Synaptotagmin

Synaptotagmins (SYTs) 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. There are 17 isoforms in the mammalian synaptotagmin family. There are several C2-domain containing protein families that are related to synaptotagmins, including transmembrane (Ferlins, Extended-Synaptotagmin (E-Syt) membrane proteins, and MCTPs) and soluble (RIMS1 and RIMS2, UNC13D, synaptotagmin-related proteins and B/K) proteins. The family includes synaptotagmin 1, a Ca2+ sensor in the membrane of the pre-synaptic axon terminal, coded by gene SYT1.

==Functions== Based on their brain/endocrine distribution and biochemical properties, in particular C2 domains of certain synaptotagmins bound to calcium, synaptotagmins were proposed to function as calcium sensors in the regulation of neurotransmitter release and hormone secretion. Although synaptotagmins share a similar domain structure and a high degree of homology in the C2 domains, not all synaptotagmins bind to calcium. In fact, only eight out of the seventeen synaptotagmins are capable of calcium binding. The calcium binding synaptotagmins include synaptotagmins 1, 2, 3, 5, 6, 7, 9, and 10. The remaining synaptotagmins do not bind to calcium due to the lack of calcium coordinating residues or spatial orientation of the acidic residues (see the section on C2 domains for details). Synaptotagmins Syt1, Syt2, and Syt9 function as quick release sensors.