Effect of α-helical peptides on liposome structure: a comparative study of melittin and alamethicin

Cryo-transmission electron microscopy was used in combination with turbidity and leakage measurements to explore and compare the membrane perturbing effects of melittin and alamethicin on POPC-based liposomes of varying composition. The results show that the two peptides, despite their differences in physico-chemical properties and proposed mode of action, induce similar structural effects on the liposomes. Importantly, whereas low peptide concentrations leave pure POPC liposomes intact and seemingly unperturbed, POPC liposomes supplemented with 40 mol.% cholesterol change their shape, rupture and fuse in response to the addition of both melittin and alamethicin. In the case of alamethicin, but not melittin, fusion is effectively prevented by inclusion of 10 mol.% POPG in the liposome membranes. By means of a competitive binding assay we furthermore show that alamethicin, in line with earlier findings for melittin, possess high affinity for positively curved lipid surfaces. Moreover, results from the present study show that magainin 2 has a similar preference for curved surfaces.