The Unique Morgue Ubiquitination Protein Is Conserved in a Diverse but Restricted Set of Invertebrates

Abstract

Drosophila Morgue is a unique ubiquitination protein that facilitates programmed cell death and associates with DIAP1, a critical cell death inhibitor with E3 ubiquitin ligase activity. Morgue possesses a unique combination of functional domains typically associated with distinct types of ubiquitination enzymes. This includes an F box characteristic of the substrate-binding subunit in Skp, Cullin, and F box (SCF)-type ubiquitin E3 ligase complexes and a variant ubiquitin E2 conjugase domain where the active site cysteine is replaced by a glycine. Morgue also contains a single C4-type zinc finger motif. This architecture suggests potentially novel ubiquitination activities for Morgue. In this study, we address the evolutionary origins of this distinctive protein utilizing a combination of bioinformatics and molecular biology approaches. We find that Morgue exhibits widespread but restricted phylogenetic distribution among metazoans. Morgue proteins were identified in a wide range of Protostome phyla, including Arthropoda, Annelida, Mollusca, Nematoda, and Platyhelminthes. However, with one potential exception, Morgue was not detected in Deuterostomes, including Chordates, Hemichordates, or Echinoderms. Morgue was also not found in Ctenophora, Cnidaria, Placozoa, or Porifera. Characterization of Morgue sequences within specific animal lineages suggests that gene deletion or acquisition has occurred during divergence of nematodes and that at least one arachnid expresses an atypical form of Morgue consisting only of the variant E2 conjugase domain. Analysis of the organization of several morgue genes suggests that exon-shuffling events have contributed to the evolution of the Morgue protein. These results suggest that Morgue mediates conserved and distinctive ubiquitination functions in specific cell death pathways.

Drosophila Morgue Associates with SkpA and Polyubiquitin In Vivo

“…Morgue is phylogenetically conserved in a diverse but discrete subset of invertebrates including members of the Lophotrochozoa, Platyzoa, and Ecdysozoa superphyla [30]. In all but one examined species, the Morgue protein exhibits the same domain organization; the 20 amino acid C4 type zinc finger exhibits an essentially invariant CX 2 CX 12 CX 2 C motif; the F box typically contains five conserved Tryptophan residues; and the UEV domain contains a Glycine replacement for the active site Cysteine present in catalytically active E2s.…”

Drosophila morgue influences cell numbers and positions in the embryonic nervous system

“…This includes an NH 2 -terminal CX 2 CX 8 CX 2 C zinc finger motif, a centrally-located F box that includes five stereotypic Tryptophan residues, and a COOHterminal variant E2 conjugase domain where the active site Cysteine is replaced by a Glycine residue. The putative Cys4 type zinc finger is of unknown activity but is well conserved in all Morgue homologs (Zhou et al 2009). F box proteins serve as adaptors in SCF E3 ubiquitin ligase complex that bind to a Skp protein and a specific substrate protein that is targeted for ubiquitination by an E2 ubiquitin conjugase (Ho et al 2006).…”

“…UEVs all lack the active site cysteine although some UEVs, such as MMS2 or UEV1, can associate non-covalently with ubiquitin and may form catalytically active heterodimers with another bona fide E2, such as Ubc13 (Eddins et al 2006;Hau et al 2006;Lewis et al 2006). All Morgue homologs share the active site Gly/Cys substitution (Zhou et al 2009), suggesting a specific functional requirement for Glycine in this position. Interestingly, no other known UEV contains a Glycine in the active site.…”