Exam 3- The ER Flashcards
What’s the translocon?
The membrane channel through which proteins are transported into the ER. It is a complex of 3 transmembrane proteins called Sec 61 proteins.
What’s the main function provided by protein glycosylation quality control for glycoproteins in the ER?
Prevents the accumulation of misfolded proteins- quality control mechanism.
What’s the role of Calnexin?
What’s the role of the Glucosyltranferase in the folding cycle regulated by Calnexin?
Facilitates folding of protein, and allows association with other proteins like PDI-in lumen of the ER.
Glycosyltransferease adds an extra glucose, so the protein can be folded properly. If the protein is never fixed, it is retrotranslocated out of the ER, then degraded by proteasome the moment if reaches the cytosol.
What is the mechanism of glycoprotein foliding by calnexin?
As the glycoprotein leaves the translocon, 2 glucose molecules are chopped off and the glycoprotein is bound by the chaperone- Calnexin. The protein then looses one more glucose and leaves Calnexin and binds to a folding sensor glycosyltransferase. If folded correctly the protein will pass to the golgi.
If not folded correctly- glycolsyltransferase adds the glucose onto the glycoprotein, the protein binds back to calnexin to be folded once more.
What about a misfold again?- The protein is recognized by EDEM1 which will remove a mannose sugar to stop calnexin binding and the protein is poly-ubiquitinated by ubiquitin ligase and degraded by the proteosome in the cytosol.
What’s the Unfolded Protein Response?
The way the cell monitors the protein concentration.
What are the three sensor receptors of the unfolded protein response?
IRE1
ATF6
PERK (kinase)
Possible outcomes of the UPR?
General inhibition of protein synthesis- PERK will phosphorylate the eIF2
Increased expression of chaperons (calnexin, calreticulin, PDI, and BiP)- ATF6 is cleaved to release a transcription factor for other UPR genes.
Increase in proteasomal activity (protein degradation)- IRE1 will cleave XBP1 to prevent translation
Which lipids are synthesized in the ER?
PGCC
Phospholipids, Glycolipids,
Cholesterol
ceramide
What’s the precursor of all Phospholipids synthesized in the ER?
diacylglycerol
Whats the precursor to all glycolipids?
Ceramide
List all phospholipids synthesized in the ER
Phosphatidyl-CESI
choline
ethanolamine
serine
inositol
Are phospholipids synthesized in both sides of the bilayer?
No, only made on the cytosolic side. To get the Plipids to the lumenal side you must transfer the polar head across the memrane using flippase.
