Figure 22-3 Model for protein-mediated membrane fusion and exocytosis. A, The release of acetylcholine from the vesicles is mediated by a series of proteins collectively called SNARE proteins. Synaptotagmin is the neuronal Ca²⁺ receptor detecting Ca²⁺ entry. Synaptobrevin (i.e., vesicle-associated membrane protein [VAMP]) is a filament-like protein on the vesicle. B, During depolarization and calcium entry, synaptobrevin on the vesicle unfolds and forms a ternary complex with syntaxin/SNAP-25. This process is facilitated by phosphorylation of synapsin, also present on the vesicle membrane. C, Assembly of the ternary complex forces the vesicle in close apposition to the nerve membrane at the active zone with release of its contents, acetylcholine. The fusion is disassembled, and the vesicle is recycled.