Reaper-induced dissociation of a Scythe-sequestered cytochrome c-releasing activity

Reaper is a potent apoptotic inducer critical for programmed cell death in the fly Drosophila melanogaster: While Reaper homologs from other species have not yet been reported, ectopic expression of Reaper in cells of vertebrate origin can also trigger apoptosis, suggesting that Reaper-responsive pathways are likely to be conserved. We recently reported that Reaper-induced mitochondrial cytochrome c release and caspase activation in a cell-free extract of Xenopus eggs requires the presence of a 150 kDa Reaper-binding protein, Scythe, We now show that Reaper binding to Scythe causes Scythe to release a sequestered apoptotic inducer. Upon release, the Scythe-sequestered factor(s) is sufficient to induce cytochrome c release from purified mitochondria, Moreover, addition of excess Scythe to egg extracts impedes Reaper-induced apoptosis, most likely through rebinding of the released factors. In addition to Reaper, Scythe binds two other Drosophila. apoptotic regulators: Grim and Hid, Surprisingly, however, the region of Reaper which is detectably homologous to Grim and Hid is dispensable for Scythe binding.