Magainin 2, a natural antibiotic from frog skin, forms ion channels in lipid bilayer membranes

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Abstract

We have examined the ion channel forming properties of magainin 2 by incorporating the peptide into artificial lipid bilayers held under voltage clamp. Magainin 2 increased lipid bilayer conductance in a concentration dependent manner with a Hill coefficient of 1.7. The magainin 2 conductance was selective for monovalent cations over anions with a ratio of 5:1 and had both voltage-sensitive and -insensitive components. Two structurally related but antibiotically less potent analogues, magainin 1 and Z-12, also increased lipid bilayer conductance with a similar ion selectivity but these peptides were less potent than magainin 2. We propose that the weak cation selectivity of the magainin channels can be accounted for by the inclusion of negatively charged lipids in the channel complex and suggest two possible structures for such a channel. The ionophoric properties of these peptides are likely to be proximal to their antibiotic activities.

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      This was added to serial twofold protein dilutions of R. temporaria RtS100A7 ranging from 128 μM to 0.125 μM. As a positive control, serial twofold dilutions of synthetic magainin 2 (CASLO Laboratory ApS, Lyngby, Denmark), a host-defense peptide from the frog Xenopus laevis known to permeabilize bacterial membranes and cause cell lysis (Cruciani et al., 1992; Ludtke et al., 1996; Matsuzaki et al., 1997), were tested in parallel (128 μM–0.125 μM). Fluorescence was measured during incubation at 37 °C in a multi-mode microplate reader (Synergy™ Mx Monochromator-Based Multi-Mode Microplate Reader, BioTek Instruments, Inc.) using an excitation wavelength of 488 nm and an emission wavelength of 523 nm.

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    Current address: Div. Human Genetics and Molecular Biology. Childrens Hospital of Philadelphia, Philadelphia, PA, USA.

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