Golgi apparatus

The primary function of the Golgi apparatus is to process and package the macromolecules such as proteins and lipids that are synthesized by the cell. It is particularly important in the processing of proteins for secretion. The Golgi apparatus forms a part of the endomembrane system of eukaryotic cells.

Structure

The Golgi is composed of membrane-bound stacks known as cisternae. Between five and eight are usually present; however, in some protists as many as sixty have been observed.^[1]

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]

A cisterna (plural cisternae) comprises a flattened membrane disk that makes up the Golgi apparatus. A typical Golgi has anywhere from 3 to 7 cisternae stacked upon each other like a stack of dinner plates, but there are usually around 6. The cisternae carry Golgi enzymes to help or to modify cargo proteins traveling through them destined for other parts of the cell.

The cisternae also carry structural proteins important for its maintenance as a flattened membrane and its stacking upon each other.

The earliest cisternae are called the cis-cisternae, followed by the medial cisternae, then the trans-cisternae (as they move away from the endoplasmic reticulum).

The formation of new cisternae is often called the cis-Golgi network and at the end of the Golgi where transport to other parts of the cell occurs is called the trans-Golgi network. Both are thought to be specialized cisternae leading in and out of the Golgi apparatus.

Function

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 phosphates (phosphorylation). In order to do so the Golgi transports substances such as nucleotide sugars into the organelle from the cytosol. Proteins are also labeled 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.

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 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 constitute 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.^[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).

Vesicular transport

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 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]

Type	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 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.	Digestive proteases destined for the lysosome

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. ^[3]

Golgi apparatus

Contents

Structure

Function

Vesicular transport

Transport mechanism

Links

References