Consequences and repair of mitochondrial protein import defects in mammals

Mitochondria are cellular hubs of energy production, signaling, apoptosis, and biogenesis of many essential metabolites. These diverse functions heavily rely on a vast repertoire of over 1,000 proteins in humans, the vast majority of which are encoded by the nuclear genome and synthesized in the cytosol. As a result, the constant import of proteins into the mitochondria is indispensable for maintaining organelle function. Defects in protein import are a feature of aged cells and associated with various neurodegenerative and metabolic disorders. When left unchecked, these defects can have negative consequences on the cell, as they not only deplete mitochondria of essential factors but also lead to aberrant accumulation of un-imported proteins outside the organelle, compromising overall cellular health. In this study, we investigate the consequences of impaired mitochondrial protein import in mammals and uncover how cells mitigate this defect. We demonstrate that un-imported proteins can clog the mitochondria by stalling inside the translocase of the outer membrane (TOM). Under these conditions, attenuation of global protein translation mediated by the integrated stress response (ISR), as well as proteasomal degradation, prevent excessive accumulation of un-imported proteins. We identify a specific mechanism that has evolved to extract stalled precursor proteins from the TOM complex and resolve the import blockage. This process is mediated by the mitochondrial AAA ATPase, ATAD1, which associates with both TOM and stalled proteins during mitochondrial stress. Deletion of ATAD1 results in extensive accumulation of mitochondrial precursors as well as a reduction in protein import. In contrast, increased ATAD1 expression improves the fitness of cells with impaired mitochondrial protein import. Overall, our work uncovers a novel quality control mechanism at the mitochondrial surface in human cells. We show that ATAD1 maintains mitochondrial protein homeostasis by surveilling and unclogging the TOM complex. This work provides important mechanistic insight into how cells cope with mitochondrial dysfunction.