Core Concepts
Dimerization of IRE1α is not sufficient for its phosphorylation and activation. Higher-order oligomerization is necessary to trigger cross-phosphorylation and unleash the full endonuclease activity of IRE1α.
Abstract
The study investigates the stepwise activation of the endoplasmic reticulum (ER) stress sensor IRE1α. The authors find that dimerization of IRE1α, while necessary, is not sufficient for its phosphorylation and full enzymatic activation.
Key insights:
IRE1α phosphorylation and RNase activity correlate with the intensity of ER stress. Under mild stress, IRE1α is only mildly activated, while unresolved stress leads to sustained high levels of phosphorylation and RNase activity.
Using a panel of IRE1α mutants, the authors show that phosphorylation does not occur within isolated IRE1α dimers. Rather, it requires collisions between dimers or formation of higher-order oligomers.
The dimerizable IRE1α chimera (Dim-IRE1α) is only phosphorylated and active when expressed at very high levels, suggesting that reaching a critical concentration of IRE1α dimers is key for triggering cross-phosphorylation.
Phosphorylation and nucleotide-binding stabilize the active, oligomeric state of IRE1α, further enhancing its RNase activity.
Stable clustering of IRE1α is not necessary for XBP1 splicing, but may modulate the potency and duration of the response.
The authors propose a model where the stepwise activation of IRE1α, from dispersed dimers to concentrated oligomers, allows cells to fine-tune the ER stress response, preventing excessive activation under mild stress conditions while unleashing full-blown IRE1α activity only upon intense and unresolved ER stress.
Stats
The fraction of phosphorylated IRE1α increases when ERAD is blocked by kifunensine, particularly during the late phases of the ER stress response.
Preventing phosphorylation of IRE1α (AAA mutant) abolishes its RNase activity even at high expression levels.
The phosphomimetic Dim-DDD IRE1α mutant is active at much lower expression levels compared to the non-phosphorylated Dim-IRE1α.
Quotes
"Reaching a critical concentration of IRE1α dimers in the ER membrane is a key event."
"Formation of stable IRE1α clusters is not necessary for RNase activity. However, clustering could modulate the potency of the response promoting interactions between dimers and decreasing the accessibility of phosphorylated IRE1α to phosphatases."