MITOL prevents ER stress‐induced apoptosis by IRE1α ubiquitylation at ER–mitochondria contact sites

Unresolved endoplasmic reticulum (ER) stress shifts the unfolded protein response signaling from cell survival to cell death, although the switching mechanism remains unclear. Here, we report that mitochondrial ubiquitin ligase (MITOL/MARCH5) inhibits ER stress‐induced apoptosis through ubiquitylation of IRE1α at the mitochondria‐associated ER membrane (MAM). MITOL promotes K63‐linked chain ubiquitination of IRE1α at lysine 481 (K481), thereby preventing hyper‐oligomerization of IRE1α and regulated IRE1α‐dependent decay (RIDD). Therefore, under ER stress, MITOL depletion or the IRE1α mutant (K481R) allows for IRE1α hyper‐oligomerization and enhances RIDD activity, resulting in apoptosis. Similarly, in the spinal cord of MITOL‐deficient mice, ER stress enhances RIDD activity and subsequent apoptosis. Notably, unresolved ER stress attenuates IRE1α ubiquitylation, suggesting that this directs the apoptotic switch of IRE1α signaling. Our findings suggest that mitochondria regulate cell fate under ER stress through IRE1α ubiquitylation by MITOL at the MAM. Synopsis Unfolded protein response sensor IRE1α controls both cell survival and apoptotic signalling. Here, chronic ER stress is shown to decrease MITOL‐mediated ubiquitylation of IRE1α at mitochondria‐ER contact sites, suggesting a role for mitochondria in cellular fate switches. E3 ligase MITOL promotes K63‐linked ubiquitination of IRE1α at mitochondria‐ER contact sites. MITOL‐mediated ubiquitination of IRE1α prevents apoptosis by inhibiting IRE1α hyper‐oligomerization and regulated IRE1α‐dependent decay of mRNA (RIDD). Prolonged ER stress attenuates IRE1α ubiquitylation by MITOL, leading to apoptosis. Nerve‐specific deletion of MITOL promotes RIDD and cell death in the spinal cord of mice under ER stress. Graphical Abstract Chronic ER stress decreases MITOL‐mediated ubiquitylation of unfolded protein response sensor IRE1α to direct cell fate and survival.