Phosphatase 1 Nuclear Targeting Subunit Is an Essential Regulator of M-phase Entry, Maintenance, and Exit

Background: Mitotic progression is regulated by reversible protein phosphorylation involving kinases and phosphatases. Results: Pnuts functions as a master regulator of mitosis by modulating PP1; Pnuts expression peaks in mitosis and is degraded at mitotic exit. Conclusion: This study reveals the function and regulation of a new and essential mitotic regulator. Significance: This study improves our understanding of M-phase regulation. Mitotic progression is regulated largely through dynamic and reversible protein phosphorylation that is modulated by opposing actions of protein kinases and phosphatases. In this study, we show that phosphatase 1 nuclear targeting subunit (Pnuts) functions as a master regulator of mitosis by modulating protein phosphatase 1 (PP1). Overexpression of Pnuts in Xenopus egg extracts inhibited both mitotic and meiotic exit. Immunodepletion of Pnuts from egg extracts revealed its essential functions in mitotic entry and maintenance. The level of Pnuts oscillates during the cell cycle and peaks in mitosis. Pnuts destruction during M-phase exit is mediated by the anaphase-promoting complex/cyclosome (APC/C)-targeted ubiquitination and proteolysis, and conserved destruction motifs of Pnuts. Disruption of Pnuts degradation delayed M-phase exit, suggesting it as an important mechanism to permit M-phase exit.