The TOM Complex in the Outer Membrane of Mitochondria: A Supramolecular Assembly for Protein Import.

Currently, there is no comprehensive physical model explaining how unfolded polypeptide chains with diverse characteristics are transported into the mitochondria. On a molecular scale, the kinetics of how transit polypeptides approach, are captured by the protein translocation machinery at the outer mitochondrial membrane, and cross the protein translocation pore to enter the intermembrane space remain unclear. This knowledge gap is primarily due to the lack of dynamic single-molecule data on the "protein-conducting channels" involved in mitochondrial protein translocation. In this chapter, we explore the recently resolved sub-nanometer cryo-EM structures, which are a prerequisite for a fundamental understanding of the translocation mechanism, and our existing knowledge of the mitochondrial two-pore outer membrane protein translocation machinery (TOM complex). Particularly intriguing are recent findings from single-molecule TIRF microscopy indicating that the TOM core complex can function as a mechanosensor, with the pores closing upon interaction with nearby membrane structures. We emphasize novel and unexpected correlations between the structural components of the TOM complexes and their dynamic behavior within the membrane environment.