Porosome: the secretory portal in cells.

Porosomes are supramolecular, cup-shaped lipoprotein structures at the cell plasma membrane, where membrane-bound secretory vesicles dock and fuse to release intravesicular contents to the outside during cell secretion. The porosome opening to the outside ranges from 150 nm in diameter in acinar cells of the exocrine pancreas to 12 nm in neurons. In the past decade, the composition of the porosome, its structure and dynamics at nanometer resolution in real time, and its functional reconstitution into an artificial lipid membrane have been described. Discovery of the universal secretory machinery in cells, the porosome, came as no surprise since porosome-like "canaliculi" structures for secretion from human platelets, the secretory machinery in single-cell organisms like the secretion apparatus in bacteria and Toxoplasma gondii, and the contractile vacuole in paramecium have been demonstrated. In this review, the discovery of the porosome complex and the molecular mechanism of its function and how this information provides a new understanding of cell secretion are discussed.