Synaptotagmins, such as SYT5, are a family of type III membrane proteins characterized by cytoplasmic repeats related to protein kinase C regulatory (C2) domains, which are thought to bind calcium. Synaptotagmins may act both as negative regulators of vesicle fusion, allowing fusion in the presence of calcium, and as calcium receptors or sensor molecules. May be involved in Ca2+-dependent exocytosis of secretory vesicles through Ca2+ and phospholipid binding to the C2 domain or may serve as Ca2+ sensors in the process of vesicular trafficking and exocytosis. Regulates the Ca2+-dependent secretion of norepinephrine in PC12 cells. Required for export from the endocytic recycling compartment to the cell surface.