Altered cytochrome c display precedes apoptotic cell death in Drosophila

Drosophila affords a genetically well-defined system to study apoptosis in vivo. It offers a powerful extension to in vitro models that have implicate d a requirement for cytochrome c in caspase activation and apoptosis. We fo und that an overt alteration in cytochrome c anticipates programmed cell de ath (PCD) in Drosophila tissues, occurring at a time that considerably prec edes other known indicators of apoptosis. The altered configuration is mani fested by display of an otherwise hidden epitope and occurs without release of the protein into the cytosol. Conditional expression of the Drosophila death activators, reaper or grim, provoked apoptogenic cytochrome c display and, surprisingly, caspase activity was necessary and sufficient to induce this alteration. In cell-free studies, cytosolic caspase activation was tr iggered by mitochondria from apoptotic cells but identical preparations fro m healthy cells were inactive. Our observations provide compelling validati on of an early role for altered cytochrome c in PCD and suggest propagation of apoptotic physiology through reciprocal, feed-forward amplification inv olving cytochrome c and caspases.