IRE1α is an endogenous substrate of endoplasmic-reticulum-associated degradation

Endoplasmic reticulum (ER)-associated degradation (ERAD) represents a principle quality control mechanism to clear misfolded proteins in the ER; however, its physiological significance and the nature of endogenous ERAD substrates remain largely unexplored. Here we discover that IRE1α, the sensor of the unfolded protein response (UPR), is a bona fide substrate of the Sel1L–Hrd1 ERAD complex. ERAD-mediated IRE1α degradation occurs under basal conditions in a BiP-dependent manner, requires both the intramembrane hydrophilic residues of IRE1α and the lectin protein OS9, and is attenuated by ER stress. ERAD deficiency causes IRE1α protein stabilization, accumulation and mild activation both in vitro and in vivo . Although enterocyte-specific Sel1L-knockout mice ( Sel1L ΔIEC ) are viable and seem normal, they are highly susceptible to experimental colitis and inflammation-associated dysbiosis, in an IRE1α-dependent but CHOP-independent manner. Hence, Sel1L–Hrd1 ERAD serves a distinct, essential function in restraint of IRE1α signalling in vivo by managing its protein turnover. Through a proteomics approach, Qi and colleagues and Long and colleagues identify the sensor of the unfolded protein response IRE1α as an endogenous substrate of the E3 ubiquitin ligase involved in ER-associated degradation, Hrd1.