Diphtheria toxin induces fusion of small unilamellar vesicles at low pH

Model membrane systems such as phospholipid vesicles have been extensively used to study the mechanism of membrane fusion at the molecular level. We report here on the capacity of diphtheria toxin to induce fusion of small unilamellar vesicles of dipalmitoylphosphatidylcholine at low pH. Fluorescence polarization and differential scanning calorimetry make it possible to demonstrate the mixing of the lipid phase. Mixing of the internal aqueous compartments of liposome was established using the terbium fluorescence technique. The analogy of structure and properties between melittin and a diphtheria toxin fragment is discussed.