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

Synaptotagmin is involved in Ca2+-regulated secretion and has been suggeste d to serve as a general Ca2+ sensor on the membrane of secretory vesicles i n neuronal cells. Insulin exocytosis from the pancreatic beta-cell is an ex ample of a Ca2+-dependent secretory process. Previous studies of pancreatic beta-cells were unable to show presence of synaptotagmin I. We now present biochemical and immunohistochemical data showing that synaptotagmin III is present in pancreatic beta-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 secre togranin I, indicating colocalization of synaptotagmin III and insulin in s ecretory granules, We could also show that blockade of synaptotagmin III by a specific antibody inhibited Ca2+-induced changes in beta-cell membrane c apacitance, suggesting that synaptotagmin III is part of the functional pro tein complex regulating beta-cell exocytosis, The synaptotagmin III antibod y did not affect the activity of the voltage-gated L-type Ca2+-channel. The se findings are compatible with the view that synaptotagmin III, because of its distinct localization in the pancreatic beta-cell, functionally modula tes insulin exocytosis, This indicates that synaptotagmin may have a genera l role in the regulation of exocytosis not only in neuronal cells but also in endocrine cells.