Congress of multiple dimers is needed for cross-phosphorylation of IRE1α and its RNase activity
The unfolded protein response can switch from a pro-survival to a maladaptive, pro-apoptotic mode. During endoplasmic reticulum (ER) stress, IRE1 sensors dimerize, are phosphorylated and activate XBP1 splicing, increasing folding capacity in the ER protein factory. The steps that turn the IRE1 endonuclease activity against endogenous mRNAs during maladaptive ER stress are still unknown. Here we show that although necessary, IRE1 dimerization is not sufficient to trigger phosphorylation. Random and/or guided collisions amongst IRE1 dimers are needed to elicit cross-phosphorylation and endonuclease activities. Thus, reaching a critical concentration of IRE1 dimers in the ER membrane is a key event. Formation of stable IRE1 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 IRE1 to phosphatases. The stepwise activation of IRE1 molecules and their low concentration at steady state prevent excessive responses, unleashing full-blown IRE1 activity only upon intense stress conditions.