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 Exocytosis of synaptic vesicles. For orientation at a synapse, see Figure 11–38. (A) The trans-SNARE complex responsible for docking synaptic vesicles at the plasma membrane of nerve terminals consists of three proteins. The v-SNARE synaptobrevin and the t-SNARE syntaxin are both transmembrane proteins, and each contributes one α helix to the complex. By contrast to other SNAREs discussed earlier, the t-SNARE SNAP25 is a peripheral membrane protein that contributes two α helices to the four-helix bundle; the two helices are connected by a loop (dashed line) that lies parallel to the membrane and has fatty acyl chains (not shown) attached to anchor it there. The four α helices are shown as rods for simplicity. (B) At the synapse, the basic SNARE machinery is modulated by the Ca2+ sensor synaptotagmin and an additional protein called complexin. Synaptic vesicles first dock at the membrane (step 1), and the SNARE bundle partially assembles (step 2), resulting in a “primed vesicle” that is already drawn close to the membrane. The SNARE bundle assembles further, but the additional binding of complexin prevents fusion (step 3). Upon arrival of an action potential, Ca2+ enters the cell and 
binds to synaptotagmin, which releases the block and opens a fusion pore (step 4). Further rearrangements complete the 
fusion reaction (step 5) and release the fusion machinery, which now can be reused. This elaborate arrangement allows the fusion machinery to respond on the millisecond time scale essential for rapid and repetitive synaptic signaling. (A, adapted from R.B. Sutton et al., Nature 395:347–353, 1998; B, adapted from J. Tang et al., Cell 126:1175–1187, 2006. With 
permission from Elsevier.)