Mitochondria play a central role in cellular energy metabolism and are essential organelles in eukaryotes. In humans, 13 proteins are encoded by the mitochondrial genome while the vast majority of mitochondrial proteins are encoded by the nuclear genome. As a result, most mitochondrial proteins are synthesized as precursors in the cytoplasm and imported across mitochondrial membranes by one or more translocase protein complexes . The translocase of the outer mitochondrial membrane (TOM complex) facilitates the import of proteins through the outer mitochondrial membrane, while the complementary translocase of the inner membrane (TIM complex) is responsible for protein transport to the mitochondrial matrix. The TOM complex consists of the receptors TOM20, TOM22, TOM70, and the channel-forming protein TOM40 . TOM20 is localized in the outer mitochondrial membrane and initially recognizes precursors with a presequence to facilitate protein import across the outer mitochondrial membrane . In a sequential process, recognition of the presequence by TOM20 is followed by tethering of the presequence to the TOM40 protein complex for efficient protein import .
