Ion specificity in fusion of phosphatidic acid-phosphatidylcholine mixed lipid vesicles.

Fusion of phosphatidylcholine-phosphatidic acid mixed lipid vesicles has been studied under the influence of the divalent Ca2+, Mg2+, Cd2+, and Ba2+, which range in size from much smaller to much larger, than Ca2+. Fusion has also been studied under the influence of Eu3+, which has a similar radius but different charge. The effects of these ions are reflected in the different degrees of fusion determined by changes in vesicle size, and in the varying fusion rates monitored by 1H-NMR spectroscopy. Cd2+, which has an ionic radius similar to Ca2+, exhibits the same effect on fusion as Ca2+, while other ions show lower efficiencies. This suggests that the vesicle fusion intermediate has a geometry ideally suited to the binding of Ca2+ or Cd2+.