Required for the import and folding of small cysteine-containing proteins (small Tim) in the mitochondrial intermembrane space (IMS). Forms a redox cycle with ERV1 that involves a disulfide relay system. Precursor proteins to be imported into the IMS are translocated in their reduced form into the mitochondria. The oxidized form of MIA40 forms a transient intermolecular disulfide bridge with the reduced precursor protein, resulting in oxidation of the precursor protein that now contains an intramolecular disulfide bond and is able to undergo folding in the IMS. Reduced MIA40 is reoxidized by FAD-linked sulfhydryl oxidase ERV1. Cu(2+) or Zn(2+). Monomer. Interacts with the FAD-linked sulfhydryl oxidase ERV1 and with the substrate proteins COX17, TIM9, and TIM13, forming transient intermolecular disulfide bridges. Interacts with FCJ1. The CHCH domain contains a conserved twin Cys-X(9)-Cys motif which is required for import and stability of MIA40 in mitochondria. Present with 5040 molecules/cell in log phase SD medium.