Analogs of the antimicrobial peptide trichogin having opposite membrane properties

Four analogs of the antimicrobial peptide trichogin GA IV were studied. The ir sequences are as follows: GT, n-octanoyl-Aib-Gly-Leu-Aib-Gly-Gly-Leu-Aib -Gly-Ile-Leu-OMe; ST, n-octanoyl-Aib-Ser-Leu-Aib-Ser-Ser-Leu-Aib-Ser-Ile-Le u-OMe; BT, n-octanoyl-Aib-Ser(tBu)Leu-Aib-Ser(tBu)-Ser(tBu)-Leu-Aib-Ser(tBu )-Ile-Leu-OMe; and DT, n-octanoyl-Aib-Ser(tBu)-Leu-Aib-Ser(tBu) Ser(tBu)-Le u-Aib-Ser(tBu)-Ile-Leu-Aib-Ser(tBu)-Leu-Aib-Ser(tBu)-Ser(tBu)-Leu-Aib-Ser(t Bu)-Ile-Leu-Ome. The tri chogin GA IV differs from GT only in the nature of the C-terminal residue, being a 1,2 aminoalcohol (leucinol) in the case of the parent peptide. Compared with GT, ST has an increased amphiphilicity. In contrast, BT has little amphiphilicity being composed only of hydrophobi c amino acids. DT is an octanoylated head-to-tail dimer of BT. We show that BT and DT lower the bilayer-to-hexagonal phase transition temperature (T-H ) of dipalmitoleoylphosphatidylethanolamine, indicating that the peptides p romote negative curvature. These two peptides, composed of only hydrophobic amino acids, have their bulkier groups on one face of the helix, suggestin g that they may penetrate membranes at an oblique angle. In contrast, GT an d ST, like trichogin itself, increase T-H, promoting positive curvature. Th ese peptides have contrasting membrane lytic activities. Whereas DT and BT did not produce leakage of aqueous contents, GT and ST, like trichogin, did cause rapid leakage. The leakage activity with liposomes also correlates w ith the greater potency of GT and ST, compared with the hydrophobic analogs , in their hemolytic and bacteriostatic action. ST has greater lytic abilit y than GT in liposomal leakage as well as hemolysis. We also measured the r ate of peptide-promoted lipid mixing as an indication of membrane fusion. B T produced lipid mixing only with large unilamellar vesicles enriched with dioleoylphosphatidylethanolamine; ST did not produce lipid mixing, as its a pparent reduction of energy transfer proved to be artifactual. Quasi-elasti c light scattering of large unilamellar vesicles was also carried out after adding ST and BT. Peptide BT, but not ST, was able to aggregate large unil amellar vesicles. Thus, one of the properties of BT that leads to the induc tion of lipid mixing is that it is able to aggregate vesicles, placing the bilayers in juxtaposition. Thus, the two pairs of peptides, BT and DT vs GT and ST, exhibit contrasting behaviour with respect to a number of membrane biophysical properties. This occurs despite the fact that the chemical str uctures of the peptides are rather similar. Such distinct behavior is also reflected in their hemolytic and bacteriostatic actions.