Selective quality control of mistargeted mitochondrial proteins at the endoplasmic reticulum

In eukaryotic cells, the function of each organelle depends on its unique protein composition. Protein targeting errors threaten organelle identity and function, yet how mistargeting errors are detected and resolved remains poorly understood. Here, we show that mitochondrial import stress drives widespread rerouting of mitochondrial proteins to the endoplasmic reticulum (ER), with strong enrichment for hydrophobic oxidative phosphorylation (OXPHOS) components. Using proximity proteomics, a split-fluorescence reporter system, and genome-wide CRISPR screening, we find that mistargeted proteins partition into distinct classes with divergent fates, ranging from stable ER residence to rapid degradation by ER-associated degradation (ERAD). The clearance of these mislocalized proteins involves partially redundant ERAD branches, with the ubiquitin ligase MARCHF6 playing a central role. Together, these findings establish the ER as a key organelle for handling mistargeted mitochondrial proteins and reveal how ER quality control maintains proteostasis during mitochondrial dysfunction.