Visualization of Redox-Controlled Protein Fold in Living Cells

•Protein import in the IMS is controlled by the cytosolic thiol-regulation systems•In-cell NMR spectroscopy allows monitoring of protein folding in living human cells•Mia40 overexpressed in the cytoplasm is folded and cannot be imported in the IMS•Glutaredoxin 1 keeps Mia40 unfolded, while thioredoxin 1 is less effective

SummaryMost mitochondrial proteins are encoded by nuclear DNA, synthesized in the cytoplasm, and imported into mitochondria. Several proteins of the intermembrane space (IMS) are imported and localized through an oxidative process, being folded through the formation of structural disulfide bonds catalyzed by Mia40, and trapped in the IMS. To be imported, these proteins need to be reduced and unfolded; however, no structural information in situ exists on these proteins in the cytoplasm. In humans, Mia40 undergoes the same mechanism, although its folding state in the cytoplasm is unknown. We provide atomic-level details on the Mia40 folding state in the human cell cytoplasm through in-cell nuclear magnetic resonance. Overexpressed cytoplasmic Mia40 is folded, and its folding state depends on the glutaredoxin 1 (Grx1) and thioredoxin 1 (Trx1) systems. Specifically, increased Grx1 levels keep most Mia40 unfolded, while Trx1 is less effective.

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