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The xenopus Suc1/Cks protein promotes the phosphorylation of G(2)/M regulators
1Division of Biology 216-76, Howard Hughes Medical Institute, California Institute of Technology, Pasadena, California 91125, USA.
Abstract:
The entry into mitosis is controlled by Cdc2/cyclin B, also known as maturation or M-phase promoting factor (MPF). In Xenopus egg extracts, the inhibitory phosphorylations of Cdc2 on Tyr-15 and Thr-14 are controlled by the phosphatase Cdc25 and the kinases Myt1 and Wee1. At mitosis, Cdc25 is activated and Myt1 and Wee1 are inactivated through phosphorylation by multiple kinases, including Cdc2 itself. The Cdc2-associated Suc1/Cks1 protein (p9) is also essential for entry of egg extracts into mitosis, but the molecular basis of this requirement has been unknown. We find that p9 strongly stimulates the regulatory phosphorylations of Cdc25, Myt1, and Wee1 that are carried out by the Cdc2/cyclin B complex. Overexpression of the prolyl isomerase Pin1, which binds to the hyperphosphorylated forms of Cdc25, Myt1, and Wee1 found at M-phase, is known to block the initiation of mitosis in egg extracts. We have observed that Pin1 specifically antagonizes the stimulatory effect of p9 on phosphorylation of Cdc25 by Cdc2/cyclin B. This observation could explain why overexpression of Pin1 inhibits mitotic initiation. These findings suggest that p9 promotes the entry into mitosis by facilitating phosphorylation of the key upstream regulators of Cdc2.
Insights
The Suc1/Cks1 protein (p9) is crucial for mitotic entry by enhancing the phosphorylation of key regulators. It counteracts Pin1, explaining how Pin1 overexpression blocks mitosis initiation.
Area of Science:
- Cell Cycle Regulation
- Molecular Biology
- Biochemistry
Background:
- Mitotic entry is orchestrated by Cdc2/cyclin B (MPF).
- Cdc25, Myt1, and Wee1 regulate inhibitory phosphorylations of Cdc2.
- Suc1/Cks1 protein (p9) is essential for mitosis but its role was unclear.
Purpose of the Study:
- To elucidate the molecular mechanism of p9's requirement for mitotic entry.
- To investigate the interaction between p9, Pin1, and mitotic regulators.
Main Methods:
- Experiments using Xenopus egg extracts.
- Analysis of protein phosphorylation and interactions.
Main Results:
- p9 significantly stimulates the phosphorylation of Cdc25, Myt1, and Wee1 by Cdc2/cyclin B.
- Pin1 overexpression inhibits mitotic initiation by antagonizing p9's effect on Cdc25 phosphorylation.
Conclusions:
- p9 promotes mitotic entry by facilitating the phosphorylation of upstream Cdc2 regulators.
- Pin1's antagonistic action on p9 provides a mechanism for mitotic inhibition.