Golgi Body Protein Synthesis
Lane Frisch
Cellshave extensive sets of intracellular membranes, which together compose the endomembrane system. The endomembrane system was first discovered inthe late 1800s when scientist Camillo Golgi noticed that a certain stainselectively marked only some internal cellular membranes. Golgi thought that theseintracellular membranes were interconnected, but advances in microscopy andbiochemical studies of the various membrane-encased organelles later made itclear the organelles in the endomembrane system are separate compartments withspecific functions. These structures do exchange membrane material, however,via a special type of transport.
Membranes and their constituent proteins are assembled in the ER. This organelle contains the enzymes involved in lipid synthesis, and as lipids are manufactured in the ER, they are inserted into the organelle's own membranes. This happens in part because the lipids are too hydrophobic to dissolve into the cytoplasm.
The proteins that will be secreted by a cell are also directed to the ER during translation, where they end up in the lumen, the internal cavity, where they are then packaged for vesicular release from the cell. The hormones insulin and erythropoietin (EPO) are both examples of vesicular proteins.
Figure 1: Co-translational synthesisA signal sequence on a growing protein will bind with a signal recognition particle (SRP). This slows protein synthesis. The SRP then binds to a location on the surface of the nearby ER. Then, the SRP is released, and the protein-ribosome complex is at the correct location for movement of the protein through a translocation channel. 2014 Nature Education All rights reserved.
TheER, Golgi apparatus, and lysosomes are all members of a network of membranes,but they are not continuous with one another. Therefore, the membrane lipidsand proteins that are synthesized in the ER must be transported through the networkto their final destination in membrane-bound vesicles. Cargo-bearing vesiclespinch off of one set of membranes and travel along microtubule tracks to thenext set of membranes, where they fuse with these structures. Traffickingoccurs in both directions; the forward direction takes vesicles from the siteof synthesis to the Golgi apparatus and next to a cell's lysosomes or plasmamembrane. Vesicles that have released their cargo return via the reversedirection. The proteins that are synthesized in the ER have, as part of theiramino acid sequence, a signal that directs them where to go, much like anaddress directs a letter to its destination.
Solubleproteins are carried in the lumens of vesicles. Any proteins that are destinedfor a lysosome are delivered to the lysosome interior when the vesicle thatcarries them fuses with the lysosomal membrane and joins its contents. Incontrast, the proteins that will be secreted by a cell, such as insulin andEPO, are held in storage vesicles. When signaled by the cell, these vesiclesfuse with the plasma membrane and release their contents into the extracellularspace.
The Golgi apparatus functions as a molecular assembly line in which membrane proteins undergo extensive post-translational modification. Many Golgi reactions involve the addition of sugar residues to membrane proteins and secreted proteins. The carbohydrates that the Golgi attaches to membrane proteins are often quite complex, and their synthesis requires multiple steps.
Figure 2: Membrane transport into and out of the cellTransport of molecules within a cell and out of the cell requires a complex endomembrane system. Endocytosis occurs when the cell membrane engulfs particles (dark blue) outside the cell, draws the contents in, and forms an intracellular vesicle called an endosome. This vesicle travels through the cell, and its contents are digested as it merges with vesicles containing enzymes from the Golgi. The vesicle is then known as a lysosome when its contents have been digested by the cell. Exocystosis is the process of membrane transport that releases cellular contents outside of the cell. Here, a transport vesicle from the Golgi or elsewhere in the cell merges its membrane with the plasma membrane and releases its contents. In this way, membranes are continually recycled and reused for different purposes throughout the cell. Membrane transport also occurs between the endoplasmic reticulum and the Golgi. 2010 Nature Education All rights reserved. Figure Detail
Figure 3: Pathways of vesicular transport by the specific vesicle-coating proteinsA protein called coat protein II (COPII; green) forms vesicles that transport from the endoplasmic reticulum (ER) to the Golgi. A different protein called coat protein I (COPI; red) forms vesicles for transport in the other direction, from the Golgi to the ER. COPI also forms vesicles for intra-Golgi transport. Clathrin (blue) forms multiple complexes based on its association with different adaptor proteins (APs). Clathrin that is associated with AP1 and AP3 forms vesicles for transport from the trans-Golgi network to the later endosomal compartments, and also for transport that emanates from the early endosomal compartments. Clathrin that is associated with AP2 forms vesicles from the plasma membrane that transport to the early endosomes. 2009 Nature Publishing Group Hsu, V. W., Lee, S. Y., & Yang, J. S. The evolving understanding of COPI vesicle formation. Nature Reviews Molecular Cell Biology 10, 360-364 (2009). All rights reserved. Figure DetailLysosomes break down macromolecules into their constituent parts, which are then recycled. These membrane-bound organelles contain a variety of enzymes called hydrolases that can digest proteins, nucleic acids, lipids, and complex sugars. The lumen of a lysosome is more acidic than the cytoplasm. This environment activates the hydrolases and confines their destructive work to the lysosome. In plants and fungi, lysosomes are called acidic vacuoles.
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, as noted earlier, glycolipids and sphingomyelin are synthesized within the Golgi. In plant cells, the Golgi apparatus further serves as the site at which the complex polysaccharides of the cell wall are synthesized. The Golgi apparatus is thus involved in processing the broad range of cellular constituents that travel along the secretory pathway.
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 structure and function. Proteins from the ER enter at its cis face (entry face), which is convex and usually oriented toward the nucleus. They are then transported through the Golgi and exit from its concave trans face (exit face). As they pass through the Golgi, proteins are modified and sorted for transport to their eventual destinations within the cell.
Distinct processing and sorting events appear to take place in an ordered sequence within different 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 most commonly viewed as consisting of four functionally distinct regions: the cisGolgi network, the Golgi stack (which is divided into the medial and trans subcompartments), and the transGolgi network (Figure 9.23). 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 place. 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.
Although the Golgi apparatus was first described over 100 years ago, the mechanism by which proteins move through the Golgi apparatus has still not been established and is an area of controversy among cell biologists. One possibility is that transport vesicles carry proteins between the cisternae of the Golgi compartments. However, there is considerable experimental support for an alternative model proposing that proteins are simply carried through compartments of the Golgi within the Golgi cisternae, which gradually mature and progressively move through the Golgi in the cis to trans direction.
Protein processing within the Golgi involves the modification and synthesis of the carbohydrate portions of glycoproteins. One 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 chapter, proteins are modified within the ER by the addition of an oligosaccharide consisting of 14 sugar residues (see Figure 9.15). Three glucose residues and one mannose are then removed while the polypeptides are still in the ER. Following transport to the Golgi apparatus, the N-linked oligosaccharides of these glycoproteins are subject to extensive further modifications.
N-linked oligosaccharides are processed within the Golgi apparatus in an ordered sequence of reactions (Figure 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 two more N-acetylglucosamines. Finally, three galactose and three sialic acid residues are added. As noted in Chapter 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 can emerge from the Golgi with a variety of different N-linked oligosaccharides.
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