Abstract
Consisting of three signaling pathways, the unfolded protein response (UPR) can be either protective or detrimental to cells that undergo ER stress. Elaborate regulation of the UPR is key to the cell-fate decision, but how it is achieved remains vague. Here, by studying cells deficient in vacuole membrane protein 1 (VMP1), a UPR regulator, we report a model of UPR regulation in which the three pathways are divergently controlled. Under basal conditions, calcium binding specifically activates PERK. Under ER stress, ER-mitochondria interaction-induced mitochondrial stress cooperates with PERK to suppress IRE1α and ATF6 by decelerating global protein synthesis. Such sophisticated regulation commits limited activation of the UPR yet refrains from UPR hyperactivation, protecting cells from chronic ER stress despite decreasing cell proliferation. Therefore, our study reveals interorganelle-interaction-dependent and calcium-dependent regulation of the UPR that dictates cell fate.