Stress-induced clustering of the UPR sensor IRE1α is driven by disordered regions within its ER lumenal domain

Abstract

Upon accumulation of unfolded proteins at the endoplasmic reticulum (ER), IRE1 activates the unfolded protein response (UPR) to restore protein-folding homeostasis. During ER stress, IRE1’s ER lumenal domain (LD) drives its clustering on the ER membrane to initiate signaling. How IRE1’s LD assembles into high-order oligomers remains largely unknown. By in vitro reconstitution experiments we show that human IRE1α LD forms dynamic biomolecular condensates. IRE1α LD condensates were stabilized when IRE1α LD was tethered to model membranes and upon binding of unfolded polypeptide ligands. Molecular dynamics simulations suggested that weak multivalent interactions are involved in IRE1α LD assemblies. Mutagenesis showed that disordered regions in IRE1α LD control its clustering in vitro and in cells. Importantly, dysregulated clustering led to defects in IRE1α signaling. Our results reveal that membranes and unfolded polypeptides act as scaffolds to assemble dynamic IRE1α condensates into stable, signaling competent clusters.