Porosome: the fusion pore revealed by multiple imaging modalities

Secretion occurs in all cells of multicellular organisms and involves the delivery of secretory products packaged in membrane-bound vesicles to the cell exterior. Specialized cells for neurotransmission, enzyme secretion, or hormone release utilize a highly regulated secretory process. Secretory vesicles are transported to specific sites at the plasma membrane, where they dock and fuse to release their contents. Similar to other cellular processes, cell secretion is found to be highly regulated and a precisely orchestrated event. It has been demonstrated that membrane-bound secretory vesicles dock and fuse at porosomes, which are specialized supramolecular structures at the cell plasma membrane. Swelling of secretory vesicles results in a buildup of pressure, allowing expulsion of intravesicular contents. The extent of secretory vesicle swelling dictates the amount of intravesicular contents expelled during secretion. The discovery of the porosome, its isolation, its structure and dynamics at nanometer resolution and in real time, and its biochemical composition and functional reconstitution into artificial lipid membrane have been determined. The molecular mechanism of secretory vesicle fusion at the base of porosomes and vesicle swelling have also been resolved. These findings reveal the molecular machinery and mechanism of cell secretion. In this chapter, the discovery of the porosome, its isolation, its structure and dynamics at nanometer resolution and in real time, and its biochemical composition and functional reconstitution into artificial lipid membrane are discussed.