Synaptotagmin III isoform is compartmentalized in pancreatic β-cells and has a functional role in exocytosis

Synaptotagmin is involved in Ca²⁺-regulated secretion and has been suggested to serve as a general Ca²⁺ sensor on the membrane of secretory vesicles in neuronal cells. Insulin exocytosis from the pancreatic β-cells is an example of a Ca²⁺-dependent secretory process. Previous studies of pancreatic β-cells were unable to show presence of synaptotagmin I. We now present biochemical and immunohistochemical data showing that synaptotagmin III is present in pancreatic β-cells as well as in the insulin-secreting cell line HIT-T15 and in rat insulinoma. By subcellular fractionation, we found synaptotagmin III in high-density fractions together with insulin and secretogranin I, indicating colocalization of synaptotagmin III and insulin in secretory granules. We could also show that blockade of synaptotagmin III by a specific antibody inhibited Ca²⁺-induced changes in β-cells membrane capacitance, suggesting that synaptotagmin III is part of the functional protein complex regulating β-cells exocytosis. The synaptotagmin III antibody did not affect the activity of the voltage-gated L-type Ca²⁺- channel. These findings are compatible with the view that synaptotagmin III, because of its distinct localization in the pancreatic β-cells, functionally modulates insulin exocytosis. This indicates that synaptotagmin may have a general role in the regulation of exocytosis not only in neuronal cells but also in endocrine cells.