Targeting and Translocation of Preproteins by the Toms of the Mitochondrial Receptor Complex

Publisher Summary This chapter discusses the targeting and translocation of preproteins by the translocase of the mitochondrial outer membrane (Toms) of the mitochondrial receptor complex. The transport of preproteins across the outer membrane involves two sequential steps: (i) recognition by specific receptors, and (ii) translocation through a general insertion pore. The Tom proteins that perform these functions are assembled in a high molecular weight complex. The steps occurring at the outer membrane are independent of a membrane potential, while the translocation of preproteins across the inner membrane strictly requires a membrane potential. It is suggested that none of the subunits of the mitochondrial receptor complex is sufficient for its own targeting. Import of a subunit requires the function of at least one other correctly assembled subunit, in most cases several other subunits may be needed. Therefore, targeting and assembly of the components of the receptor complex can only occur to preexisting complexes, ensuring a formation of receptor complexes only in the mitochondrial outer membrane. It is proposed that the dynamic nature of the receptor complex provides the molecular basis for a stepwise assembly of newly synthesized components into preformed complexes.