Synaptotagmin



Synaptotagmins (isotypes Syt1-Syt13) constitute a family of membrane-trafficking proteins that are characterized by an N-terminal transmembrane region (TMR), a variable linker, and two C-terminal C2 domains - C2A and C2B.

Functions
Synaptotagmin is Ca2+ sensor and is involved in both: It was recently shown that synaptotagmin 1 can displace complexin from the SNARE complex in the presence of calcium. This is thought to be one of the last steps in exocytosis.
 * (i) early synaptic vesicle docking to the presynaptic membrane via interaction with β-neurexin or SNAP-25
 * (ii) late steps of Ca2+ evoked synaptic vesicle fusion with the presynaptic membrane.

C-terminal C2-domains
The C2A domain regulates the fusion step of synaptic vesicle exocytosis. Consistent with this, the kinetics of $$Ca^{2+}$$-dependent phospholipid binding activity of the C2A domain in vitro are compatible with the very fast nature of neurotransmitter release (within 200 μs). The C2A domain was shown to bind negatively charged phospholipids in a $$Ca^{2+}$$-dependent fashion. $$Ca^{2+}$$-binding alters the protein-protein interactions of synaptotagmin such as increasing the affinity of synaptotagmin for syntaxin.

The C2B domain binds to phosphatidyl-inositol-3,4,5-triphosphate (PIP3) in the absence of calcium ions and to phosphatidylinositol bisphosphate (PIP2) in their presence, suggesting that a lipid-interaction switch occurs during depolarization. Ca2+-binding to the C2B domain confers synaptotagmin dimerization involved in the fusion step of synaptic vesicles by $$Ca^{2+}$$-dependent self-clustering via the C2B domain. $$Ca^{2+}$$-independent is the interaction between the C2B domain and SNAP-25, and between the C2B domain and the "synprint" (synaptic protein interaction) motif of the pore-forming subunit of voltage-gated calcium channels. The C2B domain regulates also the recycling step of synaptic vesicles by binding to the clathrin assembly protein, AP-2.