Reconstitution of Ca²⁺-Regulated Membrane Fusion by Synaptotagmin and SNAREs

We investigated the effect of synaptotagmin I on membrane fusion mediated by neuronal SNARE proteins, SNAP-25, syntaxin, and synaptobrevin, which were reconstituted into vesicles. In the presence of Ca²⁺, the cytoplasmic domain of synaptotagmin I (syt) strongly stimulated membrane fusion when synaptobrevin densities were similar to those found in native synaptic vesicles. The Ca²⁺ dependence of syt-stimulated fusion was modulated by changes in lipid composition of the vesicles and by a truncation that mimics cleavage of SNAP-25 by botulinum neurotoxin A. Stimulation of fusion was abolished by disrupting the Ca²⁺-binding activity, or by severing the tandem C2 domains, of syt. Thus, syt and SNAREs are likely to represent the minimal protein complement for Ca²⁺-triggered exocytosis.