Bilayer Interactions of Indolicidin, a Small Antimicrobial Peptide Rich in Tryptophan, Proline, and Basic Amino Acids

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Tryptophan, proline, and basic amino acids have all been implicated as being important in the assembly and structure of membrane proteins. Indolicidin, an antimicrobial 13-residue peptide-amide isolated from the cytoplasmic granules of bovine neutrophils, is highly enriched in these amino acids: five tryptophans, three prolines, three basic residues, and no acidic residues. Consistent with the likely importance of these amino acids in membrane protein assembly, indolicidin is known to be highly membrane-active and is believed to act by disruption of cell membranes. We have, therefore, examined the interactions of native indolicidin with large unilamellar vesicles (LUV) formed from palmitoyloleoylphosphatidylcholine (POPC), and palmitoyloleoylphosphatidylglycerol (POPG), in order to use it as a model system for studying membrane protein insertion and for evaluating the relative contributions of hydrophobic and electrostatic forces in peptide-bilayer interactions. Equilibrium dialysis measurements indicate that indolicidin binds strongly, but reversibly, to both neutral POPC and anionic POPG vesicles with free energies of transfer of -8.8 ± 0.2 and -11.5 ± 0.4 kcal/mol, respectively. The extremely strong partitioning into POPG vesicles necessitated the development of a new equilibrium dialysis method that is described in detail. Tryptophan fluorescence measurements show that indolicidin is located in the bilayer interface and that indole fluorescence is affected by the type of lipid used to form the LUVs. Circular dichroism (CD) measurements reveal unordered conformations in aqueous and bulk organic solutions and a somewhat more ordered, but not α-helical, conformation in SDS micelles and lipid bilayers. Fluorescence requenching measurements (Ladokhin et al. 1995. Biophys. J. 69:1964-1971) on vesicles loaded with the fluorophore/quencher pair 8-aminonapthalene-1,3,6 trisulfonic acid (ANTS)/p-xylene-bis-pyridinium bromide (DPX), show that indolicidin induces membrane permeabilization. For anionic POPG, leakage is graded with a high preference for the release of cationic DPX over anionic ANTS. For neutral POPC vesicles no such preference is observed. Leakage induction is more effective with POPG vesicles than with POPC vesicles, as judged by three quantitative measures that are developed in the Appendix.

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