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

Abstract

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 N spl (reduced eye). Ectopic M8* impairs eye development (in N spl ) 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 N spl , 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.

Functional dissection of Timekeeper (Tik) implicates opposite roles for CK2 and PP2A during Drosophila neurogenesis

“…3e). In the case of 109-68Gal4 , however, expression at 29 o C was required consistent with observations (by others and us) that expressivity of this driver is temperature-dependent (Kahali et al ., 2009; White and Jarman, 2000). Importantly, these bristle defects were not intrinsic to the UAS-mts insertion (Fig.…”

Evidence that the C‐terminal domain (CtD) autoinhibits neural repression by Drosophila E(spl)M8

“…We have recently found that loss of the MC's elicited by ectopically expressed wild type M8 is strongly rescued by coexpression of a UAS-CK2a-RNAi construct or UAS-Tik, a CK2-DN (Bose et al, 2006;Kahali et al, 2009). These results suggested that phosphorylation also regulates antagonism of ASC by M8 during SOP selection (Fig.…”

“…Moreover, these effects are unlikely to involve competition between two UAS-constructs for (rate) limiting amounts of Gal4 produced either by the G455.2 or 109-68Gal4 enhancer traps, because no modulation of the neural defects of ectopic M8SD or M8 is seen upon coexpression of a UAS-LacZ construct [data not shown; The severely reduced eye of N spl /Y; E(spl)D/1 (Fig. 5a) has provided important mechanistic insights into repression by M8 (Giebel and Campos-Ortega, 1997;Kahali et al, 2009;Karandikar et al, 2004;Tietze et al, 1992). Given the ability of M8-CtD to negate repression by ectopic M8SD or M8, we next tested whether this peptide would display similar activity in an E(spl)D mutant background.…”

New insights into the Orange domain of E(spl)-M8, and the roles of the C-terminal domain in autoinhibition and Groucho recruitment

“…Since the identification of this allele in 1956 [45], follow-up studies from a number of labs have sought to better understand why this mutant protein perturbs eye development [25, 33, 46, 47]. …”

“…Each R8 cell then recruits surrounding uncommitted cells (reviewed in [4, 53–55]), driving formation of the highly ordered adult eye in which each hexagonal facet (unit eye) contains 8 photoreceptors, pigment and cone cells. It has also been shown that E(spl)D (M8*) exhibits excessive binding to Ato, resulting in a loss of the R8 cells [25, 47]. As a result, no other retinal cell types are specified, leading to loss of virtually the entire eye field (Fig.…”