User:JeanOhm/golgi

This is a practice page for Golgi apparatus improvements

The Golgi apparatus, also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in eukaryotic cells. It was reported in 1898 by the Italian scientist Camillo Golgi and first named after him in 1910. Various aspects of the Golgi have been reviewed.

Part of the cellular endomembrane system, the Golgi apparatus is best known for packaging proteins into membrane-bound vesicles inside the cell before the vesicles are sent to their destination. It is of particular importance in processing proteins for secretion, containing a set of glycosylation enzymes that attach various sugar monomers to proteins as the proteins move through the apparatus. The Golgi apparatus resides at the intersection of the secretory, lysosomal, and endocytic pathways. It is also involved in cell cycle control and amino acid sensing. The Golgi is a dynamic structure in living cells and is highly variable in structure in different organisms.

Discovery
Owing to its large size and distinctive structure, 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. After first observing it under his microscope, he termed the structure the "internal reticular apparatus". Early references to the Golgi referred to it by various names including the "Golgi–Holmgren apparatus", "Golgi–Holmgren ducts", and "Golgi–Kopsch 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.

Structure


The Golgi apparatus is often composed of flattened lipid bilayer membrane-enclosed disks known as cisternae (singular: cisterna) which originate from vesicles that bud off of "smooth" (ribosome-free) regions of the rough endoplasmic reticulum (RER). Often 3 to 5 cisternae are present in a single Golgi stack, but many more can be present. The cis aspect of the stack is defined as that closest to the RER, while the trans aspect is on the opposite face of the stack. The cisternae have sometimes been reported to be connected by tubules. The Trans Golgi Network (TGN) is often found after the trans face of the stack. In vertebrates a structure known as the Endoplasmic Reticulum-Golgi Intermediate Complex (ERGIC) is observed between the RER and the Golgi stack.

There are structural and organizational differences in the Golgi apparatus among eukaryotes. In some yeasts, Golgi stacking is not observed. The yeast Pichia pastoris does have stacked Golgi, while the baker's (and brewer's) yeast Saccharomyces cerevisiae normally have their cisternae dispersed in the cytoplasm,. As mentioned in the legend of a movie on this page, the names early and late Golgi are used in Saccharomyces cerevisiae, and those names roughly correspond to cis and trans, respectively.

In the intracellular parasite of animals, microsporidia, there is an interlaced network of tubules (MIN) that functions like the Golgi. In contrast, Ostreococcus tauri (an alga which is the smallest known free-living eukaryote) has a single well-defined Golgi stack. In vertebrates, as shown by images on this page, Golgi stacks are sometimes connected by tubules to form ribbons. Golgi matrix proteins are important for the structure of Golgi components. In plants, individual Golgi stacks have been seen to be connected by tubules, although not as a ribbon.

In summary, the ancestral Golgi structure appears to have been a stack, with several independent losses of stacking in various lineages and the development of Golgi ribbons in vertebrates during evolution.

Subcellular localization
Among eukaryotes, the subcellular localization of the Golgi apparatus differs. In mammals, a single Golgi ribbon is usually located near the cell nucleus, close to the centrosome. This is shown especially well in mammalian cells in culture, which have their endoplasmic reticulum spread out over the extent of the cytoplasm, while the Golgi remains near the nucleus. Localization and tubular connections of the Golgi apparatus are dependent on microtubules. In mammalian cells engineered to express a particular Golgi matrix protein, GMAP210, on their mitochondria, their mitochondria become clustered near the centrosome. If microtubules are experimentally depolymerized, then the Golgi apparatus loses connections and becomes individual stacks throughout the cytoplasm. In baker's yeast, Golgi cisternae are scattered throughout the cytoplasm. In plants, Golgi stacks are not concentrated at the centrosomal region, do not form Golgi ribbons and move at several micrometers per second in relation to the ER. Until recently, organization of the plant Golgi was thought to depend on actin cables and not microtubules, but now it is believed that microtubules are involved in "fine-tuning" plant Golgi. A common, but not universal, feature among Golgi is that they are adjacent to RER exit sites.

Biochemical functions in the lumen
The best-known biochemical function of the Golgi is the continuation of sugar modifications to glycoproteins and glycolipids (collectively, the glycome) that was initiated in the endoplasmic reticulum. Glycosyltransferases are the enzymes responsible for building up larger sugar chains while glycoside hydrolase enzymes break down the bonds between the sugar residues. The production of the final product is not uniformly in the direction of increasing length or complexity of the glycan, as exemplified in the image. In cells with defined stacks, individual cisternae or groups of cisternae have different assortments of enzymes, allowing for progressive processing of cargo molecules as they travel from the cis to the trans Golgi face. That is true in plants, which also synthesize the complex polysaccharides of their cell walls in the Golgi lumen using enzymes localized to different parts of the Golgi.

There are a variety of other biochemical alterations in the Golgi. Sulfation of tyrosines and carbohydrates occurs within the TGN. Phosphorylation is important in many cellular processes, and occurs in various locations, including the Golgi,, which also contains phosphatases that can remove phosphates. Lipidation reactions such as palmitoylation, which covalently attach a lipid residue to a protein, also occur in the Golgi. Recently, a protease in the Golgi that cleaves multiple proteins has been reported to be important biochemically, in cell biology and virology.

Cargo trafficking and sorting
There are many models for how molecules are transported from the rough endoplsmic reticulum to their final destinations, but none of the models can explain all experimental observations. Most (but not all ) biologists agree, however, that vesicular transport is involved.

Vesicular transport
The 2013 Nobel Prize in Physiology or Medicine was awarded for "discoveries of machinery regulating vesicle traffic, a major transport system in our cells"  There is obviously a vast amount known about the trafficking of proteins made in the RER. The process starts with the production of a COPII vesicle from an endoplasmic reticulum exit site. The COPII vesicle then moves toward the Golgi, in some instances fusing first with the ERGIC. There is at least one report that COPII vesicles can also be generated from the ERGIC to transport materials to other parts of the cell. A special case is the transport of collagen, which is much larger than standard COPII vesicle and is instead packaged into "mega carriers".

The directed movement of vesicles can be accomplished by tethering them to molecular motors that move along microtubules as shown in the accompanying figure. The motors only move in one direction on the micotubule, but there are different motors for the "forward" and "reverse" directions.

It should be noted that not all important cargo molecules are necessarily incorporated into vesicles that migrate away from the ER. There are close contact sites that have been reported to allow lipid transfer between the ER and Golgi, Close contacts have been incorporated into a "hug and kiss" model for the transfer of cargo from the ER to Golgi.

The COPII vesicles carry materials that should remain part of the ER, as well as molecules destined for the Golgi and beyond. In order to return the ER components to the ER from the Golgi or ERGIC, COPI vesicles are formed for the retrograde transport. If that retrograde transport is inhibited, the Golgi actually swells in all dimensions. The mechanism by which the vesicles and other organelles fuse involves SNARE proteins and is explained in the SNARE article and lipid bilayer fusion.

In fairness, as of 2015, four expert botanists could not reach a consensus about transport from the ER to Golgi in higher plants.

Movement between/of the cisternae
Again, there are many models of how molecules transit from the cis to trans face, not one of which is consistent with all currently available data in all species. One of the most popular early models was that of cisternal maturation, in which cisternae at the cis face would gradually mature while migrating towards the trans face, and would be replaced by newly formed cisternae produced by maturation of the ERGIC (or similar structures in plants). That model requires transport of molecules that are known to be resident in only one part of the Golgi in a retrograde direction from more trans to more cis cisternae as the cisternae mature. The reports of transport of cargoes through Golgi cisternae that are "stapled" or "glued" argue against this model.

Another model theorizes that the Golgi cisternae stay in place, while secretory cargo molecules are somehow transported toward the trans face, either in vesicles or tubules or through direct connections. Images and videos in this section show connections at the edges of stacks. These types of connections, as well as the finding that differently-sized cargo molecules move through the Golgi at different rates, has led to the idea that transport of soluble cargo occurs by diffussion through those connections.

Another popular model is that of anterograde (cis to trans) movement of cargo in vesicles. Vesicles have been reported to form from, and "percolate" between, cisternae in a stack  and even between cisternae of different stacks.

As of 2016 there is still "heated" debate among experts about how cargo is trafficked through the Golgi and evidence in support of the the various hypotheses is "mixed at best". The authors of a 2017 review wrote that nothing about the Golgi "can be stated with absolute clarity".

Exit to other sites
The accompanying figure shows complicated trafficking pathways "downstream" from the Golgi. One complication involves the "exact" identity of the structures labeled 1 and 2. Different authors attached different names to some of them but in this article both will be described collectively as simply "trans Golgi".

The selection/sorting and packaging of cargo involve the vesicular transport Adaptor Protein complexes AP-1 through 5, retromer, exomer and other cargo adaptors. In order to avoid a significant redundancy with the other articles, only selected aspects of cargo exit will be presented.

One well studied example of specificity of sorting and exit to specific sites involves delivery of cargo molecules to the basolateral membrane of polarized epithelial cells. The selection of cargo molecules by the AP complexes involves interactions of specific AP complex subunits with specific amino acid motifs on proteins. The AP1 complex has a medium sized (mu, μ or M) subunit, but in humans there are 2 genes encoding slightly different proteins, AP1M1 and AP1M2. AP1M1 is expressed in all cells, while AP1M2 is expressed in only some cells, and the protein product of AP1M2 binds better than AP1M1 protein to basolateral signal motifs of basolateral cargo proteins.

In contrast to those specific protein-protein interactions, there is evidence that specific protein clustering into "lipid rafts" and the incorporation of those rafts into vesicles that fuse with the apical plasma membrane is important for delivery of cargo to the apical surface. While important signalling sites for a variety of apical proteins have been reported, they can be anywhere on the protein, not just residing inside the lipid bilayer.

Note that the trans Golgi can be involved in a circulation of cargo to the plasma membrane and back to the Golgi through endosomal intermediates. Cargo that needs to be degraded can be shunted away to the lysosome. The diagram may give the impression that the anterograde transport is only via vesicles, but the accompanying video clearly shows tubular transport, and as of 2015 "it is becoming increasingly accepted that together with vesicles, tubules play a major role in the transfer of cargo between different cellular compartments." In some cases, the Golgi is not even involved in transport of cargo to the plasma membrane. For example, in the stem cell shown in the accompanying figure, endoplasmic reticulum is located throughout the cell, while Golgi are absent in the basal region. Proteins still get to the basal plasma membrane, although they do not have the posttranslational modifications, indicating that there can be direct transfer from ER to the plasma membrane.

The signal that targets proteins from the trans Golgi to the lysosome is not even protein, rather it is a mannose-6-phosphate on the glycan chain.

The accompanying diagram indicates that COPI vesicles transport ER resident materials back to the ER. In fact, numerous studies have shown that all Golgi components continuously recycle back to the ER.

cell cycle and growth control
In addition to other cell cycle checkpoints, the Golgi organization also provides a mitotic checkpoint. When the "unlinking" of the stacks in a Golgi ribbon is inhibited, cell cycle progression is blocked. The unlinking is caused by phosphorylation of the Golgi matrix protein GRASP65 at residue Ser277 by one of the c-Jun N-terminal kinases, JNK2.

After unlinking, the Golgi stacks become unstacked and then form vesicular or tubular fragments that have been described as a “haze”. After mitosis, the Golgi stacks must be reformed. That reassembly is dependent on the GRASP proteins, both in vitro and in vivo, as shown in the accompanying image.

There is evidence of a Golgi-localized type of mTORC1, which may be part of an amino acid starvation signaling hub located on the Golgi and involved in cell growth control.

Relationships with cytoskeleton/other organelles
The Golgi matrix protein GMAP210 has been shown to co-purify with microtubules, and changes in the expression of GMAP210 perturbs not only Golgi, but also microtubule network organization. GMAP210 associates with the γ-tubulin small complex, which is required for nucleation of microtubules. The Golgi is known to be a microtubule organizing center, and more than one protein is involved.

Membrane contact sites (MCSs) are sites where two membranes come near to each other, usually less than 30 nm apart. MCSs have been observed between the Golgi and the phagophore, the sack-like structure involved in autophagosome formation. Approximately 20% of phagophores at any one time are closely associated with Golgi membranes, which may be a source of lipids for the expanding phagophore. There is a report that Golgi cisternae can directly develop into autophagosomes.

The Golgi "interactome" has been reported to also include the ER, mitochondria, lysosomes, lipid droplets and peroxisomes." The best characterized are those between the Golgi and ER. MCSs are held together by protein tethers,rather than ctoskeletal elements. . CASP has been reported to be such a tether for the ER-Golgi MCSs. There are several other proteins that function in the ER-Golgi MCSs. VAPs A and B are in the ER membrane and bind to CERT, FAPP2, NIR2 and OSBP which associate with the Golgi through their Pleckstrin homology domains recognizing phosphatidylinositol 4-phosphate. The 4 proteins all contain FFAT motifs that interacts with the VAPs. These proteins are involved in the transfer of lipids from one membrane to the other. For example, there is a gradient of cholesterol in mammalian cells, with higher concentration in the plasma membrane and lower in the ER membranes, and the gradient is produced by the exchange of cholesterol for phosphatidylinositol 4-phosphate.

The toxin ricin travels from the plasma membrane through the Golgi and to the ER via a retrograde movement without entering the cytoplasm. This pathway is thought to use the same mechanism as in Endoplasmic-reticulum-associated protein degradation (ERAD) of damaged proteins from the Golgi.

Rapid partitioning in a 2 phase model
A very different model of Golgi functioning has been proposed, and explains some aspects that cannot be explained by "cisernal maturation". In this model, the Golgi has 2 "phases" or "domains" (left vs right in both external images), and cargo would exit stochastically from the domains at any level (cis, medial or trans) to their next destination.

Golgipathies


The term "Golgipathies" was coined in a review of Cohen syndrome, progressive cerebello-cerebral atrophy type 2 PCCA2 syndrome (mutation in VPS53), Warburg-micro syndrome, MRT13 (mutation in TRAPPC9), A neuromuscular syndrome (mutation in GOLGA2), Dyggve-Melchior-Clausen syndrome (mutation in DYM) and Congenital disorders of glycosylation (mutations in COG1, COG2, COG7 or COG8) Many other disease associated with post-translational modifications in the Golgi have also been reviewed.

Mice engineered to lack the golgin encoded by the USO1 where found to suffer early embryonic death. A less severe, but still lethal neonatally, skeletal dysplasia was found in mice lacking GMAP-210, and human achondrogenesis type 1A is associated with homzygous loss of function mutations in the corresponding human gene.

Gallery

 * GMAP210neonatesc.jpg or homozygous for a GMAP210 mutation. ]]


 * GolgiFboxc.jpg FBXW8 is required for normal Golgi (highlighted yellow) ribbon formation in rat neurons. ]]


 * Life cycle and protein associations of connexins.jpg ]]


 * GolgiAFMc.jpg image of part of a Golgi apparatus isolated from HeLa cells. ]]


 * GolgiTethersc.jpg is thought to be more flexible and collapse onto the Golgi. ]]


 * GolgiCOGsc.jpg'' COG3 and COG8 are required for normal Golgi structure. ]]


 * Blausen 0435 GolgiApparatus.png

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 * Rab6ELKSc.jpg interacts with RAB6 to direct cargo vesicles along microtubules to melanosomes. The flourescent image is from the movie, which shows bright green vesicles moving in a human melanocyte cell line. The bottom panel shows microtubules in red, mobile vesicles in bright green, and melanosomes in the duller green.