The Golgi apparatus is one of the cell organelles and serves to modify and sort proteins. It works closely with the endoplasmic reticulum. It is also involved in secretion formation.
What is the Golgi apparatus?
The Golgi apparatus represents an important cell organelle in which proteins produced in the endoplasmic reticulum are modified and sorted. Furthermore, it forms lysosomes, which contain enzymes for the degradation of endogenous and exogenous proteins. The lysosomes are membrane-enclosed cell organelles that generate a low pH in their interior via proton pumps, thereby acidifying the enzymes. The Golgi apparatus is present in every eukaryotic cell and forms a membrane-enclosed reaction space that plays an important role in exocytosis. It was discovered by the Italian pathologist Camillo Golgi in 1898 during histological studies of the brain and named after him. Inside the Golgi apparatus, the proteins of the endoplasmic reticulum react with other proteins or with sugar residues (glycosylation) to modify them. In this way, the proteins are first converted into their transportable form. This is followed by sorting according to their destination. Within the Golgi apparatus, however, no new proteins are generated, only existing ones are modified.
Anatomy and structure
The Golgi apparatus is characterized by stacks of shallow membrane-enclosed cavities. These cavities are referred to as cisternae. Usually, the stack contains three to eight cisternae. Sometimes there can be as many as 30 cisterns. The stack has an average diameter of one micrometer. The technical term of a stack is dictyosome. The number of dictyosomes depends on the type of cell. For example, some cells can contain up to several hundred dictyosomes. Microtubules ensure that the Golgi apparatus is mostly located near the nucleus and centrosomes in animal and human cells. However, in most plant cells, the Golgi apparatus is distributed throughout the cytoplasm of the cell. An important feature of the Golgi apparatus is its polarization. The side facing the endoplasmic reticulum is convex and the side facing away from it is concave. Thereby, the Golgi apparatus receives vesicles equipped with the coat protein COP II from the endoplasmic reticulum. The convex side is also called the cis-Golgi network (CGN). The side facing away from the ER is called the trans-Golgi network (TGN). The Golgi networks represent multiple small cisternae and vesicles that are interconnected. The cisternae located between the Golgi networks are the so-called Golgi stacks, which have a specific enzymatic configuration. In this process, proteins pass from the cis-Golgi network to the trans-Golgi network. There are two models for this process, both of which probably apply. Either the vesicles move from the CGN to the TGN, in which case the proteins are retained, or the proteins undergo transport movement from vesicle to vesicle toward the TGN.
Function and Tasks
The Golgi apparatus has diverse and very complex functions. Three areas of responsibility crystallize. Thus, elements of the plasma membrane are synthesized and modified. Secretory vesicles containing transmitters and hormones are formed and stored. Finally, lysosomes are produced for the storage of digestive enzymes. Initially, the Golgi apparatus receives vesicles containing proteins or polypeptides mainly from the endoplasmic reticulum. Within the Golgi apparatus, these proteins are further modified depending on their intended use. Thus, either binding with sugar residues or with additional proteins takes place. The modified proteins are transported to the TGN, where they are sorted, packaged into Golgi vesicles, labeled with signaling substances, and forwarded to their destination via various transport mechanisms. In the process, most proteins are transported out of the cell. Outside the cell, they are used to modify the extracellular matrix. This serves intercellular communication and tissue stability. Furthermore, the Golgi apparatus forms primary lysosomes, which contain lytic enzymes. These enzymes are used to dissolve cellular and non-cellular substances.The enzymes develop their greatest activity in the acidic range at a pH of approx. 4.5. This PH value can only be achieved in membrane-enclosed reaction spaces by proton pumps. The interior of the lysosome is equipped with an acid protection of proteoglucans. In addition, lytic enzymes are modified with mannose-6-phosphates to be recognized by specific receptors on the lysosome membrane.
Diseases
The processes in the Golgi apparatus are very complex. Disruptions of the transport system can lead to serious diseases such as cancer or diabetes. At the same time, the exact mechanisms are not yet known. However, intensive research is being conducted on this problem. There is also evidence that autoimmune reactions against the elements of the Golgi apparatus can lead to rheumatic diseases. For example, more than 75 percent of patients with Sjögren’s syndrome have an antibody against a protein of the Golgi apparatus. Many patients with rheumatoid arthritis, idiopathic pulmonary fibrosis or proliferative glomerulonephritis also carry antibodies against proteins of the Golgi apparatus. Corresponding antibodies have also been discovered in the course of investigations in various infectious diseases and cancer diseases. These are additional reactions within these diseases, which are probably genetically influenced. However, the course of the corresponding disease can be significantly influenced by them. Other studies have investigated, among other things, the direct influence of chlamydiae on the Golgi apparatus. Chlamydia is sexually transmitted and often leads to infertility in women. The studies found that chlamydia fragments the Golgi apparatus and breaks it down into small mini-stacks. In doing so, the studies showed that this allows the chlamydia to multiply better and produce more infectious particles.
