On the mechanism underlying the divergent retinal and bristle defects of M8 (E(spl)D) in Drosophila

Our results, using endogenous mutants and Gal4‐UAS driven transgenes, implicate multisite phosphorylation in repression by E(spl)M8. We propose that these phosphorylations occur in the morphogenetic furrow (MF) to reverse an auto‐inhibited state of M8, enabling repression of Atonal during R8 specification. Our studies address the paradoxical behavior of M8*, the truncated protein encoded by E(spl)D. We suggest that differences in N signaling in the bristle versus the eye underlie the antimorphic activity of M8* in N+ (ectopic bristles) and hypermorphic activity in Nspl (reduced eye). Ectopic M8* impairs eye development (in Nspl) only during establishment of the atonal feedback loop (anterior to the MF), but is ineffective after this time point. In contrast, a CK2 phosphomimetic M8 lacking Groucho (Gro) binding, M8SDΔGro, acts antimorphic in N+ and suppresses the eye/R8 and bristle defects of Nspl, as does reduced dosage of E(spl) or CK2. Multisite phosphorylation could serve as a checkpoint to enable a precise onset of repression, and this is bypassed in M8*. Additional implications are discussed. genesis 47:456–468, 2009. © 2009 Wiley‐Liss, Inc.