Structure of the Bcs1 AAA-ATPase suggests an airlock-like translocation mechanism for folded proteins

Some proteins require completion of folding before translocation across a membrane into another cellular compartment. Yet the permeability barrier of the membrane should not be compromised and mechanisms have remained mostly elusive. Here, we present the structure of Saccharomyces cerevisiae Bcs1, an AAA-ATPase of the inner mitochondrial membrane. Bcs1 facilitates the translocation of the Rieske protein, Rip1, which requires folding and incorporation of a 2Fe-2S cluster before translocation and subsequent integration into the bc1 complex. Surprisingly, Bcs1 assembles into exclusively heptameric homo-oligomers, with each protomer consisting of an amphipathic transmembrane helix, a middle domain and an ATPase domain. Together they form two aqueous vestibules, the first being accessible from the mitochondrial matrix and the second positioned in the inner membrane, with both separated by the seal-forming middle domain. On the basis of this unique architecture, we propose an airlock-like translocation mechanism for folded Rip1.