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Terms in this set (22)

What happens if folding goes awry? You don't want to continue with an abnormal protein. The
ER has a quality control machinery to aid in this process to active help fold as well as monitor
progress and intervene before its too late. The presence of certain sugars is used as a method to
monitor the folded state (or oligomeric state) of a protein. Following the addition of the core sugars by OST, some trimming occurs in the ER. The
removal of two of the three outer chain glucose residues leaves a single glucose that is seen as a substrate by the membrane bound chaperone calnexin. An ER resident glucosidase (an enzyme
that removes glucose) removes the remaining glucose and the protein allowed to continue
folding. If folding is complete, the quality control machinery is done. If not, a single terminal
glucose is re-added by glucosyl transferase regenerating a calnexin substrate. This cycle of
binding and re-glucosylation-de-glucosylation is repeated until the protein is properly folded.
However, some proteins will still not properly fold. These abnormal proteins must be removed
from the ER to be degraded. They are translocated backwards through the Sec61 complex to be
ubiquitinated and degraded by the cytosolic degradation machinery (proteasome). In the event of
severe stress, the ER quality control mechanism and retrograde translocation machinery can be
overwhelmed. In this event, a novel signaling and transcriptional response is used to recover.
This process is called the Unfolded Protein Response (UPR).