Synaptotagmin I-DeltaC2B. A novel synaptotagmin isoform with a single C2 domain in the bovine adrenal medulla

Synaptotagmin I is a 65 kDa type 1 membrane glycoprotein found in secretory organelles that plays a key role in regulated exocytosis. We have characterised two forms (long and short) of synaptotagmin I that are present in the bovine adrenal medulla. The long form is a type I integral membrane protein which has two cytoplasmic C2 domains and corresponds to the previously characterised full-length synaptotagmin I isoform. The short-form synaptotagmin I-DeltaC2B has the same structure in the lumenal and transmembrane sequences, but synaptotagmin I-DeltaC2B is truncated such that it only has a single cytoplasmic C2 domain. Analysis of synaptotagmin I-DeltaC2B expression indicates that synaptotagmin I-DeltaC2B is preferentially expressed in the bovine adrenal medulla. However, it is absent from the dense core chromaffin granules. Furthermore, when expressed in the rat pheochromocytoma cell line PC12 bovine synaptotagmin I-DeltaC2B is largely absent from dense core granules and synaptic-like microvesicles. Instead, indirect immunofluorescence microscopy reveals the intracellular location of synaptotagmin I-DeltaC2B to be the plasma membrane.