PerspectiveCell Biology

Enforcing the Greatwall in Mitosis

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Science  17 Dec 2010:
Vol. 330, Issue 6011, pp. 1638-1639
DOI: 10.1126/science.1199898

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Once a eukaryotic cell commits to dividing, there is no turning back. This irreversible step relies on strict and precise regulatory mechanisms that ensure a normal transition into and through the cell division cycle. To enter mitosis, cells activate an enzyme complex called cyclin-dependent kinase 1 (Cdk1)–cyclin B through an interlocking and redundant series of positive-feedback signaling loops (1). Activated Cdk1 phosphorylates a multitude of substrates, leading to chromosome condensation and nuclear envelope breakdown, events that mark the beginning of mitosis. Because net substrate phosphorylation is determined by a balance between kinase and phosphatase activities, the transition into mitosis would be promoted more readily if counteracting protein phosphatases were turned off at the same time that Cdk1 was turned on. Two studies in this issue, by Gharbi-Ayachi et al. on page 1673 (2) and Mochida et al. on page 1670 (3), describe a feed-forward loop, which does just that, providing a mechanism that amplifies the power of Cdk1 by inhibiting a specific form of the protein phosphatase PP2A.