What Does The Golgi Bodies Do In An Animal Cell

The Golgi apparatus, or Golgi complex, functions as a factory in which proteins received from the ER are further processed and sorted for transport to their eventual destinations: lysosomes, the plasma membrane, or secretion. In addition, every bit noted before, glycolipids and sphingomyelin are synthesized within the Golgi. In constitute cells, the Golgi apparatus further serves equally the site at which the circuitous polysaccharides of the prison cell wall are synthesized. The Golgi appliance is thus involved in processing the broad range of cellular constituents that travel forth the secretory pathway.

System of the Golgi

Morphologically the Golgi is composed of flattened membrane-enclosed sacs (cisternae) and associated vesicles (Figure 9.22). A striking feature of the Golgi apparatus is its singled-out polarity in both structure and function. Proteins from the ER enter at its cis face up (entry confront), which is convex and normally oriented toward the nucleus. They are and so transported through the Golgi and exit from its concave trans face up (exit face). Equally they pass through the Golgi, proteins are modified and sorted for transport to their eventual destinations within the cell.

Effigy ix.22

Electron micrograph of a Golgi apparatus. The Golgi apparatus consists of a stack of flattened cisternae and associated vesicles. Proteins and lipids from the ER enter the Golgi appliance at its cis face and exit at its trans face. (Courtesy of Dr. 50. (more than...)

Singled-out processing and sorting events announced to take identify in an ordered sequence within unlike regions of the Golgi complex, so the Golgi is usually considered to consist of multiple discrete compartments. Although the number of such compartments has not been established, the Golgi is near ordinarily viewed as consisting of four functionally distinct regions: the cis Golgi network, the Golgi stack (which is divided into the medial and trans subcompartments), and the trans Golgi network (Effigy 9.23). Proteins from the ER are transported to the ER-Golgi intermediate compartment and and so enter the Golgi appliance at the cis Golgi network. They then progress to the medial and trans compartments of the Golgi stack, inside which about metabolic activities of the Golgi appliance have place. The modified proteins, lipids, and polysaccharides so move to the trans Golgi network, which acts as a sorting and distribution heart, directing molecular traffic to lysosomes, the plasma membrane, or the cell exterior.

Figure ix.23

Regions of the Golgi appliance. Vesicles from the ER fuse to form the ER-Golgi intermediate compartment, and proteins from the ER are then transported to the cis Golgi network. Resident ER proteins are returned from the ER-Golgi intermediate compartment (more...)

Although the Golgi apparatus was get-go described over 100 years ago, the mechanism by which proteins motility through the Golgi apparatus has still not been established and is an surface area of controversy amid cell biologists. One possibility is that send vesicles carry proteins between the cisternae of the Golgi compartments. Yet, in that location is considerable experimental support for an alternative model proposing that proteins are just carried through compartments of the Golgi within the Golgi cisternae, which gradually mature and progressively motion through the Golgi in the cis to trans direction.

Protein Glycosylation inside the Golgi

Protein processing inside the Golgi involves the modification and synthesis of the saccharide portions of glycoproteins. One of the major aspects of this processing is the modification of the Northward-linked oligosaccharides that were added to proteins in the ER. As discussed earlier in this chapter, proteins are modified within the ER past the add-on of an oligosaccharide consisting of 14 sugar residues (see Figure 9.fifteen). Three glucose residues and ane mannose are then removed while the polypeptides are still in the ER. Following transport to the Golgi appliance, the Due north-linked oligosaccharides of these glycoproteins are subject to extensive farther modifications.

N-linked oligosaccharides are processed inside the Golgi appliance in an ordered sequence of reactions (Figure 9.24). The starting time modification of proteins destined for secretion or for the plasma membrane is the removal of three additional mannose residues. This is followed by the sequential addition of an Due north-acetylglucosamine, the removal of two more mannoses, and the addition of a fucose and 2 more than Due north-acetylglucosamines. Finally, three galactose and iii sialic acrid residues are added. As noted in Chapter vii, different glycoproteins are modified to different extents during their passage through the Golgi, depending on both the structure of the protein and on the amount of processing enzymes that are present inside the Golgi complexes of different types of cells. Consequently, proteins tin emerge from the Golgi with a variety of dissimilar N-linked oligosaccharides.

Effigy 9.24

Processing of N-linked oligosaccharides in the Golgi. The N-linked oligosaccharides of glycoproteins transported from the ER are further modified by an ordered sequence of reactions in the Golgi.

The processing of the North-linked oligosaccharide of lysosomal proteins differs from that of secreted and plasma membrane proteins. Rather than the initial removal of three mannose residues, proteins destined for incorporation into lysosomes are modified by mannose phosphorylation. In the starting time step of this reaction, Northward-acetylglucosamine phosphates are added to specific mannose residues, probably while the poly peptide is notwithstanding in the cis Golgi network (Figure 9.25). This is followed by removal of the N-acetylglucosamine group, leaving mannose-half dozen-phosphate residues on the N-linked oligosaccharide. Because of this modification, these residues are not removed during further processing. Instead, these phosphorylated mannose residues are specifically recognized past a mannose-6-phosphate receptor in the trans Golgi network, which directs the transport of these proteins to lysosomes.

Figure ix.25

Targeting of lysosomal proteins by phosphorylation of mannose residues. Proteins destined for incorporation into lysosomes are specifically recognized and modified by the addition of phosphate groups to the 6 position of mannose residues. In the first (more than...)

The phosphorylation of mannose residues is thus a critical pace in sorting lysosomal proteins to their correct intracellular destination. The specificity of this process resides in the enzyme that catalyzes the first step in the reaction sequence—the selective addition of N-acetylglucosamine phosphates to lysosomal proteins. This enzyme recognizes a structural determinant that is present on lysosomal proteins but non on proteins destined for the plasma membrane or secretion. This recognition determinant is not a simple sequence of amino acids; rather, information technology is formed in the folded protein by the juxtaposition of amino acrid sequences from different regions of the polypeptide chain. In contrast to the betoken sequences that direct poly peptide translocation to the ER, the recognition determinant that leads to mannose phosphorylation, and thus ultimately targets proteins to lysosomes, depends on the three-dimensional conformation of the folded protein. Such determinants are called betoken patches, in contrast to the linear targeting signals discussed earlier in this affiliate.

Proteins can also be modified past the add-on of carbohydrates to the side chains of acceptor serine and threonine residues inside specific sequences of amino acids (O-linked glycosylation) (meet Effigy 7.28). These modifications take place in the Golgi apparatus past the sequential addition of single sugar residues. The serine or threonine is unremarkably linked directly to North-acetylgalactosamine, to which other sugars can so be added. In some cases, these sugars are further modified by the addition of sulfate groups.

Lipid and Polysaccharide Metabolism in the Golgi

In addition to its activities in processing and sorting glycoproteins, the Golgi apparatus functions in lipid metabolism—in detail, in the synthesis of glycolipids and sphingomyelin. Every bit discussed earlier, the glycerol phospholipids, cholesterol, and ceramide are synthesized in the ER. Sphingomyelin and glycolipids are then synthesized from ceramide in the Golgi apparatus (Figure 9.26). Sphingomyelin (the only nonglycerol phospholipid in cell membranes) is synthesized past the transfer of a phosphorylcholine grouping from phosphatidylcholine to ceramide. Alternatively, the addition of carbohydrates to ceramide tin yield a diversity of unlike glycolipids.

Figure 9.26

Synthesis of sphingomyelin and glycolipids. Ceramide, which is synthesized in the ER, is converted either to sphingomyelin (a phospholipid) or to glycolipids in the Golgi appliance. In the first reaction, a phosphorylcholine group is transferred from (more...)

Sphingomyelin is synthesized on the lumenal surface of the Golgi, but glucose is added to ceramide on the cytosolic side. Glucosylceramide and so apparently flips, still, and additional carbohydrates are added on the lumenal side of the membrane. Neither sphingomyelin nor the glycolipids are then able to translocate across the Golgi membrane, so they are found only in the lumenal half of the Golgi bilayer. Following vesicular transport, they are correspondingly localized to the outside half of the plasma membrane, with their polar caput groups exposed on the cell surface. As will exist discussed in Chapter 12, the oligosaccharide portions of glycolipids are important surface markers in cell-jail cell recognition.

In plant cells, the Golgi apparatus has the additional job of serving as the site where complex polysaccharides of the cell wall are synthesized. As discussed farther in Chapter 12, the institute cell wall is composed of iii major types of polysaccharides. Cellulose, the predominant constituent, is a uncomplicated linear polymer of glucose residues. It is synthesized at the cell surface by enzymes in the plasma membrane. The other cell wall polysaccharides (hemicelluloses and pectins), all the same, are complex, branched chain molecules that are synthesized in the Golgi apparatus and then transported in vesicles to the prison cell surface. The synthesis of these prison cell wall polysaccharides is a major cellular function, and as much as 80% of the metabolic activity of the Golgi apparatus in constitute cells may be devoted to polysaccharide synthesis.

Protein Sorting and Consign from the Golgi Apparatus

Proteins, equally well as lipids and polysaccharides, are transported from the Golgi appliance to their last destinations through the secretory pathway. This involves the sorting of proteins into different kinds of send vesicles, which bud from the trans Golgi network and deliver their contents to the appropriate cellular locations (Figure 9.27). Some proteins are carried from the Golgi to the plasma membrane by a constitutive secretory pathway, which accounts for the incorporation of new proteins and lipids into the plasma membrane, too every bit for the continuous secretion of proteins from the cell. Other proteins are transported to the cell surface by a distinct pathway of regulated secretion or are specifically targeted to other intracellular destinations, such equally lysosomes in fauna cells or vacuoles in yeast.

Figure 9.27

Transport from the Golgi appliance. Proteins are sorted in the trans Golgi network and transported in vesicles to their last destinations. In the absenteeism of specific targeting signals, proteins are carried to the plasma membrane by constitutive secretion. (more...)

Proteins that function inside the Golgi appliance must be retained within that organelle, rather than being transported along the secretory pathway. In contrast to the ER, all of the proteins retained within the Golgi circuitous are associated with the Golgi membrane rather than beingness soluble proteins inside the lumen. The signals responsible for retention of some proteins within the Golgi take been localized to their transmembrane domains, which retain proteins within the Golgi appliance past preventing them from beingness packaged in the transport vesicles that leave the trans Golgi network. In addition, like the KKXX sequences of resident ER membrane proteins, signals in the cytoplasmic tails of some Golgi proteins mediate the retrieval of these proteins from subsequent compartments along the secretory pathway.

The constitutive secretory pathway, which operates in all cells, leads to continual unregulated poly peptide secretion. Yet, some cells also possess a distinct regulated secretory pathway in which specific proteins are secreted in response to environmental signals. Examples of regulated secretion include the release of hormones from endocrine cells, the release of neurotransmitters from neurons, and the release of digestive enzymes from the pancreatic acinar cells discussed at the first of this chapter (run across Effigy ix.2). Proteins are sorted into the regulated secretory pathway in the trans Golgi network, where they are packaged into specialized secretory vesicles. These secretory vesicles, which are larger than other send vesicles, store their contents until specific signals direct their fusion with the plasma membrane. For example, the digestive enzymes produced by pancreatic acinar cells are stored in secretory vesicles until the presence of nutrient in the stomach and minor intestine triggers their secretion. The sorting of proteins into the regulated secretory pathway appears to involve the recognition of indicate patches shared by multiple proteins that enter this pathway. These proteins selectively aggregate in the trans Golgi network and are then released past budding as secretory vesicles.

A further complication in the transport of proteins to the plasma membrane arises in many epithelial cells, which are polarized when they are organized into tissues. The plasma membrane of such cells is divided into ii separate regions, the apical domain and the basolateral domain, that incorporate specific proteins related to their particular functions. For case, the apical membrane of intestinal epithelial cells faces the lumen of the intestine and is specialized for the efficient assimilation of nutrients; the remainder of the prison cell is covered by the basolateral membrane (Effigy ix.28). Distinct domains of the plasma membrane are nowadays not simply in epithelial cells, but also in other jail cell types. Thus, the constitutive secretory pathway must selectively transport proteins from the trans Golgi network to these distinct domains of the plasma membrane. This is accomplished by the selective packaging of proteins into at least ii types of constitutive secretory vesicles that leave the trans Golgi network targeted specifically for either the apical or basolateral plasma membrane domains of the cell.

Effigy 9.28

Send to the plasma membrane of polarized cells. The plasma membranes of polarized epithelial cells are divided into apical and basolateral domains. In this instance (intestinal epithelium), the apical surface of the cell faces the lumen of the intestine, (more...)

The best-characterized pathway of protein sorting in the Golgi is the selective transport of proteins to lysosomes. As already discussed, lumenal lysosomal proteins are marked by mannose-6-phosphates that are formed by modification of their N-linked oligosaccharides before long after entry into the Golgi apparatus. A specific receptor in the membrane of the trans Golgi network then recognizes these mannose-6-phosphate residues. The resulting complexes of receptor plus lysosomal enzyme are packaged into transport vesicles destined for lysosomes. Lysosomal membrane proteins are targeted by sequences in their cytoplasmic tails, rather than by mannose-6-phosphates.

In yeasts and institute cells, which lack lysosomes, proteins are transported from the Golgi appliance to an additional destination: the vacuole (Effigy ix.29). Vacuoles assume the functions of lysosomes in these cells likewise as performing a variety of other tasks, such as the storage of nutrients and the maintenance of turgor pressure and osmotic residual. In dissimilarity to lysosomal targeting, proteins are directed to vacuoles past curt peptide sequences instead of carbohydrate markers.

Figure 9.29

A plant cell vacuole. The large central vacuole functions equally a lysosome in addition to storing nutrients and maintaining osmotic residuum. (E. H. Newcombe/Biological Photo Service.)

Source: https://www.ncbi.nlm.nih.gov/books/NBK9838/

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