones between the membranes and the formation of transient fusion stalks in molecular dynamics simulations and a coexisting sponge phase observed by small-angle x-ray scattering. The previously reported increased leakage of lipid vesicles of identical composition in the presence of MG2a/PGLa mixtures is therefore related to a peptide-induced cross-linking of bilayers.

Biophysics Division Institute of Molecular Biosciences NAWI Graz University of Graz Graz Austria; BioTechMed Graz Graz Austria

CEITEC Central European Institute of Technology Brno Czech Republic; National Centre for Biomolecular Research Faculty of Science Masaryk University Brno Czech Republic

CEITEC Central European Institute of Technology Brno Czech Republic; National Centre for Biomolecular Research Faculty of Science Masaryk University Brno Czech Republic; Department of Condensed Matter Physics Faculty of Science Masaryk University Brno Czech Republic

Institut Laue Langevin DS LSS Grenoble France

Institute for Electron Microscopy and Nanoanalysis and Center for Electron Microscopy Graz University of Technology Graz Austria

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Magainin 2 and PGLa in bacterial membrane mimics IV: Membrane curvature and partitioning

Magainin 2 and PGLa in bacterial membrane mimics III: Membrane fusion and disruption