Publications (6)49.84 Total impact
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Article: Multisite phosphoregulation of Cdc25 activity refines the mitotic entrance and exit switches.
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ABSTRACT: Cyclin-dependent kinase 1 (Cdk1) kinase dephosphorylation and activation by Cdc25 phosphatase are essential for mitotic entry. Activated Cdk1 phosphorylates Cdc25 and other substrates, further activating Cdc25 to form a positive feedback loop that drives the abrupt G2/mitosis switch. Conversely, mitotic exit requires Cdk1 inactivation and reversal of Cdk1 substrate phosphorylation. This dephosphorylation is mediated, in part, by Clp1/Cdc14, a Cdk1-antagonizing phosphatase, which reverses Cdk1 phosphorylation of itself, Cdc25, and other Cdk1 substrates. Thus, Cdc25 phosphoregulation is essential for proper G2-M transition, and its contributions to cell cycle control have been modeled based on studies using Xenopus and human cell extracts. Because cell extract systems only approximate in vivo conditions where proteins interact within dynamic cellular environments, here, we use Schizosaccharomyces pombe to characterize, both experimentally and mathematically, the in vivo contributions of Cdk1-mediated phosphorylation of Cdc25 to the mitotic transition. Through comprehensive mapping of Cdk1 phosphosites on Cdc25 and characterization of phosphomutants, we show that Cdc25 hyperphosphorylation by Cdk1 governs Cdc25 catalytic activation, the precision of mitotic entry, and unvarying cell length but not Cdc25 localization or abundance. We propose a mathematical model that explains Cdc25 regulation by Cdk1 through a distributive and disordered phosphorylation mechanism that ultrasensitively activates Cdc25. We also show that Clp1/Cdc14 dephosphorylation of Cdk1 sites on Cdc25 controls the proper timing of cell division, a mechanism that is likely due to the double negative feedback loop between Clp1/Cdc14 and Cdc25 that controls the abruptness of the mitotic exit switch.Proceedings of the National Academy of Sciences 06/2012; 109(25):9899-904. · 9.68 Impact Factor -
Article: Switches and latches: a biochemical tug-of-war between the kinases and phosphatases that control mitosis.
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ABSTRACT: Activation of the cyclin-dependent kinase (Cdk1) cyclin B (CycB) complex (Cdk1:CycB) in mitosis brings about a remarkable extent of protein phosphorylation. Cdk1:CycB activation is switch-like, controlled by two auto-amplification loops--Cdk1:CycB activates its activating phosphatase, Cdc25, and inhibits its inhibiting kinase, Wee1. Recent experimental evidence suggests that parallel to Cdk1:CycB activation during mitosis, there is inhibition of its counteracting phosphatase activity. We argue that the downregulation of the phosphatase is not just a simple latch that suppresses futile cycles of phosphorylation/dephosphorylation during mitosis. Instead, we propose that phosphatase regulation creates coherent feed-forward loops and adds extra amplification loops to the Cdk1:CycB regulatory network, thus forming an integral part of the mitotic switch. These network motifs further strengthen the bistable characteristic of the mitotic switch, which is based on the antagonistic interaction of two groups of proteins: M-phase promoting factors (Cdk1:CycB, Cdc25, Greatwall and Endosulfine/Arpp19) and interphase promoting factors (Wee1, PP2A-B55 and a Greatwall counteracting phosphatase, probably PP1). The bistable character of the switch implies the existence of a CycB threshold for entry into mitosis. The end of G2 phase is determined by the point where CycB level crosses the CycB threshold for Cdk1 activation.Philosophical Transactions of The Royal Society B Biological Sciences 12/2011; 366(1584):3584-94. · 6.40 Impact Factor -
Article: Protein phosphatase 2A controls the order and dynamics of cell-cycle transitions.
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ABSTRACT: Bistability of the Cdk1-Wee1-Cdc25 mitotic control network underlies the switch-like transitions between interphase and mitosis. Here, we show by mathematical modeling and experiments in Xenopus egg extracts that protein phosphatase 2A (PP2A), which can dephosphorylate Cdk1 substrates, is essential for this bistability. PP2A inhibition in early interphase abolishes the switch-like response of the system to Cdk1 activity, promoting mitotic onset even with very low levels of Cyclin, Cdk1, and Cdc25, while simultaneously inhibiting DNA replication. Furthermore, even if replication has already initiated, it cannot continue in mitosis. Exclusivity of S and M phases does not depend on bistability only, since partial PP2A inhibition prevents replication without inducing mitotic onset. In these conditions, interphase-level mitotic kinases inhibit Cyclin E-Cdk2 chromatin loading, blocking initiation complex formation. Therefore, by counteracting both Cdk1 activation and activity of mitotic kinases, PP2A ensures robust separation of S phase and mitosis and dynamic transitions between the two states.Molecular cell 11/2011; 44(3):437-50. · 14.61 Impact Factor -
Article: Different effects of redundant feedback loops on a bistable switch.
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ABSTRACT: Bistable switches have important roles in cellular decision-making processes. Bistability can be the consequence of positive or double-negative feedback loops. Although necessary, such feedback is not sufficient for bistability, which also requires nonlinearity. Nonlinearity can be provided by synergy of multiple feedback loops or by an ultrasensitive response within a single feedback loop. However, these two possibilities are not mutually exclusive; a combination of them is also possible. Here we analyze a biochemical regulatory network that controls a crucial cell cycle transition in all eukaryotic cells and contains multiple redundant feedback loops and nonlinearity. We show in this realistic biological example that two redundant feedback loops have different effects on the position of one of the saddle-node bifurcations of the system, which determines where the system switches. This illustrates that even though the roles of positive and double-negative feedbacks have been regarded as equivalent, the difference in their architectures can lead to differences in their effects on the system. We speculate that this conclusion could be general for other bistable systems with redundant feedback loops.Chaos (Woodbury, N.Y.) 12/2010; 20(4):045120. · 1.80 Impact Factor -
Article: Regulated protein kinases and phosphatases in cell cycle decisions.
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ABSTRACT: Many aspects of cell physiology are controlled by protein kinases and phosphatases, which together determine the phosphorylation state of targeted substrates. Some of these target proteins are themselves kinases or phosphatases or other components of a regulatory network characterized by feedback and feed-forward loops. In this review we describe some common regulatory motifs involving kinases, phosphatases, and their substrates, focusing particularly on bistable switches involved in cellular decision processes. These general principles are applied to cell cycle transitions, with special emphasis on the roles of regulated phosphatases in orchestrating progression from one phase to the next of the DNA replication-division cycle.Current opinion in cell biology 12/2010; 22(6):801-8. · 14.15 Impact Factor -
Article: Bistability by multiple phosphorylation of regulatory proteins.
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ABSTRACT: The activity of a protein can be reversibly modulated by post-translational, covalent modifications, such as phosphorylation and dephosphorylation. In many cases, the modulated protein may be phosphorylated by the same kinase on many different amino acid residues. Such multisite phosphorylations may occur progressively (during a single binding event of kinase to substrate) or distributively (the kinase dissociates from its substrate after each phosphorylation reaction). If a protein is phosphorylated by a distributive multisite mechanism, then the net activity of a population of these protein molecules can be a highly nonlinear function of the ratio of activities of the kinase and phosphatase enzymes. If the multiply phosphorylated protein is embedded in a positive feedback loop with its kinase and/or phosphatase, then the network may exhibit robust bistable behavior. Using numerical simulations and bifurcation theory, we study the properties of a particular bistable reaction network motivated by the antagonistic relationship between cyclin-dependent kinase and its multiply phosphorylated target, Cdh1, which is involved in the degradation of cyclin molecules. We characterize the bistable switch in terms of (i) the mechanism of distributive phosphorylation (ordered or disordered), (ii) the number of phosphorylation sites on the target protein, (iii) the effect of phosphorylation on the target protein (abrupt or progressive inactivation), and (iv) the effects of stochastic fluctuations in small cells with limited numbers of kinase, phosphatase and target proteins.Progress in Biophysics and Molecular Biology 07/2009; 100(1-3):47-56. · 3.20 Impact Factor
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Institutions
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2011–2012
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University of Oxford
- Department of Biochemistry
Oxford, ENG, United Kingdom
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