Melittin-lipid bilayer interactions and the role of cholesterol

The membrane-destabilizing effect of the peptide melittin on phosphatidylcholine membranes is modulated by the presence of cholesterol. This investigation shows that inclusion of 40 mol % cholesterol in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine or 1,2-dioleoyl-sn-glycero-3-phosphocholine liposomes reduces melittin's affinity for the membrane. It is significant that the presence of cholesterol does not increase the amount of membrane-associated melittin needed to cause maximum leakage from, or major structural rearrangements of, the liposomes. Furthermore, comparison of microscopy and leakage data suggests that melittin-induced leakage occurs via different mechanisms in the cholesterol-free and cholesterol-supplemented systems. In the absence of cholesterol, leakage of carboxyfluorescein takes place from intact liposomes in a manner compatible with the presence of small melittin-induced pores. In the presence of cholesterol, on the other hand, adsorption of the peptide causes complete membrane disruption and the formation of long-lived open-bilayer structures. Moreover, in the case of cholesterol-supplemented systems, melittin induces pronounced liposome aggregation. Cryotransmission electron microscopy was used, together with ellipsometry, circular dichroism, turbidity, and leakage measurements, to investigate the effects of melittin on phosphatidylcholine membranes in the absence and presence of cholesterol. The melittin partitioning behavior in the membrane systems was estimated by means of steady-state fluorescence spectroscopy measurements.