The ER contains transmembrane proteins that function as sensors of stressful
events that occur within the ER lumen. Which statement below is false?
Oa. If the number of unfolded or misfolded proteins increase to a high level, the chaperones are recruited to aid in protein
folding, which leaves the sensors in their unbound, activated state and capable of initiating a UPR.
Ob. In one of these pathways, the release of the inhibitory BiP protein leads to the dimerization of a sensor (called PERK). In its
dimeric state, PERK becomes an activated protein kinase that phosphorylates a protein (elF2) that is required for the
initiation of protein synthesis. This translation factor is inactive in the phosphorylated state, which stops the cell from
synthesizing additional proteins in the ER, giving the cell more time to process those proteins already present in the ER
lumen.
Oc. Under normal conditions, these sensors are present in an inactive state as the result of their association with chaperones,
particularly BiP.
Od. In the second pathway that release of the active BiP protein allows the sensor (called ATF6) to move on to the Golgi
complex where the cytosolic domain of the protein is cleaved away from its transmembrane domain. The cytosolic
portion of the sensor diffuses through the cytosol and into the nucleus, where it stimulates the expression of genes
whose encoded proteins can alleviate the stress in the ER. These include chaperones, coat proteins that form on transport
vesicles, and proteins of the quality-control machinery.
e. At least three distinct UPR pathways have been identified in mammalian cells, each activated by a different protein sensor.
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