Abstract
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.
Highlights
Mitotic progression is regulated by reversible protein phosphorylation involving kinases and phosphatases
phosphatase nuclear targeting subunit (Pnuts) Overexpression Suppresses Both Meiotic and Mitotic Exit—We assessed the role of Pnuts in M-phase regulation using Xenopus egg extract, an in vitro model of cell cycle progression that has been widely used to study mitotic kinases and phosphatases [33, 34]
Pnuts Is a Quantitative Regulator of the Cell Cycle—Our results showed that the expression of Pnuts oscillates during the cell cycle and peaks in M-phase
Summary
Mitotic progression is regulated by reversible protein phosphorylation involving kinases and phosphatases. In contrast to that of protein kinases, the specific involvement of phosphatases in M-phase regulation only recently came to light [5,6,7] It has been shown in budding yeast that the dual specificity phosphatase Cdc plays a critical role in promoting mitotic exit through dephosphorylation of Cdk substrates [8]. Inhibition of the phosphatase activity of PP2A-B55␦ is essential for M-phase entry and maintenance and was later attributed to Ensa and Arpp-19, and their mitotic phosphorylation by Greatwall kinase [12, 13, 15, 16]. The cell cycle-dependent accumulation and degradation of Pnuts are tightly regulated and critical for the biochemical progression of M-phase
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