Fusion of synaptic vesicles with the plasma membrane of axon termini depends on SNAREs, the same type of proteins that mediate membrane fusion of other regulated secretory vesicles, and SM proteins (for Sec1/Munc18-like proteins). The principal v-SNARE in synaptic vesicles (VAMP) tightly binds syntaxin and SNAP-25, the principal t-SNAREs in the plasma membrane of axon termini, to form four-helix SNARE complexes. The assembly of the SNARE complex brings the synaptic vesicle membrane into close proximity to the presynaptic plasma membrane, but the formation of a fusion pore requires an additional step, association of an SM protein with syntaxin. After fusion, proteins within the axon terminus promote disassociation of VAMP from t-SNAREs, as in the fusion of secretory vesicles depicted in Figure 14-10.

The signal that triggers exocytosis of docked synaptic vesicles is a very localized rise in the Ca²⁺ concentration in the cytosol near vesicles from 0.1 µM, characteristic of resting cells, to 1–100 µM following arrival of an action potential in stimulated cells. The speed with which synaptic vesicles fuse with the presynaptic membrane after a rise in cytosolic Ca²⁺ (less than 1 ms) indicates that the fusion machinery is entirely assembled in the resting state and can rapidly undergo a conformational change leading to exocytosis of neurotransmitter (Figure 22-28). A Ca²⁺-binding protein called synaptotagmin, located in the membrane of synaptic vesicles, is a key component of the vesicle-fusion machinery that triggers exocytosis in response to Ca²⁺. A protein called complexin is thought to bind to the α-helical bundle of an assembled v-SNARE/t-SNARE complex that bridges the synaptic vesicle and plasma membranes, preventing the final fusion step. Binding of Ca²⁺ to synaptotagmin relieves this inhibition, releasing complexin and allowing the fusion event to occur very rapidly. While the mechanisms by which synaptotagmin functions are debated, Figure 22-28 depicts a widely accepted model.

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Several lines of evidence support a role for synaptotagmin as the Ca²⁺ sensor for exocytosis of neurotransmitters. Mutant embryos of Drosophila and C. elegans that completely lack synaptotagmin fail to hatch and exhibit very reduced, uncoordinated muscle contractions. Larvae with partial loss-of-function mutations of synaptotagmin survive, but their neurons are defective in Ca²⁺-stimulated vesicle exocytosis. Moreover, in mice, mutations in synaptotagmin that decrease its affinity for Ca²⁺ cause a corresponding increase in the amount of cytosolic Ca²⁺ needed to trigger rapid exocytosis. Mammals express multiple different synaptotagmin isoforms, each of which has a different binding affinity for Ca²⁺, and as a result the kinetics of exocytosis depend on the particular synaptotagmin isoform expressed in the neuron.

An important characteristic of synaptic vesicle exocytosis is its speed. Synaptic vesicle fusion occurs within a few hundred microseconds after the arrival of an action potential, which is not very different from the timescale of Ca²⁺ influx through the voltage-gated Ca²⁺ channel. What makes this speed possible is the proximity of the release machinery to the voltage-gated Ca²⁺ channels. This proximity is mediated by two scaffolding proteins called RIM (for Rab3-interacting protein) and RIM-BP (for RIM binding protein), which form a complex between Rab3-containing synaptic vesicles and voltage-gated Ca²⁺channels. In mice lacking RIM, and flies lacking RIM-BP, active zones lack voltage-gated Ca²⁺ channels, which leads to a dramatic decrease and desynchronization of neurotransmitter release.