The antimicrobial peptide trichogin and its interaction with phospholipid membranes

The interaction of the antimicrobial peptide trichogin GA IV with phospholi pid bilayers has been studied. A series of analogs of trichogin was synthes ized in which the nitroxide spin label, 4-amino-4-carboxy-2,2,6,6-tetrameth ylpiperidino- 1-oxyl (TOAC), replaced one of the three alpha-aminoisobutyri c acid (Aib) residues in the sequence. These modified peptides were used to assess the location of different residues of the peptide in a phospholipid bilayer composed of egg phosphatidylcholine containing 0.4 mol% of a fluor escently labelled phospholipid, We demonstrate that the substitution of Aib residues with TOAC does not alter the manner in which the peptide affects membrane curvature or induces vesicle leakage. The proximity of the nitroxi de group on the peptide to the 4,3-difluoro-4-bora-3a,4a-diaza-S-indacene ( BODIPY) fluorophore attached to the phospholipid was estimated from the ext ent of quenching of the fluorescence. By this criterion it was concluded th at the peptide penetrates into the bilayer and that Aib(4) is the most deep ly inserted of the Aib residues. The results suggest that the helix axis of the peptide is oriented along the plane of the membrane. All of the peptid es were shown to raise the bilayer to the hexagonal phase transition temper ature of dipalmitoleoylphosphatidylethanolamine. indicating that they promo te positive membrane curvature. This is a property observed with peptides t hat do not penetrate deeply into the bilayer or are oriented along the bila yer normal. We also demonstrate trichogin-promoted leakage of the aqueous c ontents of liposomes. These results indicate that the peptides cause bilaye r destabilization. The extent of leakage induced by trichogin is very sensi tive to the peptide to lipid ratio over a narrow range.