Fig. 7. In vivo rescue of MPF auto-amplification by Plk1 in stage IV oocytes. (A) Stage IV oocytes were injected or not with Plk1 mRNA. After overnight incubation, they were either incubated in the presence or not of progesterone, or injected with His-cyclin B1. Stage VI oocytes were used as control. Extracts were western blotted with antibodies against Cdc25, the Tyr15-phosphorylated form of Cdc2 (P-Tyr-Cdc2), Myc (indicating the presence of the Myc-tagged Plk protein), Myt1 and the active phosphorylated form of MAPK (P-MAPK). They were also assayed for H1 kinase activity of Cdc2 (lower panel shows autoradiograph of [³²P]histone H1). (B) Stage IV oocytes were injected or not with Plk1 mRNA. After overnight incubation, they were either incubated in the presence or not of progesterone (Pg), or injected with okadaic acid (OA). Extracts were western blotted with antibodies against Cdc25 and the Tyr15-phosphorylated form of Cdc2 (P-Tyr-Cdc2). (C) Stage IV oocytes were injected with mouse Mos mRNA and human Plk1 mRNA. After overnight incubation, they were incubated or not in the presence of progesterone. Oocytes were collected 4 hours after GVBD occurred in stage VI oocytes. Oocyte extracts were western blotted with antibodies against the active phosphorylated form of MAPK (P-MAPK), Cdc25, the Tyr15-phosphorylated form of Cdc2 (P-Tyr-Cdc2) and cyclin B2.