edand fred: inhibitors of EGF receptor signaling. S.A. Spencer , R.L. Cagan. Dept. Molecular Biology & Pharmacology, Washington University School of Medicine, St Louis, MO.

   Precise regulation of EGF-receptor (EGFR) activity is important in many developmental patterning processes, including the establishment of embryonic polarity, dorsal appendage formation, and wing vein differentiation. It is also an important factor in specification of the first cell fate in the developing retina, the R8 photoreceptor. R8 cells arise from the morphogenetic furrow in perfectly spaced rows, each R8 separated from the next by 10-12 undifferentiated cells, and each row of R8s exactly out of register with the row preceding it. The emergence of this precise lattice of R8 cells is dependent upon temporally and spatially restricted EGFR activity within the morphogenetic furrow. In a screen for mutations that affect R8 patterning, we identified the gene echinoid(ed), a large transmembrane protein with homology to cell adhesion molecules. Mutations in ed lead to mispatterning of R8 cells: in some ommatidia several R8's form, while in other areas R8 fails to differentiate at all. The basis of this phenotype appears to be an elevation of EGFR signaling: discs lacking edshow increased MAP kinase phosphorylation, and expression of echinoidin cultured S2 cells suppresses EGFR phosphorylation. edis ubiquitously expressed in eye imaginal discs, suggesting that, unlike other EGFR inhibitors such as argosand kekkon, its transcription is not regulated by EGFR activity. We find, however, that Echinoid becomes tyrosine-phosphorylated in response to EGFR activation, indicating that Echinoid's activity may be regulated post-translationally.
    Just upstream of ed is a highly homologous gene which we have termed Friend of Echinoid(fred). We have used RACE to determine the full-length sequence of this gene, and are examining its ability to block EGFR signaling in a manner similar to ed. Biochemical and genetic analyses of edand fred in tissue culture and in vivo will be presented.