Greatwall phosphorylates an inhibitor of protein phosphatase 2A that is essential for mitosis

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• Entry into eukaryotic mitosis requires cyclin-dependent kinase 1 (Cdk1).
• Protein phosphatases oppose Cdk1 activity by regulating key kinases (Wee1, Myt1) and phosphatase Cdc25, and by antagonizing Cdk1's phosphorylation of downstream targets.
• Protein phosphatase 2A with a B55δ subunit (PP2A-B55δ) is a major phosphatase acting on CDK substrates and possesses antimitotic activity, with its activity inversely correlating to Cdk1 activity.

Purpose of the Study:

• To elucidate the mechanism by which PP2A-B55δ activity is inhibited during mitosis.
• To identify the regulatory factors controlling PP2A-B55δ function in the context of the cell cycle.

Main Methods:

• Utilized Xenopus egg extracts as a model system.
• Investigated the interaction between α-endosulfine (Ensa) and PP2A-B55δ.
• Examined the role of the protein kinase Greatwall (Gwl) in regulating Ensa activity.

Main Results:

• Identified α-endosulfine (Ensa) as a key regulator of PP2A-B55δ.
• Demonstrated that mitosis-specific phosphorylation of Ensa by Greatwall (Gwl) converts Ensa into a potent and specific inhibitor of PP2A-B55δ.
• Established an inverse relationship between PP2A-B55δ activity and Cdk1 activity, mediated by the Gwl-Ensa pathway.

Conclusions:

• Discovered a novel regulatory pathway involving Greatwall (Gwl) and α-endosulfine (Ensa) that inhibits protein phosphatase 2A-B55δ during mitosis.
• This pathway represents a previously unrecognized mechanism controlling mitotic entry and progression.
• The Gwl-Ensa-PP2A-B55δ axis provides new insights into the intricate regulation of the eukaryotic cell cycle.