Exocytosis (ek-soh-sy-TOH-sis, Greek: Έξω - external and κύτος - cell) is the durable process by which a cell directs secretory vesicles out of the cell membrane. 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.
In multicellular organisms there are two types of exocytosis:
triggered non-constitutive and
2) non Ca2+
Exocytosis in neuronal chemical synapses
triggered and serves interneuronal signalling. 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
. Exocytosis is the opposite of endocytosis
Five steps are involved in exocytosis:
Certain vesicle-trafficking steps require the translocation of a vesicle over a significant distance. For example, vesicles that carry proteins from the Golgi apparatus
to the cell surface are likely to use motor proteins and a cytoskeletal track to get close to their target before tethering would be appropriate. Both the actin- and the microtubule-based cytoskeletons are implicated in these processes, along with several motor proteins
. 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 with their targets from the more stable, docking
interactions. Tethering involves links over distances of more than about half the diameter of a vesicle from a given membrane surface (>25 nm). Tethering interactions are likely to be involved in concentrating synaptic vesicles at the synapse.
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. In other cell types, whose secretion is constitutive (i.e. continuous, calcium ion independent, non-triggered) there is no priming.
The vesicle fusion
is driven by SNARE
proteins process of merging the vesicle membrane with the target one resulting in release of large biomolecules in the extracellular space (or in case of neurons in the synaptic cleft).
- The merging of the donor and the acceptor membranes accomplishes three tasks:
- *The surface of the plasma membrane increases (by the surface of the fused vesicle). This is important for the regulation of cell size, e.g., during cell growth.
- *The substances within the vesicle are released into the exterior. These might be waste products or toxins, or signalling molecules like hormones or neurotransmitters during synaptic transmission.
- *Proteins embedded in the vesicle membrane are now part of the plasma membrane. The side of the protein that was facing the inside of the vesicle now faces the outside of the cell. This mechanism is important for the regulation of transmembrane receptors and transporters.