APOPTOSIS INDUCED BY DROSOPHILA REAPER AND GRIM IN A HUMAN SYSTEM - ATTENUATION BY INHIBITOR OF APOPTOSIS PROTEINS (CIAPS)

Previous genetic studies have established Reaper and Grim as central r egulators of apoptosis in Drosophila melanogaster. Reaper and Grim ind uce extensive apoptosis in Drosophila, yet share no homology to known vertebrate proteins. In this study, we show for the first time that ec topic expression of Reaper or Grim induced substantial apoptosis in ma mmalian cells. Reaper- or Grim-induced apoptosis was inhibited by a br oad range of caspase inhibitors and by human inhibitor of apoptosis pr oteins cIAP1 and cIAP2. Additionally, in vivo binding studies demonstr ated that both Reaper and Grim physically interacted with human IAPs t hrough a homologous 15-amino acid N-terminal segment. Deletion of this segment from either Reaper or Grim abolished binding to cIAPs. In vit ro binding experiments indicated that Reaper and Grim bound specifical ly to the BIR domain-containing region of cIAPs as deletion of this re gion resulted in loss of binding, The physical interaction was further confirmed by immunolocalization. When co-expressed, Reaper or Grim co -localized with cIAP1. However, deletion of the N-terminal 15 amino ac ids of Reaper or Grim abolished co-localization with cIAP1, suggesting that this homologous region can serve as a protein-protein interactin g domain in regulating cell death, Moreover, by virtue of this interac tion, we demonstrate that cIAPs can regulate Reaper and Grim by abroga ting their ability to activate caspases and thereby inhibit apoptosis. This is the first function attributed to this 15-amino acid N-termina l domain that is the only region having significant homology between t hese Drosophila death inducers.