what takes proteins and moves them to the golgi apparatus

The Golgi apparatus, or Golgi circuitous, 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, as noted earlier, glycolipids and sphingomyelin are synthesized within the Golgi. In plant cells, the Golgi appliance farther serves as the site at which the complex polysaccharides of the jail cell wall are synthesized. The Golgi apparatus is thus involved in processing the wide range of cellular constituents that travel along the secretory pathway.

Organization 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 distinct polarity in both construction and function. Proteins from the ER enter at its cis confront (entry face), which is convex and usually oriented toward the nucleus. They are and so transported through the Golgi and exit from its concave trans confront (exit face). As they laissez passer through the Golgi, proteins are modified and sorted for send to their eventual destinations inside the cell.

Effigy 9.22

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 apparatus at its cis face and exit at its trans face up. (Courtesy of Dr. L. (more...)

Distinct processing and sorting events appear to take place in an ordered sequence within different regions of the Golgi complex, and then the Golgi is normally considered to consist of multiple discrete compartments. Although the number of such compartments has non been established, the Golgi is near unremarkably 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 (Figure ix.23). Proteins from the ER are transported to the ER-Golgi intermediate compartment and then enter the Golgi apparatus at the cis Golgi network. They then progress to the medial and trans compartments of the Golgi stack, within which most metabolic activities of the Golgi apparatus take identify. The modified proteins, lipids, and polysaccharides then move to the trans Golgi network, which acts as a sorting and distribution center, directing molecular traffic to lysosomes, the plasma membrane, or the cell exterior.

Figure 9.23

Regions of the Golgi apparatus. 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 first described over 100 years ago, the mechanism by which proteins motility through the Golgi apparatus has withal non been established and is an area of controversy amidst cell biologists. One possibility is that transport vesicles acquit proteins betwixt the cisternae of the Golgi compartments. However, in that location is considerable experimental support for an alternative model proposing that proteins are but 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 within the Golgi

Protein processing within the Golgi involves the modification and synthesis of the carbohydrate portions of glycoproteins. Ane of the major aspects of this processing is the modification of the N-linked oligosaccharides that were added to proteins in the ER. As discussed earlier in this affiliate, proteins are modified within the ER by the addition of an oligosaccharide consisting of fourteen sugar residues (meet Figure 9.fifteen). Three glucose residues and i mannose are then removed while the polypeptides are still in the ER. Following send to the Golgi apparatus, the Due north-linked oligosaccharides of these glycoproteins are subject to extensive further modifications.

N-linked oligosaccharides are processed inside the Golgi appliance in an ordered sequence of reactions (Effigy 9.24). The first 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 N-acetylglucosamine, the removal of two more mannoses, and the addition of a fucose and ii more Due north-acetylglucosamines. Finally, three galactose and three sialic acid residues are added. As noted in Affiliate 7, 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 within the Golgi complexes of different types of cells. Consequently, proteins tin can emerge from the Golgi with a diversity of different N-linked oligosaccharides.

Figure 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 N-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 beginning step of this reaction, N-acetylglucosamine phosphates are added to specific mannose residues, probably while the protein is yet in the cis Golgi network (Figure 9.25). This is followed past removal of the N-acetylglucosamine group, leaving mannose-half dozen-phosphate residues on the Due north-linked oligosaccharide. Because of this modification, these residues are non 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 9.25

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

The phosphorylation of mannose residues is thus a critical stride 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 Northward-acetylglucosamine phosphates to lysosomal proteins. This enzyme recognizes a structural determinant that is present on lysosomal proteins but not on proteins destined for the plasma membrane or secretion. This recognition determinant is not a simple sequence of amino acids; rather, it is formed in the folded protein by the juxtaposition of amino acid sequences from unlike regions of the polypeptide chain. In contrast to the point 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 point patches, in contrast to the linear targeting signals discussed earlier in this chapter.

Proteins can also be modified by the addition of carbohydrates to the side chains of acceptor serine and threonine residues within specific sequences of amino acids (O-linked glycosylation) (encounter Figure 7.28). These modifications take place in the Golgi apparatus by the sequential addition of unmarried sugar residues. The serine or threonine is usually linked directly to N-acetylgalactosamine, to which other sugars can and then be added. In some cases, these sugars are farther modified by the improver 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 item, in the synthesis of glycolipids and sphingomyelin. Equally discussed earlier, the glycerol phospholipids, cholesterol, and ceramide are synthesized in the ER. Sphingomyelin and glycolipids are then synthesized from ceramide in the Golgi appliance (Figure 9.26). Sphingomyelin (the only nonglycerol phospholipid in cell membranes) is synthesized by the transfer of a phosphorylcholine group from phosphatidylcholine to ceramide. Alternatively, the addition of carbohydrates to ceramide can yield a diversity of different 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, however, 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 head groups exposed on the prison cell surface. As volition be discussed in Affiliate 12, the oligosaccharide portions of glycolipids are of import surface markers in cell-cell recognition.

In constitute cells, the Golgi appliance has the additional task of serving as the site where complex polysaccharides of the cell wall are synthesized. As discussed further in Affiliate 12, the constitute prison cell wall is composed of three major types of polysaccharides. Cellulose, the predominant constituent, is a simple 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), still, are circuitous, branched chain molecules that are synthesized in the Golgi apparatus and so transported in vesicles to the prison cell surface. The synthesis of these cell wall polysaccharides is a major cellular function, and as much as eighty% of the metabolic activity of the Golgi appliance in constitute cells may be devoted to polysaccharide synthesis.

Poly peptide Sorting and Consign from the Golgi Apparatus

Proteins, too every bit lipids and polysaccharides, are transported from the Golgi apparatus to their terminal destinations through the secretory pathway. This involves the sorting of proteins into different kinds of ship vesicles, which bud from the trans Golgi network and evangelize their contents to the appropriate cellular locations (Figure nine.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, as well 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 as lysosomes in creature cells or vacuoles in yeast.

Effigy ix.27

Send from the Golgi apparatus. Proteins are sorted in the trans Golgi network and transported in vesicles to their final destinations. In the absence of specific targeting signals, proteins are carried to the plasma membrane by constitutive secretion. (more than...)

Proteins that function within the Golgi apparatus must exist 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 complex are associated with the Golgi membrane rather than being 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 apparatus by preventing them from being packaged in the transport vesicles that go out the trans Golgi network. In addition, similar 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 forth the secretory pathway.

The constitutive secretory pathway, which operates in all cells, leads to continual unregulated protein secretion. However, 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 beginning of this affiliate (encounter Effigy 9.two). 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 transport vesicles, store their contents until specific signals direct their fusion with the plasma membrane. For example, the digestive enzymes produced past pancreatic acinar cells are stored in secretory vesicles until the presence of food in the breadbasket and small 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 by budding as secretory vesicles.

A farther complexity 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 two split up regions, the apical domain and the basolateral domain, that comprise specific proteins related to their particular functions. For instance, the apical membrane of intestinal epithelial cells faces the lumen of the intestine and is specialized for the efficient absorption of nutrients; the rest of the jail cell is covered by the basolateral membrane (Figure 9.28). Distinct domains of the plasma membrane are present non only in epithelial cells, just also in other prison 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.

Figure 9.28

Transport to the plasma membrane of polarized cells. The plasma membranes of polarized epithelial cells are divided into upmost 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 poly peptide sorting in the Golgi is the selective transport of proteins to lysosomes. Equally already discussed, lumenal lysosomal proteins are marked past mannose-6-phosphates that are formed by modification of their N-linked oligosaccharides presently after entry into the Golgi apparatus. A specific receptor in the membrane of the trans Golgi network then recognizes these mannose-vi-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 plant cells, which lack lysosomes, proteins are transported from the Golgi apparatus to an boosted destination: the vacuole (Figure 9.29). Vacuoles presume the functions of lysosomes in these cells as well every bit performing a variety of other tasks, such as the storage of nutrients and the maintenance of turgor pressure and osmotic rest. In contrast to lysosomal targeting, proteins are directed to vacuoles by brusque peptide sequences instead of sugar markers.

Figure 9.29

A establish cell vacuole. The big fundamental vacuole functions every bit a lysosome in addition to storing nutrients and maintaining osmotic balance. (E. H. Newcombe/Biological Photo Service.)

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

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