Unfolded proteins in the mitochondria activate HRI and inhibit mitochondrial protein translation

The mitochondrial unfolded protein response (UPR^mt) is triggered through eIF2α phosphorylation in mammals. However, the mechanisms of UPR^mt activation and the influence of eIF2α phosphorylation on mitochondrial protein translation remain unclear. In this study, we confirmed that the UPR^mt is a rapid and specific stress response that occurs through pharmacological induction of eIF2α phosphorylation, along with the phosphorylation of eIF2α, ATF4, and CHOP. Moreover, with the upregulation of the expression of some chaperones, cytochrome P450 enzymes, and DDIT4, as determined by RNA-Seq and ribosome profiling, eIF2α phosphorylation was found to be essential for the expression of ATF4 and CHOP, after which ATF4 trafficked into the nucleus and initiated CHOP expression. In addition, the generation of ROS and mitochondrial morphology were not affected by the GTPP-induced UPR^mt. Furthermore, we investigated the mechanism by which HRI kinase-mediated UPR^mt is induced by mitochondrial unfolded proteins via CRISPR-Cas9 technology, mitochondrial recruitment of HRI and interaction with other proteins. Moreover, we confirmed that mitochondrial protein translation and mitochondrial protein import were inhibited through eIF2α phosphorylation with the accumulation of unfolded mitochondrial proteins. These findings reveal the molecular mechanism of the UPR^mt and its impact on cellular protein translation, which will offer novel insights into the functions of the UPR^mt, including its implications for human disease and pathobiology.