The connector between the kinase and RNase domains of IRE1α transmits the conformational change that underlies ER stress-induced activation.

IRE1α signals endoplasmic reticulum stress with oligomerization and activation of its kinase domain, leading to trans-auto-phosphorylation and followed by activation of the RNase domain. While these activation events are thought to be mediated by conformational changes, the intramolecular path that relays the activation from the luminal domain to the cytosolic RNase domain is unresolved. Here we show that the segment connecting the kinase and RNase domains, outside either catalytic site, is a conduit for relaying this information. Substitutions like L827P inhibit the kinase, RNase activities and oligomerization of IRE1α. L827P is dominant-negative in vivo because it associates with WT IRE1α. The less drastic substitution L827F has RNase activity, responds to the allosteric kinase inhibitor APY-29, but is still oligomerization-deficient. The connector mutants have distinct stress-induced conformations and increase the sensitivity of cells to ER stress. Therefore, the Kinase-RNase connector is an important conduit for attaining the stress-responsive conformation of IRE1α.