Protein translocation into mitochondria

The mitochondrial membranes contain translocation machineries ("preprotein translocases") which facilitate transport of proteins from the cytosol into the mitochondria. The TOM complex is localized in the outer membrane. Two translocases have been characterized in the inner membrane: the Tim23-Tim17 complex and the Tim22 complex which differ in their specificities for preproteins. The TOM and TIM complexes can act in a coordinated fashion to facilitate the insertion of preproteins into the mitochondrial membranes as well as the translocation into the internal compartments, the intermembrane space and the matrix. We discuss insights into the function of components of the translocases, illustrated by examples for mechanisms of protein sorting into the different compartments of mitochondria.