The Sec61 translocon limits IRE1α signaling during the unfolded protein response

IRE1 alpha is an endoplasmic reticulum (ER) localized endonuclease activated by misfolded proteins in the ER. Previously, we demonstrated that IRE1 alpha forms a complex with the Sec61 translocon, to which its substrate XBP1u mRNA is recruited for cleavage during ER stress (Plumb et al., 2015). Here, we probe IRE1 alpha complexes in cells with blue native PAGE immunoblotting. We find that IRE1 alpha forms a hetero-oligomeric complex with the Sec61 translocon that is activated upon ER stress with little change in the complex. In addition, IRE1 alpha oligomerization, activation, and inactivation during ER stress are regulated by Sec61. Loss of the IRE1 alpha-Sec61 translocon interaction as well as severe ER stress conditions causes IRE1 alpha to form higher-order oligomers that exhibit continuous activation and extended cleavage of XBP1u mRNA. Thus, we propose that the Sec61-IRE1 alpha complex defines the extent of IRE1 alpha activity and may determine cell fate decisions during ER stress conditions.