Two Bistable Switches Govern M Phase Entry

• Published in: Current biology Vol. 26; no. 24; pp. 3361 - 3367
• Main Authors: Mochida, Satoru, Rata, Scott, Hino, Hirotsugu, Nagai, Takeharu, Novák, Béla
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 19.12.2016; Cell Press
• Subjects: Animals; CDC2 Protein Kinase - genetics; CDC2 Protein Kinase - metabolism; Cell Cycle Checkpoints - physiology; Cell Division - physiology; cyclin-dependent kinase; Gene Expression Regulation, Enzymologic; Greatwall kinase; hysteresis; mitosis; Models, Biological; Peptides - metabolism; Phosphorylation; PP2A; Protein Phosphatase 2 - genetics; Protein Phosphatase 2 - metabolism; robustness; Signal Transduction - physiology; threshold; Greatwall kinase; robustness; PP2A; threshold; mitosis; cyclin-dependent kinase; hysteresis
• ISSN: 0960-9822; 1879-0445; 1879-0445
• DOI: 10.1016/j.cub.2016.10.022

The abrupt and irreversible transition from interphase to M phase is essential to separate DNA replication from chromosome segregation. This transition requires the switch-like phosphorylation of hundreds of proteins by the cyclin-dependent kinase 1 (Cdk1):cyclin B (CycB) complex. Previous studies have ascribed these switch-like phosphorylations to the auto-activation of Cdk1:CycB through the removal of inhibitory phosphorylations on Cdk1-Tyr15 [1, 2]. The positive feedback in Cdk1 activation creates a bistable switch that makes mitotic commitment irreversible [2–4]. Here, we surprisingly find that Cdk1 auto-activation is dispensable for irreversible, switch-like mitotic entry due to a second mechanism, whereby Cdk1:CycB inhibits its counteracting phosphatase (PP2A:B55). We show that the PP2A:B55-inhibiting Greatwall (Gwl)-endosulfine (ENSA) pathway is both necessary and sufficient for switch-like phosphorylations of mitotic substrates. Using purified components of the Gwl-ENSA pathway in a reconstituted system, we found a sharp Cdk1 threshold for phosphorylation of a luminescent mitotic substrate. The Cdk1 threshold to induce mitotic phosphorylation is distinctly higher than the Cdk1 threshold required to maintain these phosphorylations—evidence for bistability. A combination of mathematical modeling and biochemical reconstitution show that the bistable behavior of the Gwl-ENSA pathway emerges from its mutual antagonism with PP2A:B55. Our results demonstrate that two interlinked bistable mechanisms provide a robust solution for irreversible and switch-like mitotic entry. [Display omitted] •Cdk1 auto-activation loop is dispensable for switch-like mitotic entry•PP2A:B55 auto-regulation creates a bistable switch•Two bistable switches provide a robust solution for mitotic entry Mochida et al. find, using a biochemical reconstitution and mathematical simulation, that the regulation of PP2A:B55 phosphatase can create a bistable switch for mitotic phosphorylation of Cdk1 substrates. This new mechanism, together with the Cdk1 auto-activation loop, provides a robust solution for irreversible and switch-like mitotic entry.