Week 9 Review Sheet answers

Week 9 Review Sheet answers - Final Review Sheet Week 9...

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Final Review Sheet – Week 9 Lecture 22 More on Internal Membrane Systems Main Points: In addition to its role in protein synthesis, the ER is also the site of synthesis of phospholipids for the membrane. These are all made by enzymes embedded in the cytosolic side. The lipids are inserted into the membrane on that side and then some are flipped (by Flippases) to the other side. This establishes the lipid asymmetry that one sees in other membranes. Lipid composition of different membrane is different. Figure 8.15 illustrates several possible mechanism for introducing differences into different membranes. Returning to a discussion of proteins, once a protein, such as a secretory protein is finished, several events occur in the lumen of the ER: disulfide bond formation, glycosylation, proper folding. Glycosylation of the proteins occurs by the formation of a 14 sugar unit on a carrier lipid, dolichol phosphate. The sugars are added one at a time, with the first ones added on the cytosolic side, then the unit being transferred to the luminal side. There, the 14 sugar unit is completed by the addition of three glucoses and then the whole thing is moved to the protein at as asparagines (N-Linked). An additional process that occurs in the ER is the proper folding of the protein. This is mediated by a chaperone, calnexin, located in the ER membrane. The new protein binds to calnexin through a terminal glucose. If it folds properly, it leaves the ER. If not, it either exits and is degraded in a proteasome, or it goes around again after addition of a new glucose. Eukaryotic cells exhibit a “misfolded protein response” in which the cell responds to an excess of unfolded protein in the ER by activating two proteins. These proteins are resident in the ER and are normally inactivated by binding to the chaperone BiP. When too many proteins are unfolded within the ER, the Bip moves to interact with them, releasing these two proteins. One of them dimerizes and is then subjected to proteolysis on the cytoplasmic side, clipping of pieces which go to the nucleus and activate synthesis of more chaperones. The other slows protein synthesis for a while to allow the processing of proteins to catch up. The Golgi apparatus is the site of completion of sugar modifications and sorting the new proteins for their specific destinations. The Golgi consists of flattened sheets, not interconnected. Each contains unique enzymes that modify the oligosaccharide chain begun in the ER. The cis side of the Golgi is near the nucleus, and receives vesicles from the ER. The trans side is closer to the cell membrane and sorts and packages ER made proteins for their final destination. Vesicle fusion is a key aspect of transit of proteins through the cytoplasmic
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Week 9 Review Sheet answers - Final Review Sheet Week 9...

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