The Greatwall–PP2A Axis in Cell Cycle Control
Cell cycle progression is largely controlled by reversible protein phosphorylation mediated by cyclically activated kinases and phosphatases. It has long been known that cyclin B–Cdk1 activation triggers mitotic entry, and the enzymatic network controlling its activation and inactivation has been well characterized. Much more recently protein phosphatase 2A (PP2A) together with its B55 regulatory subunit has been recognized as the major activity dephosphorylating Cdk1 targets. Moreover, PP2A-B55 activity is high in late M phase and interphase, but low at mitotic entry. A series of discoveries in the fly and frog model systems have uncovered the molecular mechanism mediating this regulation. The Greatwall (Gwl) kinase activates endosulfines, which become specific inhibitors of PP2A-B55. Cdk1-dependent activation of Gwl at mitotic entry leads to PP2A-B55 downregulation, which synergizes with Cdk1 activation to promote the phosphorylated states of several mitotic substrates. Much less is known on the mechanisms inactivating Gwl and endosulfines at mitotic exit. Recent reports show the importance of spatiotemporal regulation of Gwl, endosulfines, and PP2A-B55 for cell cycle progression. The various systems and cell types differ in their dependence on the Gwl–PP2A axis for cell cycle progression. Moreover, this pathway also regulates gene expression in yeast, and this function could be conserved in metazoans.
Key wordsCell cycle Mitosis Mitotic exit Greatwall PP2A B55 Endosulfine Cdk1
Work on the Gwl–PP2A axis in V.A.’s lab is supported by the Canadian Institutes of Health Research (CIHR). V.A. also holds a New Investigator Award from the CIHR. P.W. holds a studentship from the Fonds de recherche du Québec—Santé (FRQ-S). IRIC is supported in part by the Canadian Center of Excellence in Commercialization and Research, the Canada Foundation for Innovation, and the FRQ-S. Work in M.M.’s laboratory was funded by grants from Bayer Pharma AG, Fundación Ramón Areces, MINECO (SAF2012-38215), the OncoCycle Programme (S2010/BMD-2470) from the Comunidad de Madrid, and the European Union Seventh Framework Programme (MitoSys project; HEALTH-F5-2010-241548).
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