Synaptotagmin I is a Ca2؉ -binding protein of synaptic vesicles that serves as a Ca 2؉ sensor for neurotransmitter release and was the first member found of a large family of trafficking proteins. We have now identified a novel synaptotagmin, synaptotagmin XI, that is highly expressed in brain and at lower levels in other tissues. Like other synaptotagmins, synaptotagmin XI has a single transmembrane region and two cytoplasmic C 2 -domains but is most closely related to synaptotagmin IV with which it forms a new subclass of synaptotagmins.

Synaptotagmins bind clathrin AP-2 with high affinity via their second C 2 domain, which indicates they are involved in coated pit function. We now report that expression of synaptotagmins lacking either the second C 2 domain or the entire cytoplasmic region potently inhibit endocytosis. Inhibition was dependent on two intramembrane cysteine residues that were found to be essential for synaptotagmin oligomerization. Cells expressing the wild-type, but not the mutant, truncated synaptotagmin fragment had a reduced number of clathrin-coated pits. These results suggest that the formation of synaptotagmin multimers is an important step in the regulation of coated pit assembly.