Porosome: Cells Secretory Nanomachine

Abstract

Secretion is a highly regulated fundamental cellular process in living organisms, from yeast to cells in humans. Cellular cargoes such as neurotransmitters in neurons, insulin in beta cells of the endocrine pancreas, or digestive enzymes in the exocrine pancreas are all packaged and stored in membrane-bound secretory vesicles that dock and fuse at the cell plasma membrane to release their contents during secretion. The prevailing view was that secretory vesicles completely merge with the cell plasma membrane, emptying the entire vesicular contents outside the cell during secretion. However, accumulation of partially empty secretory vesicles observed in electron micrographs in cells following a secretory episode suggested fractional release of intra-vesicular contents during cell secretion. Given the high surface tension at the secretory vesicle membrane, fractional intra-vesicular content release during cell secretion could only be possible via a plasma membrane structure capable of preventing the complete merger or collapse of secretory vesicles into the cell plasma membrane. Cup-shaped plasma membrane-embedded lipoprotein structure called porosomes was first discovered in 1996 in live pancreatic acinar cells using atomic force microscopy (AFM) and subsequently confirmed in all cells examined including neurons using AFM, electron microscopy (EM), and solution X-ray. The porosome exhibits dynamics and its chemical composition demonstrates the utilization of energy in the form of both ATP and guanosine triphosphate (GTP), the participation of molecular motors, ion channels, and soluble N-ethylmaleimide-sensitive factor activating protein receptor (SNARE) membrane fusion proteins, among others. Porosomes are composed of nearly 30 proteins, as opposed to the 120 nm nuclear pore complex comprised of nearly 1000 protein molecules. Porosomes range in size from 15 nm in neurons and astrocytes to 100–180 nm in endocrine and exocrine cells. Porosome has been functionally reconstituted into artificial lipid membrane and in live cells. During secretion, secretory vesicles dock at the base of the porosome complex via v-SNARE proteins at the secretory vesicle membrane and t-SNARE proteins at the porosome base. In the presence of calcium, the v-SNARE and t-SNARE proteins in the opposing bilayers interact in a circular array to establish conducting channels or fusion pores. An increase in volume of the docked secretory vesicle via the rapid entry of ions and aquaporin-mediated rapid entry of water molecules results in increased intra-vesicular pressure, enabling the fractional release of vesicular contents from the cell with great precision. Collectively, these observations provide a molecular understanding of the fractional release of intra-vesicular contents via the transient or kiss-and-run mechanism of cell secretion. The discovery of the porosome and the molecular mechanism of its structure–function has resulted in a paradigm shift in our understanding of the secretory process in cells.