A PP1–PP2A phosphatase relay controls mitotic progression

Abstract

SummaryThe widespread reorganisation of cellular architecture in mitosis is achieved through extensive protein phosphorylation, driven by the coordinated activation of a mitotic kinase network and repression of counteracting phosphatases. Phosphatase activity must subsequently be restored to promote mitotic exit. Although Cdc14 phosphatase drives this reversal in budding yeast, Protein Phosphatase 1 (PP1) and Protein Phosphatase 2A (PP2A) activities have each been independently linked to mitotic exit control in other eukaryotes1-6. We now describe a mitotic phosphatase relay in which PP1 reactivation is required for the reactivation of both PP2A-B55 and PP2A-B56 to coordinate mitotic progression and exit in fission yeast. The staged recruitment of PP1 to the regulatory subunits of PP2A-B55 and PP2A-B56 holoenzymes sequentially activates each phosphatase. The pathway is blocked in early mitosis because Cdk1-Cyclin B inhibits PP1 activity but declining Cyclin B levels later in mitosis permit PP1 to auto-reactivate1,7-10. PP1 first reactivates PP2A-B55; this enables PP2A-B55, in turn, to promote the reactivation of PP2A-B56 by dephosphorylating a PP1 docking site in PP2A-B56, thereby promoting the recruitment of PP1. PP1 recruitment to human, mitotic, PP2A holoenzymes and the sequences of these conserved PP1 docking motifs11,12 suggest that PP1 regulates PP2A-B55 and PP2A-B56 activities in a variety of signalling contexts throughout eukaryotes.

Greatwall dephosphorylation and inactivation upon mitotic exit is triggered by PP1

“…Given that it has recently been shown in fission yeast that PP1 association with PP2A-B55 during mitosis promotes the activation of PP2A-B55 (Grallert et al, 2015), we tested whether these two phosphatases interact in CSF extracts. We did not detect any association of the PP2A scaffolding A subunit (also known as PPP2R1A) or a PP2A catalytic subunit (PPP2CA) with PP1 in our extracts by performing either western blotting (Fig.…”

Protein phosphatase 1 is essential for Greatwall inactivation at mitotic exit

“…Our study identifies PP1 as an essential component of the Gwl inactivation pathway at mitotic exit in Xenopus embryo extract. Based on our data, we propose the following model (Fig 4D): The first event resulting ultimately in tipping the balance in favor of PP2A-B55 re-activation at mitotic exit is the inactivation of Cdk1 by APC/C-mediated destruction of cyclin B. Cdk1 inactivation enables PP1 to dephosphorylate itself at an inhibitory Cdk1 site [18,[23][24][25] resulting in PP1 activation under conditions where PP2A-B55 would still be inhibited by the molar excess of phosphorylated Arpp19. Active PP1 would then initiate Gwl inactivation by dephosphorylating it at the autophosphorylation site S883.…”

Wee1 and Cdc25: Tools, pathways, mechanisms, questions

“…Lucena and colleagues identified a role for the PP2A-B55 phosphatase complex, which was recently found to act in a multi-phosphatase relay pathway that orders the events of mitosis. 4 This places regulation of Wee1 and Cdc25 within a larger phosphatase system ordering the events of cell division. Recent work also defined a Greatwall kinase-based pathway linking nitrogen availability with PP2A activity.…”