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. An inhibitor of protein phosphatase 2A is identified as a component of the machinery controlling cell division. Entry into mitosis in eukaryotes requires the activity of cyclin-dependent kinase 1 (Cdk1). Cdk1 is opposed by protein phosphatases in two ways: They inhibit activation of Cdk1 by dephosphorylating the protein kinases Wee1 and Myt1 and the protein phosphatase Cdc25 (key regulators of Cdk1), and they also antagonize Cdk1’s own phosphorylation of downstream targets. A particular form of protein phosphatase 2A (PP2A) containing a B55δ subunit (PP2A- B55δ) is the major protein phosphatase that acts on model CDK substrates in Xenopus egg extracts and has antimitotic activity. The activity of PP2A-B55δ is high in interphase and low in mitosis, exactly opposite that of Cdk1. We report that inhibition of PP2A-B55δ results from a small protein, known as α-endosulfine (Ensa), that is phosphorylated in mitosis by the protein kinase Greatwall (Gwl). This converts Ensa into a potent and specific inhibitor of PP2A-B55δ. This pathway represents a previously unknown element in the control of mitosis.