CED-4 activates DCP-1 and drICE in Drosophila S2 cells. (A) CED-4 enhanced the caspase activities of DCP-1 and drICE in S2 cells. S2 cells were transfected with ced-4 and either drICE or dcp-1, and the caspase activities (mean ± SD) were measured 24 h after heat shock. (B) CED-4 promoted cell death induced by Drosophila caspases (mean ± SD). S2 cells were transfected with the same combinations shown in (A), and the ratio of chromatin condensed cells (% of total cells: mean ± SD) were counted 24 h after heat-shock. (C) Activation of caspases as revealed by affinity labeling (see Materials and Methods). ced-4 was cotransfected with either empty vector, drICE, or dcp-1 in S2 cells, and cell lysates for the affinity labeling were prepared 24 h after heat-shock. (D) Expression of Drosophila caspases in S2 cells and Drosophila embryos (0–24 h). RT-PCR was performed by using specific primers for dcp-1, drICE, and dcp-2 (see Materials and Methods).