The Mitochondrial Oxidase Assembly Protein1 (Oxa1) Insertase Forms a Membrane Pore in Lipid Bilayers

Abstract

The inner membrane of mitochondria is especially protein-rich. To direct proteins into the inner membrane, translocases mediate transport and membrane insertion of precursor proteins. Although the majority of mitochondrial proteins are imported from the cytoplasm, core subunits of respiratory chain complexes are inserted into the inner membrane from the matrix. Oxa1, a conserved membrane protein, mediates the insertion of mitochondrion-encoded precursors into the inner mitochondrial membrane. The molecular mechanism by which Oxa1 mediates insertion of membrane spans, entailing the translocation of hydrophilic domains across the inner membrane, is still unknown. We investigated if Oxa1 could act as a protein-conducting channel for precursor transport. Using a biophysical approach, we show that Oxa1 can form a pore capable of accommodating a translocating protein segment. After purification and reconstitution, Oxa1 acts as a cation-selective channel that specifically responds to mitochondrial export signals. The aqueous pore formed by Oxa1 displays highly dynamic characteristics with a restriction zone diameter between 0.6 and 2 nm, which would suffice for polypeptide translocation across the membrane. Single channel analyses revealed four discrete channels per active unit, suggesting that the Oxa1 complex forms several cooperative hydrophilic pores in the inner membrane. Hence, Oxa1 behaves as a pore-forming translocase that is regulated in a membrane potential and substrate-dependent manner.