FIGURE 21-36 Activation of CED-3 protease in C. elegans. EGL-1 protein, which is produced in response to developmental signals that trigger cell death, displaces an asymmetric CED-4 dimer from its association with CED-9 (step 1). The free CED-4 dimer combines with three others to form an octamer (step 2), which binds two molecules of the CED-3 zymogen (an enzymatically inactive precursor of a caspase protease) and triggers the conversion of the CED-3 zymogen into active CED-3 protease (step 3). This effector caspase then begins to destroy cell components, leading to cell death (step 4). See N. Yan et al., 2005, Nature 437:831, and S. Qi et al., 2010, Cell 141:446.