| Drosophila has long been an attractive, genetically tractable model system in which to study fundamental processes such as apoptosis which are common to higher eukaryotes. Following completion of the Drosophila genome sequence, we carried out comprehensive BLAST searches to annotate it with respect to apoptosis, and found sequence homologues of virtually all mammalian cell death genes with the exception of death receptors. The only Drosophila cell death genes for which mammalian homologues have not been identified are the cell death activators Rpr, Hid, and Grim. However, since proteins with similar activities are present in mammals and since their mechanisms are likely to be conserved even if true sequence homologues are not identified, understanding how Rpr, Hid, and Grim act to bring about death is an important area of research. To better understand their mechanisms of action, we carried out an overexpression screen to identify suppressors of Rpr-, Hid-, and Grim-induced death. We identified the strongest of these suppressors as dBruce, a large protein with an N-terminal baculovirus IAP repeat (BIR), characteristic of inhibitors of apoptosis (IAPs), and a C-terminal ubiquitin conjugation domain (E2). We show that it potently suppresses death induced by Rpr and Grim but not by Hid, and that this activity likely requires its E2 domain. It does not directly promote degradation of Rpr or Grim, but its antiapoptotic action requires that their N-termini, through which they interact with BIR2 of DIAP1, be intact. These data, combined with the inability of dBruce to block death induced by the apical caspase Dronc or the proapoptotic Bcl-2 family member Debcl/Drob-1/dBorg-1/Dbok, suggest that dBruce regulates cell death at a novel point. Interestingly, dBruce mutant males are sterile, but a lack of increased caspase activity in these mutants suggests that dBruce may also play nonapoptotic roles. A closer look at Drosophila male testes revealed the surprising observation that high levels of caspases are present in wild type testes, along with the caspase activator Ark. This provokes speculation that core components of the cell death machinery can function to regulate processes other than apoptosis, such as spermatogenesis. |
