INFLUENCE OF LIPID CHAIN UNSATURATION ON MELITTIN-INDUCED MICELLIZATION

It is well known that melittin, an amphipathic helical peptide, causes the micellization of phosphatidylcholine vesicles, In the present wor k, we conclude that the extent of micellization is dependent on the le vel of unsaturation of the lipid acyl chains. We report the results ob tained on two systems: dipalmitoylphosphatidylcholine (DPPC), containi ng 10(mol)% saturated or unsaturated fatty acid (palmitic, oleic, or l inoleic), and DPPC, containing 10(mol)% positively charged diacyloxy-3 -(trimethylammonio)propane bearing palmitic or oleic acyl chains, For both systems, the presence of unsaturation in the lipid acyl chains in hibits melittin-induced micellization. Conversely, the addition of sat urated palmitic acid to the DPPC matrix enhances the micellization. Th is modulation is proposed to be associated with the cohesion of the hy drophobic core. When the lipid chain packing of the gel-phase bilayer is already perturbed by the presence of unsaturation, it seems easier for the membrane to accommodate melittin at the interface, and the dis tribution of the peptide in the bilayer could be the origin of the inh ibition of the micellization. The cohesion of the apolar core is shown to play an unquestionable role in melittin-induced micellization; how ever, this contribution does not appear to be as important as the elec trostatic interactions between melittin and positively or negatively c harged lipids.