Melittin-phospholipid interactions: binding of the mono- and tetrameric form of this peptide, and perturbations of the thermotropic properties of bilayers.

The binding of melittin to phospholipid bilayers and micelles depends on its quaternary structure and on the state of association of lipids. Monomeric melittin only binds to lipids above their cmc, whereas tetrameric melittin exhibits a biphasic binding; the interaction with monomeric lipids being possible without dissociation of the tetramer. In lipid excess, the bound state observed by fluorescence, polarization and ORD are always very similar. We propose the following model: the presence of a lipidic interface is necessary for the binding of monomeric melittin, while the tetramer may interact with lipid monomers without any dissociation: it might increase in size by addition of lipid molecules to form a micelle-like particle. The perturbations induced by melittin on the thermotropic behaviour of charged phospholipids are detected by calorimetry (DSC) and fluorescence polarization of DPH. For the first group of lipids, constituted of mono or divalent C14 and of divalent C16 lipids, the transitions are progressively abolished in the presence of melittin, without any shift of the temperature. For a second group of lipids, essentially constituted of monovalent C16 lipids, a cooperative transition is always observed. Moreover, at lipid to protein molar ratios higher than 8, there are two distinct well-defined transitions, at the same temperature as for pure lipid and 10 degrees C to 15 degrees C lower. All these results are interpreted by a phase separation occurring between quasi-pure lipid regions and the lipid-melittin complex. These last ones either could, or not, give rise to a phase transition, according to the cohesion of the initial bilayer. In the case of binary mixtures, there would be a phase separation between enriched phosphatidylcholine regions and negative lipid-melittin complexes.