Caspase-dependent regulation of the ubiquitin-proteasome system through direct substrate targeting.
Drosophila inhibitor of apoptosis (IAP) 1 (DIAP1) is an E3 ubiquitin ligase that regulates apoptosis in flies, in large part through direct inhibition and/or ubiquitinylation of caspases. IAP antagonists, such as Reaper, Hid, and Grim, are thought to induce cell death by displacing active caspases from baculovirus IAP repeat domains in DIAP1, but can themselves become targets of DIAP1-mediated ubiquitinylation. Herein, we demonstrate that Grim self-associates in cells and is ubiquitinylated by DIAP1 at Lys136 in an UbcD1-dependent manner, resulting in its rapid turnover. K48-linked ubiquitin chains are added almost exclusively to BIR2-bound Grim as a result of its structural proximity to DIAP1's RING domain. However, active caspases can simultaneously cleave Grim at Asp132, removing the lysine necessary for ubiquitinylation as well as any existing ubiquitin conjugates. Cleavage therefore enhances the stability of Grim and initiates a feed-forward caspase amplification loop, resulting in greater cell death. In summary, Grim is a caspase substrate whose cleavage promotes apoptosis by limiting, in a target-specific fashion, its ubiquitinylation and turnover by the proteasome.
Institute for Cellular and Molecular Biology, The University of Texas at Austin, Austin, TX 78712, USA.
Ting-Chun
TC
Yeh
Shawn B
SB
Bratton
D000818:Animals; D017209:Apoptosis; D020169:Caspases / Q000378:metabolism*; D029721:Drosophila Proteins / Q000378:metabolism; D004331:Drosophila melanogaster; D004789:Enzyme Activation; D051034:Inhibitor of Apoptosis Proteins / Q000378:metabolism; D009479:Neuropeptides / Q000378:metabolism; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D059748:Proteolysis; D013379:Substrate Specificity; D025801:Ubiquitin / Q000378:metabolism*; D054875:Ubiquitination
C111891:DIAP1 protein, Drosophila; D029721:Drosophila Proteins; D051034:Inhibitor of Apoptosis Proteins; D009479:Neuropeptides; D025801:Ubiquitin; C108881:grim protein, Drosophila; D020169:Caspases; D046988:Proteasome Endopeptidase Complex