Uncovering the initial response: Intra-mitochondrial surveillance activates the UPR<sup>mt</sup>.

The mitochondrial unfolded protein response (UPR^mt) protects mitochondria from proteotoxic stress. Current models induce acute and severe mitochondrial disruption and propose cytosolic detection following the release of mitochondrial damage signals into the cytosol. However, this mode of toxicity contrasts sharply with physiological stress, such as the gradual accumulation of reactive oxygen species (ROS) during aging or chronic respiratory chain defects. Here, we employ a chemogenetic strategy in yeast to induce low levels of hydrogen peroxide (H₂O₂) in the mitochondrial matrix and show that mild oxidative stress activates the UPR^mt independently of cytosolic damage. We identify the presequence proteases MPP and Oct1 as early ROS targets, thereby linking redox imbalance to UPR^mt activation: oxidative stress induces glutathionylation of critical cysteines, impairing protease activity and causing the accumulation of unprocessed precursors in proteotoxic matrix aggregates. These aggregates are detected by intra-mitochondrial surveillance, activating UPR^mt signaling. Thus, mitochondrial self-surveillance initiates rapid protective signaling as a primary response to mitochondrial dysfunction(definition).