| Programmed cell death or apoptosis is central to numerous biological processes. Genetic studies in the nematode C. elegans have led to the identification of three central genes, CED-3, CED-4 and CED-9, that control cell death. While CED-3 and CED-4 induce cell death, CED-9 blocks cell death. I have studied the mechanism by which these genes influence the life or death decision of the cell using an insect cell line, IPLB-SF-21. I found that CED-4 promotes the proteolytic activation of CED-3 and the induction of apoptosis. CED-4-mediated activation of CED-3 processing depended on the presence of a wild-type CED-3 prodomain and a conserved lysine within a putative ATP/GTP binding site motif of CED-4. I also found that CED-9 physically associates with CED-4 and blocks the ability of CED-4 to promote CED-3 processing and the ensuing acceleration and increase in levels of apoptosis.; I have also examined the mechanism by which inhibitors of apoptosis (IAPs) block baculovirus-induced apoptosis. Pro-forms of insect Sf-caspase-1 and the closely related mammalian caspase-3 were unable to induce apoptosis in Sf-21 insect cells, although the pro-form of mammalian caspase-1 was effective in inducing apoptosis in these cells. However, expression of the pro-domain deleted form or coexpression of the two subunits of mature Sf-caspase-1 or caspase-3 induced apoptosis in Sf-21 cells. Apoptosis induced by all the active forms of the three caspases was inhibited by the baculovirus caspase inhibitor P35. In contrast, baculovirus members of the inhibitor of apoptosis (IAP) family failed to block active caspase-induced apoptosis. However, OpIAP or CpIAP effectively blocked the activation of pro-Sf-caspase-1 and the associated induction of apoptosis during virus infection. Thus, the baculovirus IAPs inhibit apoptosis by a different mechanism than P35 and act by blocking the activation of pro-caspases like Sf-caspase-1. |
