Greatwall Phosphorylates an Inhibitor of Protein Phosphatase 2Α That Is Essential for Mitosis

Beyond the Greatwall Protein phosphorylation and dephosphorylation provide a central mechanism that controls the eukaryotic cell division cycle and entry of cells into mitosis. A form of protein phosphatase 2A (PP2A) has an important role inhibiting phosphorylation-dependent activation of cyclin-dependent kinase 1 (CDK1) itself and also dephosphorylating substrates of the active CDK1 that promote mitosis. PP2A activity is inhibited when another protein kinase, known as Greatwall, is activated (see the Perspective by Virshup and Kaldis ). Mochida et al. (p. 1670 ) and Gharbi-Ayachi et al. (p. 1673 ) searched for substrates of Greatwall that might participate in the cell cycle regulatory machinery. When phosphorylated by Greatwall, a pair of small related proteins, Arpp19 and α-endosulfine, inhibited activity of PP2A. These effects were critical for regulation of mitosis in Xenopus egg extracts and in human cancer cells. Greatwall itself is phosphorylated and activated by CDK1—thus, apparently contributing to a feed-forward loop that contributes to the switchlike commitment of cells to mitosis.