A Thioredoxin reductive mechanism balances the oxidative protein import pathway in the intermembrane space of mitochondria

Mitochondria biogenesis crucially depends on the oxidative folding system in the mitochondrial intermembrane space. The oxidative capacity needs however to be balanced by a reductive pathway for optimal mitochondrial fitness. Here we report that the cytosolic thioredoxin machinery fulfils this critical reductive function by dual localisation in the mitochondrial intermembrane space (IMS) via an unconventional import pathway. We show that the presence of the Thioredoxin system in the IMS mediates a hitherto unknown communication between mitochondria biogenesis and the metabolic state of the cell via the cytosolic pool of NADPH. By a combination of complete in vitro reconstitution with purified components, import assays and protein interaction analysis we find that the IMS-localised thioredoxin machinery critically controls the redox state of Mia40, the key player in the MIA pathway in mitochondria thereby ensuring optimal mitochondria biogenesis. Intriguingly, we find that the IMS thioredoxin system fulfils a previously unknown role in the retrograde release of structurally destabilised proteins into the cytosol and protection against oxidative damage, both of which serve as critical mechanisms of mitochondrial surveillance and quality control. ### Competing Interest Statement The authors have declared no competing interest.