Fig. 2. Spindle assembly checkpoint signaling. In the presence of unattached or improperly attached kinetochores, the spindle assembly checkpoint is switched on (upper panel). Unattached kinenetochores act as catalytic sites for the activation of Mad2. Activated Mad2 (Mad2^*) then diffuses and prevents anaphase onset by inhibiting the activity of Cdc20-APC. In addition, BubR1 functions synergistically with Mad2 in inhibiting Cdc20-APC activity. After all the chromosomes are properly attached by kinetochore microtubules and aligned at the metaphase plate, the spindle assembly checkpoint is turned off (bottom panel). Mad2^* is no longer generated, and BubR1 does not interact with Cdc20-APC, resulting in the activation of Cdc20-APC. Activated Cdc20-APC catalyzes the ubiquitination of securin, leading to its degradation through proteosome-mediated proteolysis. Degradation of securin in turn causes the release of separin. The free separin is then able to cleave the SCC1 subunit of the sister-chromatid cohesion complex, triggering the separation of sister chromatids and the onset of anaphase.