Mitochondria unfold precursor proteins by unraveling them from their N-termini

Protein unfolding is a key step in the life cycle of many proteins, includi ng certain proteins that are degraded by ATP-dependent proteases or translo cated across membranes.The detailed mechanisms of these unfolding processes are not understood. Precursor proteins are unfolded and imported into mito chondria by a macromolecular machine that spans two membranes and contains at least nine different proteins. Here we examine import of a model precurs or protein derived from the ribonuclease barnase and show that mitochondria unfold this protein by unraveling it from its N-terminus. Because barnase in free-solution unfolds by a different pathway, our results demonstrate th at mitochondria catalyze unfolding in the way that enzymes catalyze reactio ns, namely by changing reaction pathways. The effectiveness of this mechani sm depends on the structure of the N-terminal part of the precursor protein .