Studies of mitochondrial protein import factors in mammals

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The whole process of synthesis of mitochondrial proteins, their translocation, processing and folding involves factors present in the cytosol, mitochondrial membranes and matrix compartments. Cytosolic and outer membrane factors involved in mitochondrial preprotein import in mammals were studied. To investigate the roles of molecular chaperones, we developed a system of chaperone depletion from and readdition to rabbit reticulocyte lysates. Using this system, we found that hsc70 and dj2 (HSDJ/hdj-2), not dj1 (hsp40/hdj-1), are involved in mitochondrial preprotein import. Recently identified dj3 (cpr3/DNJ3/HIRIP4/rdj2) was as active as dj2 in preprotein import. Human Tom20 and Tom22 were isolated and their roles were studied in vitro and in cultured cells. We revealed that the cytosolic domains of both Tom20 and Tom22, but not the intermembrane space domain of Tom22, are important for preprotein binding. Coimmunoprecipitation experiments showed that Tom20 and Tom22 form a complex via the cytosolic domains. Participation of Tom34 which is present largely in the cytosol and partly in the mitochondrial outer membrane and has no apparent counterpart in fungi, in preprotein import was found. These results indicate that the mechanism of mitochondrial protein import is highly conserved from lower eukaryotes to higher animals but some differences exist.