Жасушадан молекулалардың шығуы: Экзоцитоз процесі

Active transport and bulk transport in which a cell transports molecules out of the cell
the meaning of "exocytosis" in dermatopathology

Exocytosis (,/ɛ//k//s//oʊ//s//aɪ/'/t//oʊ//s//ᵻ//s/) is a form of active transport and bulk transport in which a cell transports molecules (e. g., neurotransmitters and proteins) out of the cell (exo + cytosis). As an active transport mechanism, exocytosis requires the use of energy to transport material. 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 portion of the cell membrane by passive means. Exocytosis is the process by which a large amount of molecules are released; thus it is a form of bulk transport. Exocytosis occurs via secretory portals at the cell plasma membrane called porosomes. Porosomes are permanent cup shaped lipoprotein structure at the cell plasma membrane, where secretory vesicles transiently dock and fuse to release intra vesicular contents from the cell. In exocytosis, membrane bound secretory vesicles are carried to the cell membrane, where they dock and fuse at porosomes and their contents (i. e., water soluble molecules) are secreted into the extracellular environment. This secretion is possible because the vesicle transiently fuses with the plasma membrane. In the context of neurotransmission, neurotransmitters are typically released from synaptic vesicles into the synaptic cleft via exocytosis; however, neurotransmitters can also be released via reverse transport through membrane transport proteins. Exocytosis is also a mechanism by which cells are able to insert membrane proteins (such as ion channels and cell surface receptors), lipids, and other components into the cell membrane. Vesicles containing these membrane components fully fuse with and become part of the outer cell membrane.

The term was proposed by De Duve in 1963.

Certain vesicle trafficking steps require the transportation of a vesicle over a moderately small distance. For example, vesicles that transport proteins from the Golgi apparatus to the cell surface area, will be likely to use motor proteins and a cytoskeletal track to get closer to their target. Before tethering would have been appropriate, many of the proteins used for the active transport would have been instead set for passive transport, because the Golgi apparatus does not require ATP to transport proteins. Both the actin and the microtubule base 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 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). Tethering interactions are likely to be involved in concentrating synaptic vesicles at the synapse.

Secretory vesicles transiently dock and fuse at the porosome at the cell plasma membrane, via a tight t /v SNARE ring complex.

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.

Retrieval of synaptic vesicles occurs by endocytosis. Most synaptic vesicles are recycled without a full fusion into the membrane (kiss and run fusion) via porosome. Non constitutive exocytosis and subsequent endocytosis are highly energy expending processes, and thus, are dependent on mitochondria. Examination of cells following secretion using electron microscopy demonstrate increased presence of partially empty vesicles following secretion. This suggested that during the secretory process, only a portion of the vesicular content is able to exit the cell. This could only be possible if the vesicle were to temporarily establish continuity with the cell plasma membrane at porosomes, expel a portion of its contents, then detach, reseal, and withdraw into the cytosol (endocytose). In this way, the secretory vesicle could be reused for subsequent rounds of exo endocytosis, until completely empty of its contents.