A role for Tim21 in membrane-potential-dependent preprotein sorting in mitochondria.

The mitochondrial inner membrane harbors complexes of the respiratory chain and translocase complexes for preproteins. The membrane potential generated by the respiratory chain is essential for ATP production by the mitochondrial ATP synthase and as a driving force for protein import. It is generally believed that the preprotein translocases just use the membrane potential without getting into physical contact with respiratory-chain complexes. Here, we show that the presequence translocase interacts with the respiratory chain. Tim21, a specific subunit of the sorting-active presequence translocase , recruits proton-pumping respiratory-chain complexes and stimulates preprotein insertion. Thus, the presequence translocase cooperates with the respiratory chain and promotes membrane-potential-dependent protein sorting into the inner mitochondrial membrane. These findings suggest a new coupling mechanism in an energy-transducing membrane.