Protein Modifications in the ER Flashcards
What are the 4 protein modifications that occur in the ER?
- Glycosylation
- Folding and assembly of multi subunit proteins
- Specific proteolytic cleavage for some
- Formation of disulfide bonds/bridges
What are the sugars used for?
Communication and interactions between the protein and other molecules. Every ER protein gets at least one sugar stuck on it
What is a consensus sequence?
A sequence where a protein modification is added
What are the two most common types of glycosylation?
N-linkages and O-linkages
What are N-linkages?
The sugars are attached to the amide nitrogen in asparagine. This results in complex branching chains
What are O-linkages?
The sugars are attached to the hydroxyl oxygen in serine or threonine. This results in a shorter and simpler sugar chain
What are the enzymes that add sugars to proteins?
Glycosyltransferases. There is an each for each monosaccharide
What is the precursor that gets sugar chains into the ER?
Dolichol phosphate
What determines what sugar is added to the chain next?
What the next enzyme is that the dolichol phosphate happens to run into next
When do the sugar chains get added to the protein?
As soon as the site becomes available. It doesn’t wait for translation to be finished
What chaperone protein is highly expressed in the ER that assists with folding in the ER? What does it do?
BiP. It binds to hydrophobic regions on the new protein and stops them from aggregating
What is the enzyme that creates disulfide bridges? What amino acid gets these?
PDI: protein disulfide isomerase. Cysteine
What kind of proteins are calnexin and calreticulin? What do they do?
They are lectins, they bind to sugars and assist with folding
What enzyme transfers the sugar chains from dolichol phosphate to a glycoslylation site?
Oligosacchryl transferase
What happens if a protein misfolds in the ER?
They undergo unfolded protein response and leave the ER. Then they get ubiquitinated and sent to a proteasome
