Protein Insertion into the Mitochondrial Inner Membrane by a Twin-Pore Translocase

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Abstract
The mitochondrial inner membrane imports numerous proteins that span it multiple times using the membrane potential Δψ as the only external energy source. We purified the protein insertion complex (TIM22 complex), a twin-pore translocase that mediated the insertion of precursor proteins in a three-step process. After the precursor is tethered to the translocase without losing energy from the Δψ, two energy-requiring steps were needed. First, Δψ acted on the precursor protein and promoted its docking in the translocase complex. Then, Δψ and an internal signal peptide together induced rapid gating transitions in one pore and closing of the other pore and drove membrane insertion to completion. Thus, protein insertion was driven by the coordinated action of a twin-pore complex in two voltage-dependent steps.