Structural organization of the TOM–CC.A, components of the TOM–CC and TOM complex (additionally containing one or two copies each of the Tom20 and Tom70 receptors) are shown as cartoon representations. B and C, ribbon diagrams of the Saccharomyces (B) (PDB ID: 6UCU, 6UCV) and human (C) (PDB ID: 7CK6) TOM–CC structures deduced with cryo-EM (26, 27, 28, 29). Pore diameters at the longest and shortest edges are shown in Tom40a and Tom40b, respectively. Both dimeric and tetrameric forms of the TOM–CC have been reported. Tetrameric TOM–CC in (C) was generated using coordinates for the dimer. Tom40 dimerization is facilitated by anchoring interactions of the Tom22 helix and sandwiched detergent/lipid molecules. Tom5, Tom6, and Tom7 associate along the three other faces of Tom40. Tetramerization of the TOM–CC in both ScTOM (B) and HsTOM (C) occurs through Tom6 and stabilized by Tom5–Tom22 interaction in ScTOM. Note how similarities in structure and organization are conserved in both ScTOM and HsTOM. HsTOM, Homo sapiens TOM; ScTOM, Saccharomyces cerevisiae TOM; TOM, translocase of the outer mitochondrial membrane; TOM–CC, TOM core complex.