INTERACTION OF MELITTIN WITH MIXED PHOSPHOLIPID-MEMBRANES COMPOSED OF DIMYRISTOYLPHOSPHATIDYLCHOLINE AND DIMYRISTOYLPHOSPHATIDYLSERINE STUDIED BY DEUTERIUM NMR

The interaction of bee venom melittin with mixed phospholipid bilayers composed of dimyristoylphosphatidylcholine deuterated in the α- and β-methylenes of the choline head group (DMPC-d4) and dimyristoylphosphatidylserine deuterated in the α-methylene and β-CH positions of the serine head group (DMPS-d3) was studied in ternary mixtures by using deuterium NMR spectroscopy. The changes in the deuterium quadrupole splittings of the head-group deuteriomethylenes of DMPC-d4 induced by DMPS in binary mixtures [DMPC-d4:DMPS (80:20 mol/mol)] were systematically reversed by increasing concentrations of melittin, so that at a melittin concentration of 4 mol % relative to total lipid the deuterium NMR spectrum from DMPC-d4 in the ternary mixture was similar to the spectrum from pure DMPC-d4 bilayers. This concentration of melittin is sufficient to neutralize the excess negative charge from DMPS. The absence of deuterium NMR signals arising from melittin-bound DMPS in ternary mixtures containing DMPS-d3 indicates that the reversal by melittin of the effects of DMPS on the quadrupole splittings of DMPC-d4 results from the response of the choline head group to the net surface charge rather than from phase separation of melittin-DMPS complexes. In mixtures containing deuterated DMPS [DMPC:DMPS-d3 (50:50 mol/mol)] melittin caused systematic changes in the quadrupole splittings of the DMPS head-group deuterons that closely matched effects observed for a cationic transbilayer polyleucyl peptide (K2GL20K2A) in similar ternary mixtures [Roux, M., Neumann, J. M., Hodges, R. J., Devaux, P. F., and Bloom, M. (1989) Biochemistry 28, 2313-2321]. The similarity in the effects of the two cationic but otherwise dissimilar peptides indicates that the DMPS head group responds to the surface charge resulting from the presence in the bilayer of charged amphiphiles, in a manner analogous to the response of the choline head group of phosphatidylcholine to the bilayer surface charge. The presence of DMPS greatly stabilized DMPC bilayers with respect to melittin-induced micellization, indicating that the latter effect of melittin may not be important for the hemolytic activity of the peptide. © 1989 American Chemical Society