Mitochondrial protein import machinery and targeting information

During evolution, eukaryotic cells have acquired subcellular compartments, such as nuclei, peroxisomes, mitochondria, and, in the case of plant cells, chloroplasts, this compartmentalization allowing the cell to function more efficiently. Although, mitochondria and chloroplasts possess their own genomes, these have a low coding content and nuclear-encoded proteins have to be imported. Since the organization, content and functioning of organelles are strictly defined, an efficient and rigorous system for importing proteins synthesized in the cytosol is required. Most mitochondrial precursors synthesized in the cytosol are recognized and/or maintained in an unfolded conformation by cytosolic chaperone proteins and are then recognized by translocases of the mitochondrial outer and inner membranes (TOM and TIM, respectively), which transport them across the two membranes. The mechanism of mitochondrial protein import is thought to be well conserved across species. However, while many studies have investigated mitochondrial import in fungi, few have been carried out in plants, although it is clear that differences exist between fungi, mammals, and plants as regards the mitochondrial import system. In plants, for instance, the presence of both chloroplasts and mitochondria, with similar import mechanisms, might have rendered the import system more stringent. We will review the literature concerning the mitochondrial import machinery and the structure of mitochondrial presequences, paying particular attention to those features specific to plants. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved.