Here, we show that the widely used strategy of employing ER stress-eliciting drugs obscures how ER homeostatic readjustment may be achieved, and instead, we present a He La cell model that allows us to evaluate just that.
By inducible overexpression of orphan immunoglobulin M (Ig M) secretory heavy chain (µ levels again, while the ER expands in the process.
A consequence of client retention is that when the protein folding machinery is not adequate to facilitate maturation of clients, they accumulate in the ER, which causes stress and activates the unfolded protein response (UPR) (Walter and Ron, 2011).
The circuitry of the UPR has been mapped to impressive detail: the three well-studied branches of the UPR are initiated by the UPR transducers IRE1α, PERK and ATF6α, each of which has an ER stress-sensing domain in the ER lumen and a UPR signaling effector domain in the cytosol.
Upon its religation, the spliced m RNA encodes the XBP1 transcription factor (Yoshida et al., 2001; Calfon et al., 2002).
Activated PERK transiently attenuates protein synthesis through phosphorylation of the translation initiation factor e IF2α (Harding et al., 1999).
This step triggers IRE1α to assume endonuclease activity, such that it removes an intron from XBP1 m RNA.Insufficient folding capacity of the endoplasmic reticulum (ER) activates the unfolded protein response (UPR) to restore homeostasis.Yet, how the UPR achieves ER homeostatic readjustment is poorly investigated, as in most studies the ER stress that is elicited cannot be overcome.At the same time, e IF2α phosphorylation favors the expression of a few transcripts, in particular ATF4, a transcription factor that activates further downstream effectors, such as CHOP (Walter and Ron, 2011).The third UPR branch is activated by ATF6α, which undergoes regulated intramembrane proteolysis in the Golgi and thus a transcriptionally active N-terminal portion of 50 k Da is liberated that acts as a transcription factor (Ye et al., 2000).