Structural organization of the APC/C. (A) Schematic representation of the Ub activation, conjugation, and ligation pathway and of the different players in the context of APC/C function. The asterisk on Mnd2 indicates that the indicated position of this protein in the density maps is hypothetical. (B–D) A pseudoatomic model was generated by fitting known high-resolution structures (currently available only for a subset of the APC/C subunits or for closely related sequences) into an ∼10-Å cryo-EM map of the APC/C^{Cdh1–D box} ternary complex from S. cerevisiae (Reprinted by permission from Macmillan Publishers Ltd: Nature, Schreiber et al., 2011). Four different orientations of the complex are shown. The subunits are labeled with both the yeast and vertebrate names. The approximate expected position of Mnd2/Apc15, for which a molecular model is currently missing, is indicated. The figure is reproduced with permission and with minor adaptations from Barford (2011b). The diamond identifies local twofold symmetry axes of Cdc27 and Cdc23. Protein Data Bank (PDB) accession numbers for deposited high-resolution models are as follows: Apc10/Doc1, 1JHJ; APC7 N-terminal domain, 3FFL; Cdc26–Apc6 complex, 3HYM; Apc10/Doc1, 1GQP; Cdc16/Cut9–Cdc26/Hcn1 complex, 2XPI; Apc8/Cdc23, 3ZN3; and Cdc27, 3KAE.