Golgi Apparatus

A major organelle in most eukaryotic cells is the structure of membrane-bound sacs called the Golgi apparatus (or Golgi body, Golgi complex, dictyosome). It acts to process and package the macromolecules such as proteins and lipids that are synthesized by the cell. It has been likened to the cell's post office. A major function is the processing of proteins for secretion.

The sacs or folds of the Golgi apparatus are called cisternae. Typically there are five to eight cisternae but as many as sixty have been observed. 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 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.

Cells synthesize 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. The Golgi also plays an important role in the synthesis of proteoglycans, molecules present in the extracellular matrix of animals, and it 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 its 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 its 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] The Golgi also has a putative role in apoptosis, with several Bcl-2 family members localised there, as well as to the mitochondria. In addition a newly characterised anti-apoptotic protein, GAAP (Golgi anti-apoptotic protein), which almost exclusively resides in the Golgi, protects cells from apoptosis by an as-yet undefined mechanism (Gubser et al., 2007).

Lodish, et al.
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