Synaptotagmin 13: Structure and expression of a novel synaptotagmin

Synaptotagmins represent a family of putative vesicular trafficking protein s. With synaptotagmin 13, we have now identified a novel synaptotagmin, mak ing this one of the largest families of trafficking proteins. Similar to sy naptotagmins 3, 4, 6, 7, 9, and 11, synaptotagmin 13 is expressed at highes t levels in brain but is also detectable at lower levels in non-neuronal ti ssues. Synaptotagmin 13 is composed of the canonical domains of synaptotagm ins that include an N-terminal transmembrane region and two C-terminal cyto plasmic C-2-domains (C(2)A- and C2B-domain) and a connecting sequence betwe en the transmembrane region and the C-2-domains. Different from most other synaptotagmins, however, synaptotagmin 13 does not have an N-terminal seque nce preceding the transmembrane region, and features an unusually long conn ecting sequence that is proline-rich, Furthermore, the C-2-domains of synap totagmin are degenerate and lack almost all of the residues involved in Ca2 + binding, suggesting that synaptotagmin 13 is not a Ca2+-binding protein u nlike most other synaptotagmins, Our data demonstrate that synaptotagmins r epresent a larger and more complex gene family than previously envisioned.