Golgi apparatus

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Diagram of the endomembrane system in a typical eukaryote cell

Micrograph of Golgi apparatus, visible as a stack of semicircular black rings near the bottom. Numerous circular vesicles can be seen in proximity to the organelle

Diagram of secretory process from endoplasmic reticuli (orange) to Golgi apparatus (pink). Please click for full labels.

The Golgi apparatus (also called the Golgi body, Golgi complex, or dictyosome) is an organelle found in typical eukaryotic cells. It was identified in 1898 by the Italian physician Camillo Golgi and was named after him. The primary function of the Golgi apparatus is to process and package macromolecules synthesised by the cell, primarily proteins and lipids. The Golgi apparatus forms a part of the endomembrane system present in eukaryotic cells.

[edit] Structure

The Golgi is composed of membrane-bound sacs known as cisternae. Between five and eight are usually present, however as many as sixty have been observed.^[1] Surrounding the main cisternae are a number of spherical vesicles which have budded off from the cisternae. The cisternae stack has five functional regions: the cis-Golgi network, cis-Golgi, medial-Golgi, trans-Golgi, and trans-Golgi network. Vesicles from the endoplasmic reticulum (via the vesicular-tubular cluster) fuse with the cis-Golgi network and subsequently progress through the stack to the trans-Golgi network, where they are packaged and sent to the required destination. Each region contains different enzymes which selectively modify the contents depending on where they are destined to reside.^[2] Please remember that as mentioned below, the precise process of how the structure of the actual lumen, or content of the cisternae of the Golgi apparatus, has not yet been determined. However, it is also supposed to be a multi joined membrane-bound accumulation.

[edit] Function

Cells synthesise a large number of different macromolecules required for life. The Golgi apparatus is integral in modifying, sorting, and packaging these substances for cell secretion (exocytosis) or for use within the cell. 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. In this respect it can be thought of as similar to a post office; it packages and labels "items" and then sends them to different parts of the cell.

Enzymes within the cisternae are able to modify substances by the addition of carbohydrates (glycosylation) and phosphate (phosphorylation) to them. In order to do so the Golgi transports substances such as nucleotide sugars into the organelle from the cytosol. Proteins are also labelled with a signal sequence of molecules which determine their final destination. For example, the Golgi apparatus adds a mannose-6-phosphate label to proteins destined for lysosomes.

A specific example of modifying tasks of the Golgi include the important role it plays in the synthesis of proteoglycans, molecules present in the extracelluar matrix of animals. The Golgi is a major site of carbohydrate synthesis.^[3]

This includes the productions of glycosaminoglycans or GAGs, long unbranched polysaccharides which the Golgi then attaches to a protein synthesized in the endoplasmic reticulum to form the proteoglycan.^[4]Enzymes in the Golgi will 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 it’s lumen via sulphotranferases that gain their sulphur molecule from a donor called PAPs. This process occurs on the GAGs of proteoglycans as well as on the core protein. The level of sulfation is very important to the proteoglycans signalling abilities as well as giving the proteoglycan it’s overall negative charge.^[3]

The Golgi is also capable of phosphorylating molecules. To do so it transports ATP into the lumen.^[5] The Golgi itself contains resident kinases, such as casein kinases. One molecule that is phosphorylated in the Golgi is Apolipoprotein, which forms a molecule known as VLDL that is a constitute of blood serum. It is thought that the phosphorylation of these molecules is important to help aid in their sorting of secretion into the blood serum.^[6]

Vesicles which 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 they are modified, sorted, and shipped towards their final destination. As such, the Golgi apparatus tends to be more prominent and numerous in cells synthesising and secreting many substances: plasma B cells, 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).^[2] 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:^[2]

[edit] 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.^[3]

• 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.^[3] 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.^[7]. Additional evidence comes from the fact that COP1 vesicles move in the retrograde direction,. transporting ER proteins back to where they belong by recognizing a signal peptide.^[8]

• Vesicular transport model: Vesicular transport views the Golgi as a very stable organelle, divided into compartments is the cis to trans direction. Membrane bound carriers transported material between the ER and Golgi and the different compartments of the Golgi.^[9] Experimental evidence inlcudes the abundance of small vesicles (known technically as shuttle vesicles) in proximity to the Golgi apparatus. Directionality is achieved by packaging proteins into either forward-moving or backward-moving (retrograde) transport vesicles, or alternatively this directionality may not be necessary as the constant input of proteins from the endoplasmic reticulum on the cis face of the Golgi would ensure flow. Irrespectively, it is likely that the transport vesicles are connected to a membrane via actin filaments to ensure that they fuse with the correct compartment.^[3]

It has also been proposed that the cisternae are interconnected, and the transport of cargo molecules within the Golgi is due to diffusion, while the localisation of Golgi-resident proteins is achieved by an unknown mechanism.^{[citation needed]}

[edit] Footnotes

[edit] References