ATAD1 and the integrated stress response prevent clogging of TOM and damage caused by un-imported mitochondrial proteins

Mitochondria require the constant import of nuclear-encoded proteins for proper functioning. Impaired protein import not only depletes mitochondria of essential factors but also leads to toxic accumulation of un-imported proteins outside the organelle. Defects in mitochondrial protein import are associated with neurodegenerative and bioenergetic diseases. Here, we investigated the consequences of mitochondrial protein import stress in human cells. We demonstrated that un-imported proteins can clog the mitochondria by stalling inside the translocase of the outer membrane (TOM). We found that the integrated stress response (ISR) acted as the first line of defense to mitochondrial clogging by attenuating global protein translation and preventing excessive accumulation of un-imported proteins. A second mechanism, mediated by a mitochondrial ATPase, ATAD1, acted specifically to remove proteins from TOM and clear the entry gate into the mitochondria. ATAD1 interacted with both TOM and stalled proteins, and its knockout resulted in extensive accumulation of mitochondrial precursors as well as decreased protein import. Increased ATAD1 expression improved tolerance of cells to defective mitochondrial protein import, demonstrating the importance of this quality control pathway in surveilling protein import and its contribution to cellular health.