Protein phosphatases and their regulation in the control of mitosis

Abstract

Cell cycle transitions depend on protein phosphorylation and dephosphorylation. The discovery of cyclin-dependent kinases (CDKs) and their mode of activation by their cyclin partners explained many important aspects of cell cycle control. As the cell cycle is basically a series of recurrences of a defined set of events, protein phosphatases must obviously be as important as kinases. However, our knowledge about phosphatases lags well behind that of kinases. We still do not know which phosphatase(s) is/are truly responsible for dephosphorylating CDK substrates, and we know very little about whether and how protein phosphatases are regulated. Here, we summarize our present understanding of the phosphatases that are important in the control of the cell cycle and pose the questions that need to be answered as regards the regulation of protein phosphatases.

Figure 1. Ensuring productive cycles.

A protein kinase adds, whereas a phosphatase removes, phosphate residues on substrates. If these mutually antagonistic enzymes work simultaneously, it not only results in a waste of ATP, but also renders impossible a full switch-like interconversion of the phosphorylation state of the substrate. To avoid this, the two enzymes should work alternatively, ideally while communicating with each other.

Figure 2. α-Endosulfine and ARPP-19 are Greatwall-dependent inhibitors of PP2A-B55.

(A) Schematic diagram of CDK1 and PP2A-B55 activity during the cell cycle. The patterns of CDK1 and PP2A-B55 activity are complementary to each other; CDK1 activity is shown in red and PP2A-B55 activity in green. The phosphorylation status of Apc3/Cdc27 reflects the ratio of kinase to phosphatase activity (upper bands indicate mitotic hyperphosphorylation). (B) Sequence alignment of the Ensa subfamily from yeast to human. Three possible phosphorylation sites are indicated with arrows. The CDK consensus site is found only in Xenopus Ensa, but is well conserved in the ARPP-19 subfamily. (C) Protein phosphatase (PPase) assay using a model CDK substrate and a catalytic C monomer, A+C dimer or heterotrimer holocomplex containing B55δ. Ensa phosphorylated by Gwl (red bars) inhibits PP2A trimeric holocomplexes that contain B55δ, but not dimeric or monomeric PP2A complexes. Fig 2c is a modified version of Figure 2A from Mochida et al [46]. APC3, anaphase-promoting complex subunit 3; ARPP-19, cyclic-AMP-regulated phosphoprotein of 19 kDa; CDK, cyclin-dependent kinase; Ensa, α-endosulfine; Gwl, Greatwall; PKA, cyclic-AMP-activated protein kinase A.

Figure 3. Factors that control the Greatwall pathway during the cell cycle.

Red arrows show factors that promote mitosis, whereas blue ones support interphase. +PO₄ and –PO₄ denote phosphorylation and dephosphorylation, respectively. PPase-X and PPase-Y are the as-yet-unidentified protein phosphatases that deactivate the Gwl pathway. ARPP-19, cyclic-AMP-regulated phosphoprotein of 19 kDa; CDK1, cyclin-dependent kinase 1; Ensa, α-endosulfine; Gwl, Greatwall.

Satoru Mochida & Tim Hunt