User:Stan.Mikita/sandbox

Vesicle fusion is the process of a vesicle approaching a membrane (or another vesicle) and then fusing with it. The SNARE complex, a four-protein complex, is involved in bringing the vesicle in contact with the membrane. Two proteins are bound to the vesicle, and two proteins are bound to the plasma membrane. One of the two plasma membrane proteins involved in the formation of the exocytotic fusion complex in neurons is SNAP-25, a Q-SNARE protein contributing two α-helices. SNAP-25 is a membrane-bound protein anchored to the cytosolic face of membranes via palmitoyl side chains in the middle of the molecule. This means that SNAP-25 does not contain a trans-membrane domain. SNAP-25 assembles with the other three SNARE complex proteins syntaxin-1, synaptotagmin, and synaptobrevin. The selective binding of these proteins enables vesicle docking and fusion to occur at the correct place.

To form the SNARE complex, the two proteins on the vesicle membrane and the two proteins on the plasma membrane bind to each other and begin to wrap around each other and form a coiled coil. Both synaptobrevin and syntaxin bind to SNAP-25. The two proteins bind to different regions on the SNAP-25 protein. Syntaxin binds near SNAP-25's N-terminus side.

SNAP-25 inhibits P/Q- and L-type voltage-gated calcium channels located presynaptically and interacts with the synaptotagmin C2B domain in Ca2+-independent fashion. In glutamatergic synapses SNAP-25 decreases the Ca2+ responsiveness, while it is naturally absent in GABAergic synapses.

There are two known forms of the SNAP-25 protein: SNAP-25A and SNAP-25B. The differences between these two forms are outlined in the table below.