Two Bistable Switches Govern M Phase Entry.

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• The transition from interphase to M phase is crucial for cell division, requiring precise regulation of protein phosphorylation.
• Cyclin-dependent kinase 1 (Cdk1):cyclin B (CycB) complex drives this transition, historically thought to rely on Cdk1 auto-activation via Tyr15 dephosphorylation.
• This auto-activation creates a bistable switch, ensuring irreversible commitment to mitosis.

Purpose of the Study:

• To investigate the mechanisms underlying switch-like and irreversible mitotic entry.
• To determine if Cdk1 auto-activation is essential for this process.
• To elucidate the role of the Greatwall (Gwl)-endosulfine (ENSA) pathway in regulating mitotic entry.

Main Methods:

• Biochemical reconstitution using purified components.
• Mathematical modeling of pathway dynamics.
• Analysis of Cdk1 phosphorylation thresholds in a luminescent substrate assay.

Main Results:

• Cdk1 auto-activation is dispensable for irreversible, switch-like mitotic entry.
• The Gwl-ENSA pathway, which inhibits the phosphatase PP2A:B55, is necessary and sufficient for switch-like mitotic phosphorylations.
• A reconstituted Gwl-ENSA system demonstrated a distinct Cdk1 threshold for initiating, but not maintaining, mitotic phosphorylation, indicating bistability.

Conclusions:

• Two interlinked bistable mechanisms ensure robust and irreversible mitotic entry.
• The Gwl-ENSA pathway provides a critical, previously unrecognized, bistable switch for mitotic commitment.
• This finding redefines our understanding of cell cycle regulation and mitotic commitment.