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 IRE1 alpha at the mitochondria-associated ER membrane (MAM). MITOL promotes K63-linked chain ubiquitination of IRE1 alpha at lysine 481 (K481), thereby preventing hyper-oligomerization of IRE1 alpha and regulated IRE1 alpha-dependent decay (RIDD). Therefore, under ER stress, MITOL depletion or the IRE1 alpha mutant (K481R) allows for IRE1 alpha 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 IRE1 alpha ubiquitylation, suggesting that this directs the apoptotic switch of IRE1 alpha signaling. Our findings suggest that mitochondria regulate cell fate under ER stress through IRE1 alpha ubiquitylation by MITOL at the MAM.