Golgi apparatus
The Golgi apparatus (/ˈɡoʊldʒiː/), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic cells.[1] It was identified in 1897 by the Italian physician Camillo Golgi and named after him in 1898.[2]
Part of the cellular endomembrane system, the Golgi apparatus
packages proteins inside the cell before they are sent to their destination; it
is particularly important in the processing of proteins for secretion.
Discovery
Owing
to its large size, the Golgi apparatus was one of the first organelles to be
discovered and observed in detail. It was discovered in 1898 by Italian
physician Camillo Golgi during an investigation of the nervous
system.[2]
After first observing it under his microscope,
he termed the structure the internal reticular apparatus. Some doubted
the discovery at first, arguing that the appearance of the structure was merely
an optical illusion created by the observation technique used by Golgi. With
the development of modern microscopes in the 20th century, the discovery was
confirmed.[3]
Early references to the Golgi referred to it by various names including the
"Golgi–Holmgren apparatus", "Golgi–Holmgren ducts", and
"Golgi–Kopsch apparatus".[2]
The term "Golgi apparatus" was used in 1910 and first appeared in
scientific literature in 1913.[2]
Structure
Found within the cytoplasm of
both plant and animal cells, the Golgi is composed of stacks of membrane-bound
structures known as cisternae (singular: cisterna). An individual stack
is sometimes called a dictyosome (from Greek dictyon: net + soma:
body),[4]
especially in plant cells.[5]
A mammalian cell typically contains 40 to 100 stacks.[6]
Between four and eight cisternae are usually present in a stack; however, in
some protists
as many as sixty have been observed.[3]
Each cisterna comprises a flat, membrane enclosed disc that includes special
Golgi enzymes which modify or help to modify cargo proteins that travel through
it.[7]
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The cisternae stack has four
functional regions: the cis-Golgi network, medial-Golgi, endo-Golgi, and
trans-Golgi network. Vesicles from the endoplasmic reticulum (via the vesicular-tubular clusters) fuse with
the network and subsequently progress through the stack to the trans Golgi
network, where they are packaged and sent to their destination. Each region
contains different enzymes which selectively modify the contents depending on
where they reside.[8]
The cisternae also carry structural proteins important for their maintenance as
flattened membranes which stack upon each other.[9]
Function
Cells synthesize a large number
of different macromolecules. The Golgi apparatus is integral in modifying,
sorting, and packaging these macromolecules for cell secretion[10]
(exocytosis)
or use within the cell.[11]
It primarily modifies proteins delivered from the rough endoplasmic reticulum but is also
involved in the transport of lipids around the cell, and the creation of lysosomes.[11]
In this respect it can be thought of as similar to a post office; it packages
and labels items which it then sends to different parts of the cell.
Enzymes within the cisternae are
able to modify the proteins by addition of carbohydrates (glycosylation)[12]
and phosphates (phosphorylation). In order to do so, the Golgi
imports substances such as nucleotide sugars from the cytosol. These
modifications may also form a signal
sequence which determines the final destination of the protein. For
example, the Golgi apparatus adds a mannose-6-phosphate
label to proteins destined for lysosomes.
The Golgi plays an important
role in the synthesis of proteoglycans, which are molecules present in the extracellular matrix of animals. It is also a
major site of carbohydrate synthesis.[13]
This includes the production of glycosaminoglycans
(GAGs), long unbranched polysaccharides which the Golgi then attaches to a
protein synthesised in the endoplasmic reticulum to form proteoglycans.[14]
Enzymes in the Golgi polymerize several of these GAGs via a xylose link onto
the core protein. Another task of the Golgi involves the sulfation of
certain molecules passing through its lumen via sulfotranferases that gain
their sulfur molecule from a donor called PAPS. This process occurs on the GAGs
of proteoglycans as well as on the core protein. Sulfation is generally
performed in the trans-Golgi network. The level of sulfation is very important
to the proteoglycans' signalling abilities as well as giving the proteoglycan
its overall negative charge.[13]
The phosphorylation of molecules
requires that ATP is imported into the lumen
of the Golgi[15]
and utilised by resident kinases such as casein
kinase 1 and casein kinase 2. One molecule that is
phosphorylated in the Golgi is Apolipoprotein,
which forms a molecule known as VLDL that is a constituent of blood serum.
It is thought that the phosphorylation of these molecules is important to help
aid in their sorting for secretion into the blood serum.[16]
The Golgi has a putative role in
apoptosis,
with several Bcl-2
family members localised there, as well as to the mitochondria.
A newly characterized protein, GAAP (Golgi anti-apoptotic protein), almost
exclusively resides in the Golgi and protects cells from apoptosis by an as-yet
undefined mechanism.[17]
Vesicular transport
Diagram of
secretory process from endoplasmic reticulum (orange) to Golgi apparatus
(pink). 1. Nuclear membrane; 2. Nuclear pore; 3. Rough endoplasmic reticulum
(RER); 4. Smooth endoplasmic reticulum (SER); 5. Ribosome attached to RER; 6.
Macromolecules; 7. Transport vesicles; 8. Golgi apparatus; 9. Cis face
of Golgi apparatus; 10. Trans face of Golgi apparatus; 11. Cisternae of
the Golgi Apparatus
The vesicles that leave the
rough endoplasmic reticulum are transported
to the cis face of the Golgi apparatus, where they fuse with the Golgi
membrane and empty their contents into the lumen.
Once inside the lumen, the molecules are modified, then sorted for transport to
their next destinations. The Golgi apparatus tends to be larger and more
numerous in cells that synthesize and secrete large amounts of substances; for
example, the plasma B cells and the antibody-secreting
cells of the immune system have prominent Golgi complexes.
Those proteins destined for
areas of the cell other than either the endoplasmic reticulum or Golgi apparatus are
moved towards the trans face, to a complex network of membranes and
associated vesicles known as the trans-Golgi network (TGN). This area of
the Golgi is the point at which proteins are sorted and shipped to their
intended destinations by their placement into one of at least three different
types of vesicles, depending upon the molecular marker they carry.
Types
|
Description
|
Example
|
Exocytotic vesicles (continuous)
|
Vesicle contains proteins destined for
extracellular release. After packaging, the vesicles bud off and immediately
move towards the plasma membrane, where they fuse and release the
contents into the extracellular space in a process known as constitutive secretion.
|
Antibody release by activated plasma
B cells
|
Secretory vesicles (regulated)
|
Vesicle contains proteins destined for
extracellular release. After packaging, the vesicles bud off and are stored
in the cell until a signal is given for their release. When the appropriate
signal is received they move towards the membrane and fuse to release their
contents. This process is known as regulated secretion.
|
Neurotransmitter
release from neurons
|
Lysosomal vesicles
|
Vesicle contains proteins and ribosomes destined
for the lysosome,
an organelle of degradation containing many acid hydrolases,
or to lysosome-like storage organelles. These proteins include both digestive
enzymes and membrane proteins. The vesicle first fuses with the late endosome,
and the contents are then transferred to the lysosome via unknown mechanisms.
|
Transport mechanism
The transport
mechanism which proteins use to progress through the Golgi apparatus is not
yet clear; however a number of hypotheses currently exist. Until recently, the
vesicular transport mechanism was favoured but now more evidence is coming to
light to support cisternal maturation. The two proposed models may actually
work in conjunction with each other, rather than being mutually exclusive. This
is sometimes referred to as the combined model.[13]
- Cisternal maturation model: the cisternae of the Golgi apparatus move by being built at the cis face and destroyed at the trans face. Vesicles from the endoplasmic reticulum fuse with each other to form a cisterna at the cis face, consequently this cisterna would appear to move through the Golgi stack when a new cisterna is formed at the cis face. This model is supported by the fact that structures larger than the transport vesicles, such as collagen rods, were observed microscopically to progress through the Golgi apparatus.[13] This was initially a popular hypothesis, but lost favour in the 1980s. Recently it has made a comeback, as laboratories at the University of Chicago and the University of Tokyo have been able to use new technology to directly observe Golgi compartments maturing.[18] Additional evidence comes from the fact that COPI vesicles move in the retrograde direction, transporting endoplasmic reticulum proteins back to where they belong by recognizing a signal peptide.[19]
- Vesicular transport model: Vesicular transport views the Golgi as a very stable organelle, divided into compartments in the cis to trans direction. Membrane bound carriers transport material between the endoplasmic reticulum and the different compartments of the Golgi.[20] Experimental evidence includes the abundance of small vesicles (known technically as shuttle vesicles) in proximity to the Golgi apparatus. To direct the vesicles, actin filaments connect packaging proteins to the membrane to ensure that they fuse with the correct compartment.[13]
Fate during mitosis
In animal cells, the Golgi
apparatus will break up and disappear following the onset of mitosis, or
cellular division. During the telophase of mitosis, the Golgi apparatus reappears. As of
December 2009 it is uncertain how this occurs.[21]
In contrast, Golgi stacks have been observed to remain intact in plant or yeast
cells throughout the cell cycle. The reason for this difference is not yet
known, but it may, in part, be a consequence of golgin proteins.
From:
http://en.wikipedia.org/wiki/Golgi_apparatus
https://www.youtube.com/watch?v=m3aRWCyxyno
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