Fig. 6. Echinoid is phosphorylated in response to Egfr. (A) Immunoprecipitated EdFLAG from S2 cells transfected with EdFLAG and/or Egfr were analyzed by western blot using an antibody specific for phosphotyrosine. Echinoid was phosphorylated in response to Egfr transfection. This phosphorylation was further increased by addition of an active form of the Egfr ligand Spitz (sSpi) to the tissue culture medium. The clipped C-terminal form of Echinoid also becomes phosphorylated (arrow). (B) Echinoid lacking the intracellular domain (EdFLAGC) was not phosphorylated in response to Egfr signaling, but Echinoid lacking the extracellular motifs (EdFLAGN) was phosphorylated. (C) Echinoid immunoprecipitated from imaginal discs was also tyrosine-phosphorylated in response to Egfr signaling: Echinoid from w¹¹¹⁸ imaginal discs shows a low-level of tyrosine phosphorylation. Activation of Egfr signaling by transient overexpression of Rhomboid increases phosphotyrosine associated with Echinoid; inhibition of Egfr by transient overexpression of Argos decreases phosphotyrosine.