Osm1 facilitates the transfer of electrons from Erv1 to fumarate in the redox-regulated import pathway in the mitochondrial intermembrane space.

Prokaryotes have aerobic and anaerobic electron acceptors for oxidative folding of periplasmic proteins. The mitochondrial intermembrane space has an analogous pathway with the oxidoreductase Mia40 and sulfhydryl oxidase Erv1, termed the mitochondrial intermembrane space assembly (MIA) pathway. The aerobic electron acceptors include oxygen and cytochrome c , but an acceptor that can function under anaerobic conditions has not been identified. Here we show that the fumarate reductase Osm1, which facilitates electron transfer from fumarate to succinate, fills this gap as a new electron acceptor. In addition to microsomes, Osm1 localizes to the mitochondrial intermembrane space and assembles with Erv1 in a complex. In reconstitution studies with reduced Tim13, Mia40, and Erv1, the addition of Osm1 and fumarate completes the disulfide exchange pathway that results in Tim13 oxidation. From in vitro import assays, mitochondria lacking Osm1 display decreased import of MIA substrates, Cmc1 and Tim10. Comparative reconstitution assays support that the Osm1/fumarate couple accepts electrons with similar efficiency to cytochrome c and that the cell has strategies to coordinate expression of the terminal electron acceptors. Thus Osm1/fumarate is a new electron acceptor couple in the mitochondrial intermembrane space that seems to function in both aerobic and anaerobic conditions.