Exocytosis is the durable, energy-consuming process by which a cell directs the contents
of secretory vesicles out of the cell membrane and into the extracellular space.
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.
However, 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.
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, expel a portion of its contents, then detach, reseal, and
withdraw into the cytosol.
In this way, the secretory vesicle could be reused for subsequent rounds of exo-endocytosis,
until completely empty of its contents.
Types In multicellular organisms there are two types
of exocytosis: 1) Ca2+ triggered non-constitutive and 2) non-Ca2+ triggered constitutive.
Exocytosis in neuronal chemical synapses is Ca2+ 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.
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.
Exocytosis is the opposite of endocytosis. 3) 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 for translocating microbial biochemical signals into eukaryotic host cells or other
microbes located nearby, accomplishing control of the secreting microbe on its environment
- including invasion of host, endotoxemia, competing with other microbes for nutrition,
etc.
This finding of membrane vesicle trafficking occurring at the host-pathogen interface,
also breaks the myth that exo-cytosis is purely a eukaryotic cell phenomenon.
Steps Five steps are involved in exocytosis:
Vesicle trafficking Certain vesicle-trafficking steps require
the transportation of a vesicle over a moderately small distance.
For example, vesicles that have the duty to transport the proteins from the Golgi apparatus
to the cell surface area, will be likely to use motor proteins and a cytoskeletal track
to get closer than previously stated 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, due to the fact that 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.
Vesicle tethering It is useful to distinguish between the initial,
loose tethering of vesicles with 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.
Tethering interactions are likely to be involved in concentrating synaptic vesicles at the
synapse.
The vesicles are also involved in regular cell's transcription processes.
Vesicle docking 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.
Vesicle priming 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 there is no priming.
Vesicle fusion
Transient vesicle fusion is driven by SNARE proteins, resulting in release of vesicle
contents into the extracellular space.
The merging of the donor and the acceptor membranes accomplishes three tasks:
The surface of the plasma membrane increases.
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 signaling 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 and transporters.
Vesicle Retrieval Retrieval of synaptic vesicles occurs by endocytosis.
Some synaptic vesicles are recycled without a full fusion into the membrane, while others
require a complete reformation of synaptic vesicles from the membrane by a specialized
complex of proteins.
Non-constitutive exocytosis and subsequent endocytosis are highly energy expending processes,
and thus, are dependent on mitochondria.
References
External links Exocytosis at the US National Library of Medicine
Medical Subject Headings See also
Secretory pathway Synapse
Kiss-and-run fusion Endocytosis
Phagocytosis Endocytic cycle
Membrane nanotube Active Transport
Viral shedding Presynaptic active zone
Residual body Bacterial outer membrane vesicles
Membrane vesicle trafficking Host-pathogen interface