We believe that divalent cat- ions catalyze the process by increasing both the probability of vesicle-membrane contact and the duration of that contact, thus allowing time for the os- motic swelling to occur. For phospholipids, millimolar amounts of any diva- lent cation are required, whereas the presence of calcium-binding protein in the planar membrane lowers this requirement to the micromolar range of Ca2+, specifically. Still lower Ca2+ concentra- tions might produce fusion if calciumbinding protein were present in both planar and vesicular membranes. Limita- tions in the amount of protein available to us thus far have precluded experiments along this line.
The calcium-binding protein used here was obtained from synaptic membranes, but it is not clear at present whether it either promotes transmitter release at synapses or is particularly unique in its properties. Conceivably, many such binding proteins exist in neuronal or oth- er membranes and function in related or different capacities. Its molecular weight of -16,000, four calcium-binding sites, and Km for Ca2+ of -15 PM are strikingly similar to the properties of calmodulin (19); further work is required to deter- mine if the two molecules are related.