ER Quality control Flashcards
What is the process of perceiving misfolded proteins to degrading them called?
ER-associated protein degradation (ERAD)
What does it mean when a protein is glycosylated?
It receives a premade complex carbohydrate structure containing 2 N-glucosamine residues, 9 manose and 3 glucose.
It acts as a label for whether the protein is folded or not.
When does N-linked glycosylation happen?
co-translationally, so oligosaccharide transferase passes the preformed sugar from a lipid doner/carrier to the protein as it is being made
How is the folding/quality of the protein monitored through the ER?
Through the trimming of the glycan, a mechanism which relies on a number of chaperones.
What is the Calnexin cycle?
Calnexin is a chaperone in the ER membrane. It recognises and binds proteins which are partially folded (so have 2/3 glucoses removed i.e has one remaining glucose.
This gives it time to finish folding properly.
What happens when a protein is fully folded?
The final glucose is removed by glucosidase. The protein can now leave the ER and continue down the secretory pathway.h
What reason could proteins not fold completely?
The system could be under stress so there’s an unusual amount of incoming protein, resulting in everything slowing down /there being a backlog of proteins that need to be folded.
Or the protein has a mutation which prevents correct folding.
What happens to terminally misfolded proteins bound to calnexin?
They are passed to glucosyl transferase, which decides if it is folded correctly. If misfolded, it attaches another glucose residue to give it another chance to refold.
How was glucosyl transferase (GT) activity discovered
When Roversi et al 2017 found the crystal structure despite flexible parts. They found flexible thioredoxin-like domains which wrap around the protein and senses its folding state by looking for free cysteines which should be in disulphide bonds.
How does the calnexin cycle determine when a protein has had enough opportunities to fold and should be degraded?
A mannosidase sits in the ER with a very slow catalytic rate. If a protein hangs around in the ER for a long time, it will eventually be caught by it and a mannose will be trimmed off so it will go from 9 mannose residues to 8.
How are misfolded proteins detected to be kicked out of the ER?
Glc1 Man8 glycans are detected by ER mannose binding lectins. These lectins remove misfolded glycoproteins from the calnexin cycle and hand it over to retrotranslocation machinery to have it leave the ER.
Why are proteins degraded in the cytosol and not the ER?
Because you don’t want proteases where you are synthesising proteins
How are misfolded proteins translocated out of the ER?
They go through a PCC operating in the reverse direction.
While going through this channel, multiple ubiquitins are added by an ATPase called p67 so it pulls the Ub tagged protein out of the ER.
What is the protein involved in removing proteins from the ER?
Hrd1 is an E3 Ubiquitin ligase so it self-ubiquitinates.
Baldridge and Rapoport 2016 found that Hrd1 alone is the minimum requirement for in vitro retrotranslocation of ERAD substrates - but only when it is auto-ubiquitinated on its RING-finger domain
How does retrotranslocation happen?
Substrate binds to Hrd1. It autoubiquitinates. A channel opens in Hrd1 for the substrate to travel to, out of the ER where it is degraded by a proteosome.
