Vg1RBP phosphorylation is dependent on the MAPK pathway. (A) Inhibition of Mos synthesis prevents Vg1RBP phosphorylation. Stage VI oocytes were injected with 50 nL of water (W), 0.5 mM anti-Mos morpholino antisense (Mos), or 0.5 mM control morpholino oligonucleotide (M3), and incubated overnight in the presence (E, E′; duplicate samples) or absence (VI) of progesterone. Cells were then subjected to Western blot analysis using antisera directed against the proteins indicated on the left. (B) Inhibition of Mek1, but not p90Rsk or GSK3β, prevents phosphorylation of Vg1RBP. Stage VI oocytes were preincubated with 5 μM Ro318220, 10, 20, or 50 μM U0126, 10 mM LiCl, or DMSO at comparable concentrations for 4 h and then left overnight in the presence or absence (VI) of progesterone. Cells were then subjected to Western blot analysis using antisera directed against the proteins indicated on the left. (C) Vg1RBP phosphorylation in embryogenesis correlates with MAPK activity. Protein equivalents of one-stage VI oocyte (VI), progesterone-matured egg (E), laid egg (LE), fertilized egg (FE), MBT embryo, and stage 19/20 or 22/23 neurulas were subjected to Western blot analysis using antisera against the proteins indicated on the left.
