The cell. 5. Vesicular traffic.
|« From reticulum to Golgi||Exocytosis »|
Golgi complex is made up of stacks of cisternae (flat sacs). In the animal cells, it is located close to the centrosome.
Golgi complex is a polarized organelle: there is a cis domain, where molecules included in vesicles or in the ERGIC arrive from the endoplasmic reticulum, intermediate cisternae where these molecules are processed, and a trans domain where molecules are distributed in vesicles and are sent to other cellular compartments.
Cisternae are formed in the cis face and move to the trans face. During this journey the molecules are processed and cisternae mature.
Golgi complex is a central place of the cell for glycosylation and synthesis of sphingolipids. Furthermore, sulphate and phosphate groups are added to some molecules.
Different types of molecules are distributed from the Golgi complex to other cellular compartments: plasma membrane and endosomes.
Golgi complex was discovered by Camillo Golgi in 1989, but it was no completely accepted as a true organelle for decades. Golgi complex can be observed at light microscopy by normal stainings, such as Schiff-periodic acid staining, which makes visible the carbohydrate rich areas, such as the Golgi complex. By using transmission electron microscopy, the Golgi membrane organization was described in detail by Dalto and Felix (1954). They proposed the name Golgi complex.
In animal cells, Golgi complex is an organelle located close to the centrosome, which in turn is located near the nucleus. This position is determined by interactions with microtubules. Golgi complex membranes are moved toward the minus end of the microtubules. Minus ends are kept together in the centrosome. Golgi complex is made up of flat cisternae piled in several groups. Every stack is known as dictyosome. Several dictyosomes are present in one cell and some cisternae belonging to nearby dictyosomes may be laterally connected by their membranes. The number (normally 3 to 8) and the size of the cisternae in dictyosomes are variable and depend on the cell type, as well as the cell physiological state. In one cell, the Golgi complex includes all the dictyosomes and their lateral connections.
Transmission electron microscopy image of Golgi complex showing several dictyosomes (arrows).
In animal cells, Golgi complex is made up of several dictyosomes, located close to the centrosome, and therefore close to the nucleus. Some nearby dictyosomes are laterally connected.
In animal cells, there is a fibrous protein matrix where Golgi cisternae are located that could help in keeping the Golgi complex structure. However, it has been shown that the structural integrity of the Golgi complex mainly relies on microtubule organization. During cell division, the Golgi complex is disorganized and cisternae are no longer observed. During mitosis, Golgi cisternae are broken in small vesicles, which are divided into the two new cells after cytokinesis. Once the cell division is completed and the microtubular system is assembled, Golgi complex is rebuilded again. Furthermore, a continuous flux of vesicles from the endoplasmic reticulum is needed to maintain the Golgi complex. If this flux is stopped the Golgi complex disappears. The cellular location of the Golgi complex depends on those microtubules with the minus end located in the centrosome, but the integrity of every dictyosome is maintained by microtubules that nucleate from the cisternae themselves. The myosin and actin proteins also participate in the final location of Golgi stacks.
In plant cells, which do not have centrosome, there are structures resembling a poorly developed Golgi complex, such as small stacks of cisternae or even individual cisternae, all of them dispersed through the cytoplasm. Thus, the Golgi complex looks like divided in many parts distributed through the cytoplasm. Every of these stacks works as independent units, and it is not known if they develope exactly the same functions or not, i.e. if they are specialized in processing of certain type of molecules. Cisternae are smaller than in animal cells, although the total number of cisternae could be from dozens to more than hundred. In plant cells, ERGIC (see below) has not been observed, but the trans Golgi network (TGN) is well developed, so much that some authors endorse it as a different organelle. Individual cisternae or dictyosomes can move through the cytoplasm thanks to actin filaments. They move around the places where there is vesicle formation by the endoplasmic reticulum, as if they were collecting new formed vesicles. The morphology of dictyosomes does not change during these movements. Vesicles that must fuse with vacuolas are moved by actin filaments. One more feature of Golgi complex organization in plant cells is that the dictyosomes do not disappear during mitosis, as occurs in animal cells, because Golgi stacks are needed for synthesizing the new cell wall during cytocinesis, which will separate the two new cells. There are some rare Golgi complex organizations in some species. For example, fruit fly, although it has centrosome, shows a Golgi organization similar to that of plant cells. Yeasts, eukaryote cells, do not show any membrane organization similar to Golgi stacks observed in animal or cell plants.
Organization of the Golgi complex in animal and plant cells. In animal cells, dictyosomes are located close to the centrosome, near to the nucleus, and are dragged mainly by microtubules. In plant cells, dictyosomes are distributed through the cytoplasm, they are moved by actin filaments. Arrows indicate the movement direction of the vesicles.
Golgi complex is a polarized organelle. Dictyosomes show two domains: cis and trans domains. Intermediate cisternae are located between them. For example, cis cisternae contain higher concentration of N-acetylgalactosamine transferase, intermediate cisternae contain more N-acetylglucosamine transferase I, whereas trans Golgi has more galactosyl transferase and sialyltransferase. In the cis domain, there is an ongoing addition of new material coming from the endoplasmic reticulum via the ERGIC (endoplasmic reticulum Golgi intermediate compartment). In the trans domain, there is a tubulo-vesicular arrangement of the membranes, referred as TGN (trans Golgi network), where molecules are distributed in vesicles and tubules in their way toward different destinations. Thus, there is a traffic of molecules starting in the cis cisternae, going through intermediate cisternae, and ending in the trans domain. There is also a recycling pathway carried out by COPI coated vesicles, which bud from the lateral part of the cisternae and are directed toward the endoplasmic reticulum. The Golgi complex is permanently renewing and its organization and size is affected by the trafficking of molecules. It is particularly well developed in the cells showing a strong secretion.
Golgi complex is divided in domains. Cis domain is where ERGIC bodies and vesicles coming from the endoplasmic reticulum fused between each other to form the first Golgi cisterna. ERGIC is not a Golgi compartment, but a transient one between the endoplasmic reticulum and Golgi complex. Cisternae in the middle of dictyosomes are known as intermediate cisternae. Trans domain is where cisternae are transformed in vesicles and tubules containing the molecules to be delivered. TGN (trans Golgi network) is this network of vesicles and tubules. From the lateral part of cisternae, COPI coated vesicles arise and are moved toward the endoplasmic reticulum. This is a recycling pathway. Arrows located laterally indicated the direction of the recycling vesicles and arrows located in the center indicate the pathway for molecules which are being processed by the Golgi.
Models to explain the movement of molecules from cis domain to trans domains:
a) Cisternal maturation model. It proposes that ERGIC bodies get fused between each other to form de first cisterna at the Golgi cis domain. Then, this cisterna moves toward the trans domain, where it is eventually splitted in vesicles. The internal material of the cisterna is progressively processed during this journey. Nowadays, this is the most accepted model because it can explain some observations that do not fit in other models. For example, procollagen molecules are 300 nm in length and do not fit in a conventional vesicle, but can travel inside a cisterna. Furthermore, COPI coated vesicles are intermingled with Golgi cisternae, but they mostly contain proteins targeted to the endoplasmic reticulum and do not contain molecules that need to be processed in the Golgi. So, these vesicles are not supposed to be a communication mechanism between cisternae. This model has been demonstrated in yeasts.
b) Vesicular transport model. In this model, ERGIC bodies are fused with cis cisternae, and vesicles bud from these cisternae and get fused with other cisternae of the Golgi stack. In this way, the molecules to be processed in the Golgi are transported by vesicles from one cisterna to the next. Cisternae remain in the same position in the stack. The cisterna of the trans domain does not disappear and molecules are packaged into differente vesicles. Vesicles are released from the trans domain in the same way as in other organelles.
c) Connection by tubules. Tubular connections between adjoining cisternae have been observed at electron microscopy. This connections are like bridges and appear to be transient. These bridges depend on the material being processed. This proposal is not incompatible with the other models, and this direct communication between cisternae could happen simultaneously with the movement of cisternae.
Models for molecular transport (colored in orange) and processing through a Golgi stack. In the cisternal maturation model (upper), cisternae are generated in the cis domain and then are moved toward the trans domain, while molecules are being processed. In the trans domain, cisternae get divided in many vesicles. In the vesicular transport model (lower), molecules are transported from one cisterna to the next by vesicles (orange colored vesicles).
a) GLycosylation center. Golgi complex is one of the main glycosylation centers of the cell by modifying and adding carbohydrates that will be part of glycoproteins, proteoglycans, glycolips and other polysaccharides, such as hemicellulose in plants. Many of the carbohydrates which are modified in the Golgi cisterane have been previously synthesized and assembled in the endoplasmic reticulum. But there are specific carbohydrates which are added in the Golgi complex, such as sialic acid. There are two types of protein glycosilation. N-type glycosylation: in the endoplasmic reticulum, mannose rich carbohydrates are attached to nitrogen atoms of the asparragine amino acids, and are later modified in the Golgi complex by adding more new saccharides. O-type glycosylation: carbohydrates are added in the Golgi complex to the hydroxil groups of the amino acids, such as serine, threonine and hydroxylysine, which is the type of glycosylation of proteoglycans. Sulfation of proteoglycans also happens in the Golgi complex, as well as phosphorylation, palmitoylation, methylation, and other chemical modifications. In plants, the glycosilation function of the Golgi complex is esential because many glycoconjugates will be part of the cell wall (except cellulose, which is sinthesized at the cell membrane).
Carbohydrates are added by glycosyltransferases and removed by glycosidases. About 200 different types of these enzymes can be found in the Golgi complex. Different cisternae have different roles in the carbohydrate processing, which is an ordered process. Some evidences point to a gradient of enzymes concentration, from cis- to trans-domain, so that those enzymes in charge on the first steps of the carbohydrate processing are concentrated in the cis domain. Assembling a branched chain of carbohydrates is far more complicated than for example a polypeptide or a nucleotide chain. Polypeptides and nucleotide chains just need a sequence of nucleotides and one enzymatic complex (ribosomes or DNA polymerase). Carbohydrates, however, need one enzyme for every step of adding or removing one monosaccharide, and furthermore it should be done in a proper order. Because carbohydrate processing is very expensive and complex, the function of the saccharides in glycoproteins and glycolipids, as well as glycosaminoglycans of the extracellular matrix of animal cells, and hemicellulose of cell wall of plant cells, must have an outstanding function in the cell and that is why it has been conserved during evolution.
b) The synthesis of sphingomyelins and glycosphingolipids is accomplished in the Golgi complex. Ceramide, synthesized in the endoplasmic reticulum, is the molecular base for the synthesis of these lipids by enzymes located in the Golgi complex. Sphingolipids have recently attracted much attention because they are thought to be involved, together with cholesterol, in the lipid rafts formation. Lipid rafts are domains in the outer leaflet of the plasma membrane with particular properties and functions.
c) Golgi complex is a center for shipping molecules coming from the endoplasmic reticulum or synthesized in the Golgi complex. Once processed, molecules are distributed in different types of vesicles which will be targeted to other cell compartments. In the trans domain, TGN is the structure where this process takes place (see figure ). From the trans domain, vesicles will be moved to and fuse with the plasma membrane in a process known as exocytosis. There are two types of exocytosis: constitutive and regulated (see next page). This delivering is more complicated in cells having apical and a basolateral domains in the plasma membrane, because vesicles leaving the trans domain toward the plasma membrane should be transferred to the appropriate place of the plasma membrane. Furthermore, there are vesicles leaving the trans domain that must be delivered to the late endosomes/multivesicular bodies/lysosomes, or to the vacuoles in plants. Trans domain is also a target for vesicles coming from endosomes, where some receptors are recycled. In plants, Golgi complex can receive vesicles from the plasma membrane, and thus Golgi complex participates in both, endocytosis and exocytosis.
Vesicles coming out from the TGN are not typically rounded vesicles, but show different morphology. They arise from tubular expansions of the TGN membrane complex. Molecules must be selected and included in different vesicles to be delivered to different targets. Those transmembrane protein heading toward late endosomes (and basolateral membranes in epithelial cells) are selected by a particular amino acid sequence of the cytosol domain. This sequence is recognized by adaptor proteins that allow the concentration of cargoes at one site of the membrane, where clathrin nets will be assembled, and the vesicle will be formed. However, in exocytosis, molecules targeted to plasma membrane (or to the apical membrane of the epithelial cells) are selected by selectins which recognize O and N glycosidic bonds. In animal cells, these vesicles are moved to their target fusion places by microtubules, whereas actin filaments are in charge of this transport in plant cells.
|« From reticulum to Golgi||Exocytosis »|
Actualizado: 11-07-2016. 09:44
Atlas of Plant and Animal Histology
Dep. of Functional Biology and Health Sciences.
Faculty of Biology.
University of Vigo