Davis Ryan J, Swanger Jherek, Hughes Bridget T, Clurman Bruce E
Divisions of Human Biology and Clinical Research, Fred Hutchinson Cancer Research Center, Seattle, Washington, USA.
Molecular and Cellular Biology Program, University of Washington, Seattle, Washington, USA.
Mol Cell Biol. 2017 Mar 31;37(8). doi: 10.1128/MCB.00657-16. Print 2017 Apr 15.
Cyclin E, in conjunction with its catalytic partner cyclin-dependent kinase 2 (CDK2), regulates cell cycle progression as cells exit quiescence and enter S-phase. Multiple mechanisms control cyclin E periodicity during the cell cycle, including phosphorylation-dependent cyclin E ubiquitylation by the SCF ubiquitin ligase. Serine 384 (S384) is the critical cyclin E phosphorylation site that stimulates Fbw7 binding and cyclin E ubiquitylation and degradation. Because S384 is autophosphorylated by bound CDK2, this presents a paradox as to how cyclin E can evade autocatalytically induced degradation in order to phosphorylate its other substrates. We found that S384 phosphorylation is dynamically regulated in cells and that cyclin E is specifically dephosphorylated at S384 by the PP2A-B56 phosphatase, thereby uncoupling cyclin E degradation from cyclin E-CDK2 activity. Furthermore, the rate of S384 dephosphorylation is high in interphase but low in mitosis. This provides a mechanism whereby interphase cells can oppose autocatalytic cyclin E degradation and maintain cyclin E-CDK2 activity while also enabling cyclin E destruction in mitosis, when inappropriate cyclin E expression is genotoxic.
细胞周期蛋白E与其催化伴侣细胞周期蛋白依赖性激酶2（CDK2）共同作用，在细胞退出静止期并进入S期时调节细胞周期进程。细胞周期中有多种机制控制细胞周期蛋白E的周期性，包括SCF泛素连接酶介导的磷酸化依赖性细胞周期蛋白E泛素化。丝氨酸384（S384）是关键的细胞周期蛋白E磷酸化位点，可刺激Fbw7结合以及细胞周期蛋白E的泛素化和降解。由于S384会被结合的CDK2自身磷酸化，这就产生了一个悖论，即细胞周期蛋白E如何能逃避自催化诱导的降解，以便磷酸化其其他底物。我们发现，S384的磷酸化在细胞中受到动态调节，并且细胞周期蛋白E在S384处被PP2A - B56磷酸酶特异性去磷酸化，从而使细胞周期蛋白E的降解与细胞周期蛋白E - CDK2的活性解偶联。此外，S384去磷酸化的速率在间期较高，而在有丝分裂期较低。这提供了一种机制，通过该机制间期细胞可以对抗细胞周期蛋白E的自催化降解并维持细胞周期蛋白E - CDK2的活性，同时在有丝分裂期（此时不适当的细胞周期蛋白E表达具有基因毒性）也能使细胞周期蛋白E被降解。
