Kinetics of melittin-induced fusion of dipalmitoylphosphatidylcholine small unilamellar vesicles.

We have studied the kinetics of fusion of dipalmitoylphosphatidylcholine small unilamellar vesicles at 51 degrees C which is induced by bee venom melittin at a protein-to-lipid molar ratio of 1/60. This was done by following with a stopped-flow fluorometer the reduction in the ratio of the excimer to monomer fluorescence intensities of 1-palmitoyl-2-(10-pyrenyldecanoyl)-sn-glycero-3-phosphorylcholine that accompanies fusion. At a low melittin concentration and low ionic strength, for which case the protein is monomeric, the value of the rate constant for fusion is 0.006 s-1. This is much smaller than that of 0.06 s-1 obtained for a high melittin concentration at low ionic strength, i.e. for the protein in the tetrameric form which is not induced by a high salt concentration. The value of the rate constant for fusion for a low melittin concentration in the presence of 2 M NaCl, i.e. for the protein in the tetrameric form which is induced by a high salt concentration, is 0.12 s-1. This is twice as large as that for fusion induced by the tetramer in a low ionic strength solution. These findings show that the state of aggregation of the protein in solution and, to a lesser extent, electrostatic interactions play an important role in the kinetics of melittin-induced fusion of vesicles.