Fig. 6. Phosphorylated Groucho is a nuclear and stable protein that persists after MAPK activation has been extinguished. (A,A') Posterior terminus of stage 5 wild-type embryo, stained with pGro antibodies (red) and Lamin antibody (green) demarcating the nuclear membrane. Superficial (A) and transverse (A') confocal sections show the nuclear localisation of pGro, encircled by Lamin staining. (B-D) Ventral view of wild-type stage 6 gastrulating embryo (note the invagination of the ventral furrow), stained for pGro (B; red) and dpERK (C; green). (D) Merge. pGro staining is detected at the termini even after MAPK activation has been turned off. Strong dpERK and pGro staining on both sides of the ventral furrow correlates with EGFR activation in this region. (E-E'') Model depicting possible implications of Gro phosphorylation to RTK target gene regulation. Prior to RTK activation (E), Gro is associated with its partner DNA-binding repressors (R), mediating repression of RTK target genes. Upon RTK pathway activation (E'), Gro is phosphorylated by MAPK. Modification of Gro downregulates its repressor activity, causing derepression of pathway target genes. MAPK is no longer active after RTK signalling has been turned off (E''), yet Gro remains stably phosphorylated and its activity attenuated, allowing for sustained RTK target gene expression.