Hoffmann I, Draetta G, Karsenti E
Cell Biology Programme, European Molecular Biology Laboratory, Heidelberg, Germany.
EMBO J. 1994 Sep 15;13(18):4302-10. doi: 10.1002/j.1460-2075.1994.tb06750.x.
Progression through the cell cycle is monitored at two major points: during the G1/S and the G2/M transitions. In most cells, the G2/M transition is regulated by the timing of p34cdc2 dephosphorylation which results in the activation of the kinase activity of the cdc2-cyclin B complex. The timing of p34cdc2 dephosphorylation is determined by the balance between the activity of the kinase that phosphorylates p34cdc2 (wee1 in human cells) and the opposing phosphatase (cdc25C). Both enzymes are regulated and it has been shown that cdc25C is phosphorylated and activated by the cdc2-cyclin B complex. This creates a positive feed-back loop providing a switch used to control the onset of mitosis. Here, we show that another member of the human cdc25 family, cdc25A, undergoes phosphorylation during S phase, resulting in an increase of its phosphatase activity. The phosphorylation of cdc25A is dependent on the activity of the cdc2-cyclin E kinase. Microinjection of anti-cdc25A antibodies into G1 cells blocks entry into S phase. These results indicate that the cdc25A phosphatase is required to enter S phase in human cells and suggest that this enzyme is part of an auto-amplification loop analogous to that described at the G2/M transition. We discuss the nature of the in vivo substrate of the cdc25A phosphatase in S phase and the possible implications for the regulation of S phase entry.
G1/S期转换和G2/M期转换期间。在大多数细胞中，G2/M期转换由p34cdc2去磷酸化的时间来调控，这会导致cdc2-细胞周期蛋白B复合物的激酶活性被激活。p34cdc2去磷酸化的时间由磷酸化p34cdc2的激酶（人类细胞中的wee1）和相对的磷酸酶（cdc25C）的活性平衡所决定。这两种酶都受到调控，并且已经表明cdc25C被cdc2-细胞周期蛋白B复合物磷酸化并激活。这形成了一个正反馈环，提供了一个用于控制有丝分裂起始的开关。在此，我们表明人类cdc25家族的另一个成员cdc25A在S期发生磷酸化，导致其磷酸酶活性增加。cdc25A的磷酸化依赖于cdc2-细胞周期蛋白E激酶的活性。将抗cdc25A抗体显微注射到G1期细胞中会阻断细胞进入S期。这些结果表明cdc25A磷酸酶是人类细胞进入S期所必需的，并提示该酶是一个类似于在G2/M期转换中所描述的自放大环的一部分。我们讨论了cdc25A磷酸酶在S期体内底物的性质以及对S期进入调控的可能影响。
