Drosophila morgue and the intersection between protein ubiquitination and programmed cell death.

In Drosophila, cell survival decisions are mediated by the integrated functions of the Grim-Reaper death activators and Inhibitor-of-Apoptosis-Proteins (IAPs), such as DIAP1, to regulate caspase activities. We recently identified a gene that enhances the actions of the Grim-Reaper proteins and negatively regulates the levels of DIAP1 protein. This gene, morgue, encodes a novel protein that contains both an F box and a ubiquitin conjugase domain. Interestingly, the Morgue conjugase domain lacks the active site cysteine required for covalent linkage to ubiquitin. Morgue could target IAPs and other proteins for ubiquitination and proteasome-dependent turnover by acting either in an SCF ubiquitin E3 ligase complex, or as a ubiquitin E2 conjugase enzyme variant (UEV) in conjunction with a catalytically active E2 conjugase. Morgue is evolutionarily conserved, as a Morgue ortholog was identified from the mosquito, Anopheles gambiae. Elucidation of morgue function should provide novel insights into the mechanisms of ubiquitination and programmed cell death.