The AAA+ family in eukaryotes has many members in various cellular compartments with a role in protein unfolding and degradation. We show that the mitochondrial AAA-ATPase Bcs1 has an unusual function in protein translocation. Bcs1 mediates topogenesis of the Rieske protein, Rip1, a component of respiratory chains in bacteria, mitochondria, and chloroplasts. The oligomeric AAA-ATPase Bcs1 is involved in export of the folded Fe-S domain of Rip1 across the inner membrane and insertion of its transmembrane segment into an assembly intermediate of the cytochrome bc(1) complex, thus revealing an unexpected mechanistical concept of protein translocation across membranes. Furthermore, we describe structural elements of Rip1 required for recognition and export by as well as ATP-dependent lateral release from the AAA-ATPase. In bacteria and chloroplasts Rip1 uses the Tat machinery for topogenesis; however, mitochondria have lost this machinery during evolution and a member of the AAA-ATPase family has taken over its function.
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