Processes

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Author:
DesLee26
ID:
246245
Filename:
Processes
Updated:
2013-11-12 07:38:12
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Cell Bio
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Mickle
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  1. signal hypothesis
    the idea that a signal peptide is synthesized (explain the process) and translated cotranslationally
  2. Steps of glycosylation:
    • (1)    Core segment made in the cytoplasm on a molecule of dolichol phosphate; glycosyltransferases add sugars
    • a.       order:
    •                                                                i.      in the cytoplasm: 7 subunits are added: five mannose and two N-
    •                                                              ii.      in the ER lumen: the remaining seven (4 mannose and 3 glucose)
    • (2)    oligosaccharyltransferase transfers it from dolichol phosphate to asparagines residues of the polypeptide as the polypeptide is being translocated into the ER lumen
    • (3)    after transferred to the polypeptide, the oligosaccharide chain undergoes modification:
    • a.       begins in the ER with the enzymatic removal of two of the three glucose residuesà screening that decides whether fit to go on or not
  3. Screening of the Polypeptide and Dealing with a Misfolded polypeptide
    Step 1
    • 1)      begins with the enzymatic removal of two of the three terminal glucose residues
    • 2)      each glycoprotein, which contains only one remaining glucose, binds to the ER chaperone, such as calnexin
    • 3)      the last glucose is removed by glucosidase II, causing release from the chaperone
    • a.       if folding is incorrect/ incomplete, it is recognized by a conformation-sensing enzyme (UGGT) that adds the single glucose back to allow recognition by the same ER chaperone
    • b.      after some time with the same ER chaperone, the glucose residue is removed once again and the conformation-sensing enzyme checks it again
    • c.       the process continues (if still incorrect) until eventually another slow acting enzyme in the ER lumen acts on the protein and removes a mannose residue instead of a glucose, causing the glycoprotein to be “unfixable” and signaled to death
  4. What happens if folding is incorrect?
    • a.       if folding is incorrect/ incomplete, it is recognized by a conformation-sensing enzyme (UGGT) that adds the single glucose back to allow recognition by the same ER chaperone
    • b.      after some time with the same ER chaperone, the glucose residue is removed once again and the conformation-sensing enzyme checks it again
    • c.       the process continues (if still incorrect) until eventually another slow acting enzyme in the ER lumen acts on the protein and removes a mannose residue instead of a glucose, causing the glycoprotein to be “unfixable” and signaled to death
  5. Glycoprotein death is called __
    ER associated degradation (ERAD)
  6. ERAD process
    • (1)    First dislocation occurs out into the cytoplasm
    • (2)    oligosaccharide chains are removed and the misfolded proteins are destroyed in proteasomes (protein-degrading machines) to prevent transport to other parts of the cell
  7. UPR
    • (1)    If an overaccumulation of unfolded proteins occursà unfolded protein response
    • - originally the senors that monitor stressful events are inactive due to BiP
    • a.       BiP, a molecular chaperone, activaties the sensors that monitor the concentration of misfolded or unfolded proteins in the ER lumen by leaving them and being called into service to aid in protein folding elsewhere; thus, BiP molecules are no longer able to inhibit the sensors
    • c.       Signals occur that cause expression of hundreds of different genes whose encoded proteins have the potential to alleviate stressful conditions
    • d.      Phosphorylation of a key protein (eIFalpha) helps because it inhibits protein synthesis and gives the cell time to remove what is in the ER lumen
  8. PERK
    a UPR pathway in which release of BiP enables dimerization of a sensor called PERK which becomes activated and phosphorylates eIFalpha, required for the initiation of protien synthesis. Phosphorylation inactivates it

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