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Fig. 5. Presence of Plk1 restores MPF auto-amplification induced by cyclins or
okadaic acid in stage IV oocyte extracts. (A) Stage IV and stage VI oocyte
extracts were incubated in the presence or in the absence of either His-cyclin
B1 (B1) or GST-cyclin A (A) and ATP for 3 hours. They were western blotted
with antibodies against Cdc25 and the Tyr15-phosphorylated form of Cdc2
(P-Tyr-Cdc2). (B) Stage IV and stage VI oocyte extracts were incubated for 3
hours in the presence of increasing amounts of human His-cyclin B1 and were
then assayed for H1 kinase activity of Cdc2. (C) Stage IV oocytes were
injected with human Plk1 mRNA. After overnight incubation, oocyte extracts
were prepared and supplemented with 1 µM okadaic acid (OA) or His-cyclin B1
(B1) and ATP. Three hours later, extracts were western blotted with antibodies
directed against Myc (indicating the presence of the Myc-tagged Plk protein),
Cdc25 and the Tyr15-phosphorylated form of Cdc2 (P-Tyr-Cdc2). They were also
assayed for H1 kinase activity of Cdc2 (lower panel shows autoradiograph of
[32P]histone H1).
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