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

Abstract

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.

Genome-wide siRNA Screen Identifies the Radiosensitizing Effect of Downregulation of MASTL and FOXM1 in NSCLC

“…The radiosensitizing effect of MASTL knockdown did not seem to be driven by ENSA, the currently known substrate for MASTL (22,33), as we could not confirm an altered phosphorylation state of this protein after MASTL knockdown. Our phosphoproteomics analysis further suggests that the initial phosphorylation of DNA damage response proteins is not altered in irradiated cells with lowered MASTL expression.…”

“…Endosulfine alpha (ENSA) has been shown to be a substrate for MASTL kinase in Xenopus laevis, where it appears to play a role in the mitosis by modifying cyclinB-CDK1 activity (22). As the G 2 -M checkpoint is critically important in the response to DNA damage, we wondered whether the regulation of ENSA activity by MASTL is involved in the observed phenotype after irradiation.…”

LB100, a small molecule inhibitor of PP2A with potent chemo- and radio-sensitizing potential

“…25,26 PP2A also negatively regulates Cdk1 through activating wee1 and myt1 and inhibiting cdc25 through dephosphorylation. 24 Furthermore, PP2A silencing has been shown to upregulate downstream Cdk1 transcriptional targets and promote mitosis. 27,28 As such, the Greatwall-PP2A network has been proposed as a key signaling axis that promotes normal Cdk1-driven entry through mitosis.…”

“…In particular, PP2A is regulated by Greatwall kinase (called microtubule-associated serine/threonine kinase-like [Mastl] in mammals), which when depleted is associated with severe mitotic defects. 21,22 Greatwall has been shown to inhibit PP2A via the small proteins, a-endosulphine (ENSA) and cyclic AMP-regulated phosphoprotein 19 (ARPP19), 23,24 relieving PP2A-mediated dephosphorylation of various Cdk1 substrates and promoting mitotic progression. 25,26 PP2A also negatively regulates Cdk1 through activating wee1 and myt1 and inhibiting cdc25 through dephosphorylation.…”

The Renaissance or the cuckoo clock

“…Thus, Cyclin B-Cdk1 activity must reach a critical threshold before it triggers the amplification loop, whereupon the bi-stable switch from inactive to fully active is rapid and complete. The lock that sets this switch is provided by PP2A-B55d, because fully activated Cyclin B-Cdk1 is not sufficient to drive Xenopus extracts into mitosis unless the PP2A-B55d form of the PP2A phosphatase is inhibited by phosphorylated endosulphine [18] or phosphorylated Arpp19 [19].…”