How Many AMP Molecules Kill a Bacterium? Spectroscopic Determination of PMAP-23 Binding to E. Coli

Investigations of the synergistic enhancement of antimicrobial activity in mixtures of magainin 2 and PGLa

Magainins are antimicrobial peptides isolated from the African clawed frog Xenopus laevis. They interact with bacterial membranes where they cause channel formation and membrane disruption. When added as a cocktail magainin 2 and PGLa are considerably more efficient when compared to the corresponding amounts of individual components. In order to investigate this synergistic interaction of PGLa and magainin a number of magainin variants have been prepared and investigated in biological and biophysical assays. In particular we report on the antimicrobial activities and solid-state NMR investigations of magainins that have been extended by a carboxyterminal GGC tripeptide to form covalently linked dimers. Notably, when the formation of the covalent linkage is prevented by exchanging the cystein by serine or alanine no loss in efficiency was observed indicating that the covalent interaction is not necessary for synergistic interaction. In a next step peptides labelled with (15)N and (2)H were reconstituted into oriented membranes and their topology studied by solid-state NMR spectroscopy. The tendency of some of these peptides to adopt membrane-spanning alignments does not correlate with their synergistic activities in antimicrobial assays. In contrast, the stable alignment of PGLa parallel to the surface of membranes made of Escherichia coli lipid extracts is strongly suggestive that the peptides develop synergistic activities when in an in-planar configuration. Notably, the phospholipid head groups of these samples show a high degree of disturbance suggesting that the synergistic interactions between the magainin peptides could be mediated through the lipid phase.

The SMART model: Soft Membranes Adapt and Respond, also Transiently, in the presence of antimicrobial peptides

Biophysical and structural studies of peptide-lipid interactions, peptide topology and dynamics have changed our view on how antimicrobial peptides insert and interact with membranes. Clearly, both the peptides and the lipids are highly dynamic, change and mutually adapt their conformation, membrane penetration and detailed morphology on a local and a global level. As a consequence, the peptides and lipids can form a wide variety of supramolecular assemblies in which the more hydrophobic sequences preferentially, but not exclusively, adopt transmembrane alignments and have the potential to form oligomeric structures similar to those suggested by the transmembrane helical bundle model. In contrast, charged amphipathic sequences tend to stay intercalated at the membrane interface where they cause pronounced disruptions of the phospholipid fatty acyl packing. At increasing local or global concentrations, the peptides result in transient membrane openings, rupture and ultimately lysis. Depending on peptide-to-lipid ratio, lipid composition and environmental factors (temperature, buffer composition, ionic strength, etc.), the same peptide sequence can result in a variety of those responses. Therefore, the SMART model has been introduced to cover the full range of possibilities. With such a view in mind, novel antimicrobial compounds have been designed from amphipathic polymers, peptide mimetics, combinations of ultra-short polypeptides with hydrophobic anchors or small designer molecules.