(C) Model for the effects of
dpl-1 and
mcd-1 on cell killing. Top, in a cell specified to die in wild-type animals, the CED-3 caspase is activated and the MCD-1 Zn finger, DPL-1 DP, LIN-35 Rb, EFL-1 E2F, LIN-37 Mip40, and LIN-52 dLin52 proteins mediate transcriptional regulation of unknown targets to allow cell death to occur. Middle, in the absence of the CED-3 caspase, cells fail to display the morphological alterations characteristic of cell death. Bottom, in the absence of
mcd-1 or
dpl-1, CED-3 is still activated and initiates the cell-death process. The execution of cell death occasionally fails, and cells can survive and differentiate. (D) Multiple activities function independently and additively to promote cell death. The MCD-1 Zn finger, DPL-1 DP, LIN-35 Rb, EFL-1 E2F, LIN-37 Mip40, and LIN-52 dLin52 proteins define a transcriptional regulatory activity that promotes cell death. This activity functions in an additive manner with the cell-killing activity of the CED-9 Bcl-2 and CED-8 XK proteins, as well as with the genes that control the process of engulfment to promote cell death. Multiple other activities could exist and act in a similar additive manner to control cell-death execution.