B1100 Cytology - Golgi Complex PDF

91 CH 7. GOLGI COMPLEX I. GENERAL STRUCTURE In the late nineteenth century, Camillo Golgi discovered through the manipulation of specific dyes a membranous complex in nerve cells. The complex took the name of its discoverer and was later identified in all eukaryoti...

91 CH 7. GOLGI COMPLEX I. GENERAL STRUCTURE In the late nineteenth century, Camillo Golgi discovered through the manipulation of specific dyes a membranous complex in nerve cells. The complex took the name of its discoverer and was later identified in all eukaryotic cells. The Golgi complex or Golgi body or Golgi apparatus consists of several flattened, disk-like, cisternae with dilated rims and associated vesicles and tubules. Golgi apparatus is located towards the center of the cell (near the centrosome). Functionally, it lies between the RER and the plasma membrane. Cisternae which are typically 0.5 to 1 µm in diameter are curved and arranged by microtubules in an orderly stack resembleing a shallow bowl (Figure 7.1 ). Each stack of cisternae is referred to as dictyosome cis face and comprises several cisternae in animal and at least cells, twenty in plant CIS :,. [ ne ,,ark cisterna--_;;;,..,.;; medial cells. The cisterna number of trans dictyosomes cisterna rrans- that constitute Golg1 [ the Golgi body ne 10rk in a cell is seer tory v sicle trans face variable. The Golgi body may Figure 7.1: Scheme of a dictyosome of Golgi apparatus (left) and its TEM image (right). consist of one single dictyosome or more, it could be up to several thousands depending on cell type and cell physiology. Cells that actively secrete proteins and carbohydrates have an extensive Golgi. The dictyosomes in a cell are not independent; rather, they are interconnected by tubules and work in a coordinated way. Golgi apparatus is a part of the endomembrane system. A dictyosome is polarized and divided into functionally distinct compartments that differ in terms of enzyme content, membrane thickness and composition. The cisternae that are convex (cis face or cis-forming face, oriented to the ER) differ from those at the concave one (trans face trans­ maturing face oriented to the plasma membrane). In addition, cis and trans compartments differ from the middle one. The difference is functional since each compartment has a specific set of ,,'-.l( 1Yl11lut1 mint M.lnno,,(! enzymes involved in synthesis and maturation of many cell components (glycoproteins, glycolipids, and polysaccharides). In addition to the well delimited cisternae, there are tubular networks at Figure 7.2: Processing of N-linked oligosaccharides in both cis and trans sides. They are named the Golgi apparatus. Lebanese University-FS1. B1100 - Cell Biology by Pr Ziad ABDEL-RAZZAK (2022)  92 cis-Golgi Network (CGN) and trans-Golgi Network (TGN). TGN and CGN are thought to play a key role in sorting molecules. For instance, CGN sorts the molecules that must be sent back to the RER form those that must continue their export to the cis cisternae of dictyosomes. TGN segregates proteins into different vesicles which are directed to the diverse cell compartments such as the lysosomes, the plasma membrane, the ER membranes, the nuclear membranes, or the extracellular medium. Like the other organelles, the Golgi apparatus is not static but rather is in a continuous state of flux. Vesicles are continuously added by fusion with the cis face and others are detached from the trans face and the edges of the other Golgi cisternae. II. FUNCTIONS# The Golgi apparatus is responsible for the maturation and sorting of many proteins, glycoproteins and carbohydrates as well as the maturation of glycolipids. As proteins, carbohydrates and lipids pass from one cisterna to another in a dictyosome, they undergo specific reactions of maturation. The most common maturation reactions are (1) trimming by proteolytic enzymes where an inactive protein is cleaved by proteases into its active form, by removal of certain aminoacid sequences from the polypeptide chain (2) modification of the N-glycosylation state (Figure 7.2) by addition and removal of specific sugar motifs and by phosphorylation of mannose (3) hydroxylation of the "aa" chain at lysine by hydroxylases and (4) sulfation of the "aa" chains by sulfotransferases (e.g. proteoglycans). Moreover, there is the 0-glycosylation reaction that occurs for many proteins in this organelle. 1. Golgi complex roles in glycosylation# Assembly of the carbohydrate motifs of glycoproteins is started in the RER lumen and continued in the Golgi complex. In fact, some of the monosaccharides of the N-linked oligosaccharide are removed by specific enzymes (Figure 7.2) and replaced by different molecules. These reactions are catalyzed by the diverse enzymes of the three Golgi compartments. In addition, the 0- glycosylation starts only in the Golgi complex. During 0- glycosylation, the carbohydrate motifs are linked to OH group of serine or threonine at certain specific positions in the polypeptide chain (Figure 6.6). Glycosylation is necessary for protein maturation Figure 7.3: Separation of daughter plant cells during cytokinesis by vesicles derived from Golgi apparatus and directed by the phragmoplast to the cell equator. and folding and TEM image (right) and schematic model (left). mediates sorting and targeting of proteins to their right destinations. Lebanese University-FS1. B1100 - Cell Biology by Pr Ziad ABDEL-RAZZAK (2022)  93 Glycosylation of lipids occurs in the Golgi complex. For instance, glycolipids exposed at the red blood cells surface, and which determine blood groups (Figure 4.4) are produced by the Golgi complex and then transported as components of secretory vesicles membranes in order to be integrated in the plasma membrane. 2. Polysaccharides synthesis Most of the complex carbohydrates are produced by the Golgi apparatus. For example, glycosaminoglycans of extracellular matrix are produced in Golgi complex and released by exocytosis outside the cell. Hemicellulose and pectin of plant cell wall are also heteropolysaccharides that are synthesized by the same organelle. In plant cells, the number of Golgi bodies increases during cell division because these organelles are responsible for formation and secretion of the cell wall components which separate the two daughter cells. In fact, Golgi-derived vesicles migrate due to microtubules of the phragmoplast toward the equatorial plate during cytokinesis of plant Secretory cells (Figure 7.3). These vesicles gran e which contain cell wall material fuse together and form a plate that grows toward the old cell wall by fusion of additional vesicles. Finally the edges of the plate fuse with the ancient cell wall thereby separating the two daughter cells. The new cell wall portion requires maturation by addition of other components produced by Golgi complex or the synthesis in situ by enzymes (cellulose synthase) in the plasma memrane. t Go gi cisterna 3. Sorting process# The Golgi complex is well developed in cells which are involved in active secretion (e.g. goblet intestine cells) since the secretory products are matured, sorted and packed in this organelle. Golgi complex is also responsible 0 for preparation of lysosomes that ERGIC are rich in hydrolytic enzymes. Therefore, there are different types of vesicles that bud from or merge with the cisternal membranes. Some of these vesicles are coated (Figure 7.4). There are at least several types of coat which help sorting and targeting vesicles. Interaction between coat proteins and target membrane components are involved Figure 7.4: Traffic of vesciles and sorting of maturing molecules by the Golgi complex. in the dispatching process. Three Lebanese University-FS1. B1100 - Cell Biology by Pr Ziad ABDEL-RAZZAK (2022)  94 among these coats are listed below: - COP-II coated vesicles which are involved in the import of molecules from the RER to ERGIC (ER Golgi intermediate complex). They are enveloped by five types of proteins. - COP-I coated vesicles which are involved in sending back the RER-specific components from the ERGIC to the RER (retrograde movement). Moreover, they are thought to play a key role in transport within Golgi complex cisternae from one compartment to another. Note that retrograde transport within Golgi complex is necessary to maintain specificity of the cis and trans citernae. - Clathrin-coated vesicles which play a key role in targeting lysosomal enzymes as well as plant vacuole enzymes. It is evident that each type of protein is packed in one of these vesicles or in another depending on its specific address code (e.g. a sugar motifs such as mannose-6- phosphate) that has specific receptors in the membrane of the vesicle that contains it. It should be noted that retrograde movement of vesicles occurs also from the plasma membrane to Golgi apparatus (via endosomes). 4. Proliferation of cell membranes When proteins are sorted in the Golgi apparatus, those destined to be secreted or to be incorporated in the plasma membrane are included into vesicles detaching from the edges of the middle cisternae and the trans face. These vesicles are to their destinations (e.g. plasma membrane). Upon fusion with the plasma membrane or other membranes, the membranes of these vesicles contribute to proliferation of the targeted membranes (Figure 6.9). Lebanese University-FS1. B1100 - Cell Biology by Pr Ziad ABDEL-RAZZAK (2022)