Fig. 4. Okadaic acid is unable to induce MPF auto-amplification in stage IV oocyte extracts. Stage IV and stage VI oocyte extracts were incubated in the presence (+) or in the absence (–) of 1 µM okadaic acid (OA) and an ATP-regenerating system for 3 hours. (A) Histone H1 kinase assay of Cdc2. (B) Western blots revealed with antibodies against the active phosphorylated form of MAPK (P-MAPK), Cdc25 and the Tyr15-phosphorylated form of Cdc2 (P-Tyr-Cdc2). (C) Oocyte extracts were supplemented with [³⁵S]methionine labeled Myt1 at time of OA addition. Extracts were then submitted to SDS-PAGE and to autoradiography.