IRE1 drives a homeostatic response to reduced protein influx into the endoplasmic reticulum
IRE1, alongside ATF6 and PERK, orchestrates the Unfolded Protein Response, a network of signaling pathways that maintains endoplasmic reticulum (ER) homeostasis. Two modes of IRE1 activation are known: i) in response to an accumulation of unfolded proteins in the ER lumen and ii) in response to compositional changes to the ER membrane that alter its physical properties. Here we identify a third, independent mode of IRE1 activation: ER co-translational translocation deficits activate IRE1 through a mechanism that relies on the release of IRE1 molecules from unoccupied translocons. We define this mechanism as TRES for "TRanslocon Engagement Surveillance". TRES leads to spontaneous activation of IRE1 and bypasses its unfolded protein- and ER membrane composition-sensing functions. Inhibiting translation initiation similarly activates IRE1 by TRES, as it leads to a decline in ER protein import, thus linking the Integrated Stress Response to IRE1 signaling. TRES drives IRE1 activation without activating ATF6 or PERK, resulting in a distinct gene expression program that feeds back by boosting the co-translational translocation machinery to rebalance the ER protein load. Our findings thus demonstrate that monitoring and adjusting the rates of protein translocation are critical for maintaining ER homeostasis.