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Posttranslational Protein Modifications
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| How to get into the ER.... |
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| Transmembrane Proteins | |
| Glycosylation | |
| In the Golgi apparatus |
Every (eukaryotic) protein's life begins in the nucleus. In this compartment of the cell DNA is transcribed into mRNA which leaves the nucleus. At this point of the synthesis the first differentiation into two groups of different protein types can be made: mRNAs which do not encode for a certain N-terminal amino acid signal sequence, the leader, are translated by free ribosomes in the cytosol. These proteins stay in the cytosol, associate with macromolecules as filaments and microtubules or they pass as nuclear proteins into the nucleus. The second type of proteins is integral membrane or secretory proteins. |
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They have a signal or leader sequence which is 13 to 36 amino acids long, consisting of at least one positively charged amino acid at the terminus and a large hydrophobic region. After synthesis of about 70 amino acids a protein called signal recognition particle (SRP) binds to the signal sequence. By connecting to the leader sequence SRP inhibits translation. Then SRP associates to a receptor in the ER membrane, the ribosome binds to the membrane (now forming the rough ER) and translation resumes while SRP and its receptor are released and free to bind to another signal sequence.Now the nascent protein is channeled into the lumen of the rER during its translation which is called co-translational translocation. A secretory protein is simply in full length synthesized into the ER lumen, folded due to the chemical interactions of the single amino acids and then processed. This is another important function of the SRP: it does not only help the ribosome connect to the ER it also prevents the nascent protein from being released into the aqueous environment of the cytosol which would lead to malfolding.
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Cotranslational protein-translocation (1)
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But integral membrane proteins are formed another way. Because they have to stay in the membrane translation is halted when a stop-transfer signal, the anchor region, leaves the ribosome and enters the membrane. An anchor located at the very end of the polypeptide chain the protein leads to a type I integral membrane protein which stays in the membrane the way it was produced: the N-terminus first in the lumen, and the C-terminus staying in the cytosol. In type II integral membrane proteins the leader and anchor sequence are combined. This sequence enters the membrane but does not pass through it. It stays in the membrane while the rest of the protein loops into the. All amino acids that precede the signal-anchor region N-terminal are exposed to the cytosol; the C-terminus is in the ER lumen. A change in the amino acid sequence may cause the protein to insert to the lumen (leading to its secretion) or incorrectly positioned within the membrane. In both cases the protein is not able to fulfill its task.
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The protein inserted in the ER membrane now undergoes further processing. There are two types of glycosylation (the adding of carbohydrates): the first is the addition of the carbohydrate to the NH2 group of asparagine (N-linked glycosylation) or to the OH group of serine, threonine or hydroxylysine (O-linked glycosylation). N-linked glycosylation begins in the ER and is completed in the Golgi apparatus; O-linked glycosylation takes place only in the Golgi apparatus. But before the protein can be translocated from the ER to the Golgi apparatus it has to be glycosylated correctly. |
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core glycosylation (2)
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As aforementioned only N-linked glycosylation is performed in the ER The oligosaccharide is synthesized on a special lipid - dolichol - which resides in the ER membrane and presents the oligosaccharide to the ER lumen where it is added by glycosyl transferase to the asparagine residue of the protein. The added oligosaccharide is trimmed afterwards; some sugars are removed and added by ER glucosidases and mannosidases; the inner core consisting of two N-acetyl glucosamines and three mannoses is never stripped off. How exactly each protein is processed in its characteristic pattern is not known yet. A failure in glycosylation causes the new formed glycoprotein to stay in the ER. This is another control mechanism which new proteins have to pass in order to move on to the Golgi apparatus. During the whole transport from ER to the plasma membrane the orientation of the membrane protein is adhered
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The Golgi apparatus is an important organelle in which further glycosylation takes place and where the new proteins are targeted to their destinations. It consists in most mammal cells of three to four membrane stacks and changes it’s constitution from the nucleus to the plasma membrane. Closest to the ER the cis-Golgi stack is located where the transport vesicles coming from the ER enter. The trans-Golgi stack is located near to the plasma membrane. The membrane structure changes from cis- to trans-Golgi stack as does the composition of enzymes. Nascent proteins are modified as they pass the different enzymes from cis to trans-Golgi apparatus. In the Golgi apparatus the modification of the N-linked oligosaccharide chains proceeds and new O-linked sugars are added. The mode of modification influences glycoprotein sorting. The correct transport of proteins is important for the cellular and organismal metabolism. Several lysosomal storage disorders for example result from defects in lysosomal function triggered by wrong protein-transport. |
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