Fig. 1.

Schematic of Spindle Assembly Checkpoint (SAC) – dependent control of mitosis. (A) During mitosis, unattached kinetochores (orange) catalyze formation of an inhibitor that targets the APC/C^Cdc20, a ubiquitin ligase whose activity is essential for sister chromatid separation and mitotic exit. The SAC thereby ensures that the metaphase-anaphase transition occurs only after all kinetochores are attached to spindle microtubules. (B) The SAC functions by catalyzing the formation of the Mitotic Checkpoint Complex (MCC), which then binds APC/C^Cdc20 to block its activity. Shown here are the crystal structure of the MCC from S. pombe (PDB 4aez, left) [12] and the cryo-EM structure of human APC/C^Cdc20 bound to the MCC (PDB 5lcw, right) [15]. Note that Bub3, a component of the MCC in most species, is absent in these structures. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)