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.