Drosophila Morgue is an F box/ubiquitin conjugase domain protein important for grim-reaper mediated apoptosis

Abstract

In Drosophila melanogaster, apoptosis is controlled by the integrated actions of the Grim-Reaper (Grim-Rpr) and Drosophila Inhibitor of Apoptosis (DIAP) proteins (reviewed in refs 1 4). The anti-apoptotic DIAPs bind to caspases and inhibit their proteolytic activities. DIAPs also bind to Grim-Rpr proteins, an interaction that promotes caspase activity and the initiation of apoptosis. Using a genetic modifier screen, we identified four enhancers of grim-reaper-induced apoptosis that all regulate ubiquitination processes: uba-1, skpA, fat facets (faf), and morgue. Strikingly, morgue encodes a unique protein that contains both an F box and a ubiquitin E2 conjugase domain that lacks the active site Cys required for ubiquitin linkage. A reduction of morgue activity suppressed grim-reaper-induced cell death in Drosophila. In cultured cells, Morgue induced apoptosis that was suppressed by DIAP1. Targeted morgue expression downregulated DIAP1 levels in Drosophila tissue, and Morgue and Rpr together downregulated DIAP1 levels in cultured cells. Consistent with potential substrate binding functions in an SCF ubiquitin E3 ligase complex, Morgue exhibited F box-dependent association with SkpA and F box-independent association with DIAP1. Morgue may thus have a key function in apoptosis by targeting DIAP1 for ubiquitination and turnover.

Regulation of the Drosophila ubiquitin ligase DIAP1 is mediated via several distinct ubiquitin system pathways

“…It should be noted that a role has been suggested for Morgue in promoting the degradation of DIAP1, although it has not been shown that it serves as its ligase. 42,43 Also, a recent study demonstrates that phosphorylation of DIAP1 by the Drosophila IKK-related kinase leads to its destruction under nonapoptotic conditions. 44 Thus, it appears that multiple pathways and mechanisms regulate the IAP group of proteins that are positioned in a critical cross roads along the developmental and apoptotic pathways.…”

Genetic characterization of two gain-of-function alleles of the effector caspase DrICE in Drosophila

“…[16][17][18][19][20] In GMR-reaper eye imaginal discs, DIAP1 degradation is very prominent in cells immediately posterior to the column of R8 photoreceptor neurons, which are very resistant to apoptosis 52 and where DIAP1 is not degraded (Figure 7a and b; R8 photoreceptor columns are marked by arrows; the zones of DIAP1 degradation by asterisks (*)). Interestingly, reduced DrICE activity in heterozygous loss-of-function alleles partially protects DIAP1 from reaper-induced degradation (Figure 7c and d), which is consistent with the suppression of GMR-reaper by these alleles (Figure 6a).…”

“…47 If reaper is in excess, it triggers degradation of DIAP1 (Figure 7b) and thus apoptosis. [16][17][18][19][20] Therefore, owing to the failure of DIAP1 to bind to the class C subunit, the wt/ class C heterotetramer may shift the equilibrium towards binding of DIAP1 to reaper such that DIAP1 is more efficiently degraded by reaper, resulting in higher enzymatic activity of the wt/class C heterotetramer. Consistent with this notion, reduced DrICE activity in heterozygous loss-of-function alleles partially protects DIAP1 from reaper-induced degradation (Figure 7c and d), whereas the class C alleles of DrICE fail to protect DIAP1 from degradation (Figure 7e and f), suggestive of decreased binding of the wt/class C heterotetramer to DIAP1.…”

“…10,11 In cells committed to apoptosis, IAP-mediated inhibition of caspases is counteracted by IAP antagonists. Specifically, the IAP antagonists encoded by reaper, hid and grim (RHG) [12][13][14][15] in Drosophila trigger proteolytic degradation of DIAP1 [16][17][18][19][20] which releases caspases from DIAP1 inhibition and triggers apoptosis. The overexpression of the RHG genes in the fly eye using the eye-specific GMR promoter causes an eye ablation phenotype due to massive apoptosis (see, for example, GMR-hid in Figure 1a).…”

Fhos encodes a Drosophila Formin‐Like Protein participating in autophagic programmed cell death

“…While the first pulse triggers midgut cell death and differentiation of the cen-tral nervous system, the second triggers salivary gland autophagic cell death through the activation of a set of early ecdysone responsive genes namely Broad Complex (BR-C), E74A and E75 (Burtis et al, 1990;DiBello et al, 1991;Jiang et al, 1997;Lee and Baehrecke, 2001;Seagraves and Hogness, 1990;Thummel, 2001). Early genes, in turn, induce the transcriptional activation of late genes, including Drosophila cell death genes reaper (rpr), head involution defective (hid), and grim (Cristich et al, 2002;Goya et al, 2000;Greether et al, 1995;Srinivasula et al, 2002;White et al, 1994;Wing et al, 2002). Also, in response to the second hormonal pulse, the transcription of the stage-specific gene E93 is induced.…”