drICE is an essential caspase required for apoptotic activity in Drosophila cells

Caspases are involved in the execution of cell death in all multicellular organisms so far studied, including the nematode worm, fruit fly and vertebrates. While Caenorhabditis elegans has only a single identified caspase, CED-3, whose activity is absolutely required for all developmental programmed cell deaths, most mammalian cell types express multiple caspases with varying specificities. The fruit fly Drosophila melanogaster is genetically tractable, less complex than vertebrates and possesses two known caspases, DCP-1 and drICE. The fly may therefore provide a good model system for examining the hierarchy and relative roles of individual caspases in the execution of apoptosis. We have examined the role of drICE in in vitro apoptosis of the D.melanogaster cell line S2. We show that cytoplasmic lysates made from S2 cells undergoing apoptosis induced by either reaper (rpr) expression or cycloheximide treatment contain a caspase activity with DEVD specificity which can cleave p35, lamin DmO, drICE and DCP-1 in vitro, and which can trigger chromatin condensation in isolated nuclei. Using antibodies specific to drICE, we show that immunodepletion of drICE from these lysates is sufficient to remove most measurable in vitro apoptotic activity, and that re-addition of exogenous drICE to such immunodepleted lysates restores apoptotic activity. We conclude that, at least in S2 cells, drICE can be the sole caspase effector of apoptosis.