Apocytochrome c requires the TOM complex for translocation across the mitochondrial outer membrane

The import of proteins into the mitochondrial intermembrane space differs i n various aspects from the classical import pathway into the matrix. Apocyt ochrome c defines one of several pathways known to reach the intermembrane space, yet the components and pathways involved in outer membrane transloca tion are poorly defined. Here, we report the reconstitution of the apocytoc hrome c import reaction using proteoliposomes harbouring purified component s. Import specifically requires the protease-resistant part of the TOM comp lex and is driven by interactions of the apoprotein with internal parts of the complex (involving Tom40) and the 'trans-side receptor' cytochrome c ha em lyase. Despite the necessity of TOM complex function, the translocation pathway of apocytochrome c does not overlap with that of presequence-contai ning preproteins. We conclude that the TOM complex is a universal preprotei n translocase that mediates membrane passage of apocytochrome c and other p reproteins along distinct pathways. Apocytochrome c may provide a paradigm for the import of other small proteins into the intermembrane space such as factors used in apoptosis and protection from stress.