Dual phosphorylation controls Cdc25 phosphatases and mitotic entry.

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• Cdc25C protein phosphatase activity is negatively regulated by phosphorylation at Serine 216, a key 14-3-3 binding site.
• This regulation is crucial for blocking cell cycle progression into mitosis under normal conditions and after DNA damage.
• During mitosis, Cdc25C is typically not phosphorylated at Serine 216, and ionizing radiation does not induce this phosphorylation or 14-3-3 binding.

Purpose of the Study:

• To investigate the regulatory mechanisms of Cdc25C during mitosis.
• To identify novel phosphorylation sites on Cdc25C that influence its activity and binding partners.
• To elucidate the role of Serine 214 phosphorylation in controlling Cdc25C function and cell cycle progression.

Main Methods:

• Site-directed mutagenesis to create Cdc25C mutants (S214A, S214A/S216A).
• Introduction of exogenous Cdc25C mutants into HeLa cells depleted of endogenous Cdc25C.
• Analysis of phosphorylation status, 14-3-3 binding, and mitotic entry timing.

Main Results:

• Cdc25C is phosphorylated on Serine 214 during mitosis, which inhibits Serine 216 phosphorylation.
• Mutation of Serine 214 to Alanine restores Serine 216 phosphorylation and 14-3-3 binding during mitosis.
• Expression of Cdc25C(S214A) mutant delays mitotic entry, an effect dependent on Serine 216 phosphorylation.

Conclusions:

• Mitotic phosphorylation of Cdc25C at Serine 214 acts as a negative regulator, preventing Serine 216 phosphorylation and 14-3-3 binding.
• This pathway ensures Cdc25C remains active after mitosis initiation, maintaining proper cell cycle transitions.
• A similar mechanism may regulate other mitotic phosphatases, including Cdc25B and orthologs in other species.