Department of Molecular, Cell, and Developmental Biology, University of California, Santa Cruz, Santa Cruz, CA 95064, USA.
Mol Biol Cell. 2011 Oct;22(19):3595-608. doi: 10.1091/mbc.E11-04-0340. Epub 2011 Aug 17.
Entry into mitosis is initiated by synthesis of cyclins, which bind and activate cyclin-dependent kinase 1 (Cdk1). Cyclin synthesis is gradual, yet activation of Cdk1 occurs in a stepwise manner: a low level of Cdk1 activity is initially generated that triggers early mitotic events, which is followed by full activation of Cdk1. Little is known about how stepwise activation of Cdk1 is achieved. A key regulator of Cdk1 is the Wee1 kinase, which phosphorylates and inhibits Cdk1. Wee1 and Cdk1 show mutual regulation: Cdk1 phosphorylates Wee1, which activates Wee1 to inhibit Cdk1. Further phosphorylation events inactivate Wee1. We discovered that a specific form of protein phosphatase 2A (PP2A(Cdc55)) opposes the initial phosphorylation of Wee1 by Cdk1. In vivo analysis, in vitro reconstitution, and mathematical modeling suggest that PP2A(Cdc55) sets a threshold that limits activation of Wee1, thereby allowing a low constant level of Cdk1 activity to escape Wee1 inhibition in early mitosis. These results define a new role for PP2A(Cdc55) and reveal a systems-level mechanism by which dynamically opposed kinase and phosphatase activities can modulate signal strength.
有丝分裂的起始是通过细胞周期蛋白的合成来启动的，细胞周期蛋白与细胞周期蛋白依赖性激酶 1（Cdk1）结合并使其激活。细胞周期蛋白的合成是逐渐进行的，而 Cdk1 的激活则是分阶段进行的：最初产生低水平的 Cdk1 活性，从而引发早期有丝分裂事件，随后 Cdk1 被完全激活。关于 Cdk1 的分阶段激活是如何实现的，目前知之甚少。Cdk1 的一个关键调节因子是 Wee1 激酶，它使 Cdk1 磷酸化并抑制其活性。Wee1 和 Cdk1 相互调节：Cdk1 磷酸化 Wee1，激活 Wee1 抑制 Cdk1。进一步的磷酸化事件使 Wee1 失活。我们发现一种特定形式的蛋白磷酸酶 2A（PP2A(Cdc55)）可以拮抗 Cdk1 对 Wee1 的初始磷酸化。体内分析、体外重建和数学建模表明，PP2A(Cdc55)设定了一个阈值，限制了 Wee1 的激活，从而使低水平的 Cdk1 活性在早期有丝分裂中能够逃脱 Wee1 的抑制。这些结果为 PP2A(Cdc55)定义了一个新的作用，并揭示了一种系统水平的机制，即动态拮抗的激酶和磷酸酶活性可以调节信号强度。
