Fig. 6

a, b The expression of AtErv1 in the temperature-sensitive Mia40 strains mia40-3 and mia40-4 allows cell growth at a restrictive temperature (a) and restores the level of Mia40 substrates in the IMS of mitochondria (b). c Δmia40 cells containing MIA40 on an URA3 plasmid were transformed with plasmids to express AtErv1 alone (upper row), AtErv1 in combination with Mia40-SPS (middle row), or wild-type Mia40 as a positive control (lower row). The ability of the strains to grow upon loss of URA3 plasmid was tested on 5-fluoroorotic acid. Co-expression of AtErv1 and Mia40-SPS resulted in viable cells. d, e Many ‘primitive’ eukaryotic phyla (such as euglenozoa, alveolata, and stramenopila) lack Mia40 and protein oxidation should be carried out by Erv1 directly. In plants, a gene for Mia40 is present but not essential, as substrate oxidation can still occur in a Mia40-independent manner. In opisthokonts, both Erv1 and Mia40 are crucial for protein oxidative folding. We propose that the initial disulfide relay only employed an Erv1-like oxidase and was thus similar to the situation still found in some protists