Exocytosis (Exocytosis is the opposite of endocytosis) is a form of active transport in which a cell transports molecules (such as proteins) out of the cell(exo- + cytosis) by expelling them in an energy-using process.
Exocytosis and its counterpart, endocytosis, are used by all cells because most chemical substances important to them are large polar molecules that cannot pass through the hydrophobic plasma or cell membrane by passive means.
These membrane-bound vesicles contain soluble proteins to be secreted to the extracellular environment, as well as membrane proteins and lipids that are sent to become components of the cell membrane.
The mechanism of the secretion of intravesicular contents out of the cell is very different from the incorporation in the cell membrane of ion channels, signaling molecules, or receptors.
While for membrane recycling and the incorporation in the cell membrane of ion channels, signaling molecules, or receptors complete membrane merger is required, for cell secretion there is transient vesicle fusion with the cell membrane in a process called exocytosis, dumping its contents out of the cell’s environment. This suggested that during the secretory process, only a portion of the vesicular content is able to exit the cell.
- Constitutive exocytosis is performed by all cells and serves the release of components of the extracellular matrix, or just delivery of newly synthesized membrane proteins that are incorporated in the plasma membrane after the fusion of the transport vesicle.
- Regulated exocytosis, on the other hand, requires an external signal, a specific sorting signal on the vesicles, a clathrin coat, as well as an increase in intracellular calcium.
- Vesicular exocytosis in prokaryote gram negative microbes is the latest finding in exocytosis. Herein, periplasm of gram negative microbes is pinched off as bacterial outer membrane vesicles (OMVs) for translocating microbial biochemical signals into eukaryotic host cells or other microbes located nearby, accomplishing control of the secreting microbe on its environment.
The 5 Steps Of Exocytosis
Certain vesicle-trafficking steps require the transportation of a vesicle over a moderately small distance. Once the vesicles reach their targets, they come into contact with tethering factors that can restrain them.
It is useful to distinguish between the initial, loose tethering of vesicles to their objective from the more stable, packing interactions. Tethering involves links over distances of more than about half the diameter of a vesicle from a given membrane surface (>25 nm). Tethered vesicles are also involved in regular cell’s transcription processes.
Secretory vesicles transiently dock at the cell plasma membrane, preceding the formation of a tight t-/v-SNARE complex, leading to priming and the establishment of continuity between the opposing bilayers.
In neuronal exocytosis, the term priming has been used to include all of the molecular rearrangements and ATP-dependent protein and lipid modifications that take place after initial docking of a synaptic vesicle but before exocytosis, such that the influx of calcium ions is all that is needed to trigger nearly instantaneous neurotransmitter release.
Transient vesicle fusion is driven by SNARE proteins, resulting in release of vesicle contents into the extracellular space.