echinus, required for interommatidial cell sorting and cell death in the Drosophila pupal retina, encodes a protein with homology to ubiquitin-specific proteases

BACKGROUND: Programmed cell death is used to remove excess cells between ommatidia in the Drosophila pupal retina. This death is required to establish the crystalline, hexagonal packing of ommatidia that characterizes the adult fly eye. In previously described echinus mutants, interommatidial cell sorting, which precedes cell death, occurred relatively normally. Interommatidial cell death was partially suppressed, resulting in adult eyes that contained excess pigment cells, and in which ommatidia were mildly disordered. These results have suggested that echinus functions in the pupal retina primarily to promote interommatidial cell death. RESULTS: We generated a number of new echinus alleles, some likely null mutants. Analysis of these alleles provides evidence that echinus has roles in cell sorting as well as cell death. echinus encodes a protein with homology to ubiquitin-specific proteases. These proteins cleave ubiquitin-conjugated proteins at the ubiquitin C-terminus. The echinus locus encodes multiple splice forms, including two proteins that lack residues thought to be critical for deubiquitination activity. Surprisingly, ubiquitous expression in the eye of versions of Echinus that lack residues critical for ubiquitin specific protease activity, as well as a version predicted to be functional, rescue the echinus loss-of-function phenotype. Finally, genetic interactions were not detected between echinus loss and gain-of-function and a number of known apoptotic regulators. These include Notch, EGFR, the caspases Dronc, Drice, Dcp-1, Dream, the caspase activators, Rpr, Hid, and Grim, the caspase inhibitor DIAP1, and Lozenge or Klumpfuss. CONCLUSION: The echinus locus encodes multiple splice forms of a protein with homology to ubiquitin-specific proteases, but protease activity is unlikely to be required for echinus function, at least when echinus is overexpressed. Characterization of likely echinus null alleles and genetic interactions suggests that echinus acts at a novel point(s) to regulate interommatidial cell sorting and/or cell death in the fly eye.