Organization of the MCC. (A) Schematic organization of the Mad2–BubR1/Mad3–Cdc20 complex (PDB ID: 4AEZ; Chao et al., 2012). The KEN1 motif of BubR1/Mad3 binds on the Cdc20 propeller. Mad2 binds to the MIM motif and makes extensive contacts with BubR1/Mad3. The Cdc20 linker between the MIM and the entry point in the propeller was disordered in the crystal structure (Chao et al., 2012). The KEN2 motif of BubR1/Mad3 was not included in the crystallized construct. (B) Domain organization of BubR1/Mad3. The Cdc20-binding domain in the N-terminal region consists of three TPR repeats. The TPR motif is preceded by a helix–loop–helix domain that embeds the KEN1 motif. The KEN2 motif is C-terminal to the TPR repeats. N, N terminus; C, C terminus. (C) Cartoon model of the MCC (PDB ID: 4AEZ). (D) An enlargement of the area boxed in C and showing the interaction of the KEN1 motif with Cdc20. The KEN motif lies within a folded region. (E) A view of the MCC rotated about a vertical axis ∼180° with respect to C shows that Mad2 does not make direct contacts with the Cdc20 propeller. (F) After an additional 90° rotation about a horizontal axis, the extensive interactions between Mad2 and BubR1/Mad3 become evident. The Mad2 interface engaged in the interaction with BubR1/Mad3 overlaps significantly with the interface required to interact with O-Mad2 and p31^comet (Mapelli et al., 2007; Yang et al., 2007). The box encloses a region of the MCC complex in which extensive contacts between Mad2 and Mad3/BubR1 are formed. (G) A series of EM reconstructions showing APC/C devoid of coactivator or MCC subunits (APC/C^apo), APC/C^Cdc20, and APC/C^MCC (Herzog et al., 2009). The position and orientation of Cdc20 varies significantly in the presence of the MCC.