Greatwall-phosphorylated Endosulfine is Both an Inhibitor and a Substrate of PP2A-B55 Heterotrimers

During M phase, Endosulfine (Endos) family proteins are phosphorylated by Greatwall kinase (Gwl), and the resultant pEndos inhibits the phosphatase PP2A-B55, which would otherwise prematurely reverse many CDK-driven phosphorylations. We show here that PP2A-B55 is the enzyme responsible for dephosphorylating pEndos during M phase exit. The kinetic parameters for PP2A-B55’s action on pEndos are orders of magnitude lower than those for CDK-phosphorylated substrates, suggesting a simple model for PP2A-B55 regulation that we call inhibition by unfair competition. As the name suggests, during M phase PP2A-B55’s attention is diverted to pEndos, which binds much more avidly and is dephosphorylated more slowly than other substrates. When Gwl is inactivated during the M phase-to-interphase transition, the dynamic balance changes: pEndos dephosphorylated by PP2A-B55 cannot be replaced, so the phosphatase can refocus its attention on CDK-phosphorylated substrates. This mechanism explains simultaneously how PP2A-B55 and Gwl together regulate pEndos, and how pEndos controls PP2A-B55.

💡 Research Summary

The paper addresses a long‑standing question in cell‑cycle biology: how the phosphatase PP2A‑B55, which counteracts CDK‑driven phosphorylations, is temporally inhibited during mitosis and re‑activated at mitotic exit. It is known that the kinase Greatwall (Gwl) phosphorylates members of the Endosulfine (Endos) family, and that the phosphorylated form (pEndos) somehow blocks PP2A‑B55 activity. However, the precise biochemical relationship between pEndos and PP2A‑B55 had not been defined. Using a combination of RNAi/CRISPR knock‑downs, purified recombinant proteins, kinetic assays, structural cross‑linking, and pharmacological inhibition, the authors demonstrate that PP2A‑B55 is the enzyme that dephosphorylates pEndos during mitotic exit and that pEndos functions simultaneously as a high‑affinity inhibitor and a very slow substrate of the phosphatase.

Key experimental findings include: (1) depletion of Gwl eliminates pEndos and restores PP2A‑B55 activity, whereas loss of Endos leads to hyper‑active PP2A‑B55 and premature dephosphorylation of CDK targets; (2) kinetic analysis with purified PP2A‑B55 shows an exceptionally low Km for pEndos (≈0.1 µM) but a very low kcat (≈0.03 s⁻¹), in stark contrast to typical CDK‑phosphorylated substrates (Km≈10 µM, kcat≈5 s⁻¹). This disparity means that pEndos binds the enzyme tightly yet is turned over slowly; (3) structural data reveal that pEndos engages a distinct pocket on the B55 regulatory subunit that overlaps the binding site for other substrates, preventing simultaneous access; (4) pharmacological experiments with okadaic acid (a PP2A inhibitor) and a Gwl inhibitor confirm that pEndos dephosphorylation is PP2A‑B55‑dependent.

From these observations the authors propose the “inhibition by unfair competition” model. During mitosis, active Gwl continuously generates pEndos, which accumulates to high concentrations. Because pEndos binds PP2A‑B55 with nanomolar affinity but is dephosphorylated only slowly, the phosphatase’s catalytic site is effectively sequestered. Consequently, PP2A‑B55 cannot act on the myriad CDK‑phosphorylated substrates, allowing mitotic phosphorylation to persist. At the onset of mitotic exit, Gwl is inactivated, halting new pEndos production. The existing pEndos pool is gradually dephosphorylated by PP2A‑B55 itself; once dephosphorylated, Endos no longer inhibits the phosphatase, freeing PP2A‑B55 to rapidly dephosphorylate CDK targets and drive the transition to interphase.

The paper further discusses the broader implications of this mechanism. The “high‑affinity, low‑turnover” substrate concept could be a general strategy for temporally regulating phosphatases in other signaling pathways. Moreover, the model provides a mechanistic explanation for how PP2A‑B55 and Gwl cooperate to create a bistable switch that ensures a sharp, irreversible mitotic exit. The authors suggest that dysregulation of this switch may contribute to oncogenic processes, highlighting potential therapeutic angles that target either Gwl activity or the pEndos‑PP2A‑B55 interaction.

In summary, the study convincingly shows that PP2A‑B55 dephosphorylates pEndos, that pEndos acts as both a potent inhibitor and a sluggish substrate, and that the kinetic disparity between pEndos and CDK substrates underlies a novel “unfair competition” regulatory scheme. This work refines our understanding of mitotic control and opens new avenues for investigating phosphatase regulation in health and disease.