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

The unfolded protein response (UPR) plays an evolutionarily conserved role in homeostasis, and its dysregulation often leads to human disease, including diabetes and cancer. IRE1 is a major transducer that conveys endoplasmic reticulum (ER) stress to biochemical signals, yet major gaps persist in our understanding of how the detection of stress is converted to one of several molecular outcomes. It is known that upon sensing unfolded proteins via its ER luminal domain, IRE1 dimerizes and oligomerizes (often visualized as clustering), and then trans-autophosphorylates. The IRE1 kinase activity is required for activation of its RNase effector domain and for clustering of IRE1. It is not yet clear if IRE1 clustering is a platform for the RNase activity, or if the two represent distinct biological functions. Here, we uncover a previously unrecognized role for helix K between IRE1 kinase and RNase domains in conveying critical conformational changes. Using mutants within this inter-domain helix, we show for the first time that: 1) distinct substitutions (specifically, of Leu827) selectively affect oligomerization, RNase activity, and, unexpectedly, the kinase activity of IRE1; 2) RNase activation can be uncoupled from IRE1 oligomerization, and phosphorylation of S729 marks the former but not the latter; 3) The nature of residue 827 determines the conformation that the IRE1 protein adopts, leading to different patterns of biochemical activities. In summary, this work reveals a previously unappreciated role for the inter-domain helix as a pivotal conduit for attaining the stress-responsive conformation of IRE1.