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

Abstract

The unfolded protein response (UPR) plays an evolutionarily conserved role in homeostasis, and its dysregulation often leads to human disease, including diabetes and cancer. IRE1α 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, IRE1α dimerizes and oligomerizes (often visualized as clustering), and then trans-autophosphorylates. The IRE1α kinase activity is required for activation of its RNase effector domain and for clustering of IRE1α. It is not yet clear if IRE1α 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 αK between IRE1α 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 IRE1α; 2) RNase activation can be uncoupled from IRE1α oligomerization, and phosphorylation of S729 marks the former but not the latter; 3) The nature of residue 827 determines the conformation that the IRE1α 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 IRE1α.

Orexins in apoptosis: a dual regulatory role

“…It's categorized as a type I transmembrane protein with dual functionalities, serving as both an endoribonuclease and a serine/ threonine kinase. When GRP78 disengages, IRE1α experiences homooligomerization and trans-autophosphorylation events, which then activate its endoribonuclease and kinase domains (Ricci et al, 2021). As a result of this activation, IRE1α becomes engaged in facilitating the splicing of the mRNA intron responsible for encoding XBP1.…”

Splicing HAC1/XBP1 mRNAs in Cytoplasm: The Non-Conventional mRNA Splicing Reaction in the Unfolded Protein Response

“…The oligomerized Ire1 undergoes trans-autophosphorylation, which activates the RNase domain [53,[57][58][59][60]. A recent study discovered that the RNase activation is mediated by an interdomain helix between the Ire1's kinase and RNase domains [61]. The activated RNase domain cleaves the HAC1/XBP1 mRNA at the two splice sites, separating the two exon fragments from the intron fragment [16,62].…”