No abstract

Protein phosphorylation is a common mechanism for the regulation of cell cycle progression. The opposing functions of cell cycle kinases and phosphatases are crucial for accurate chromosome segregation and exit from mitosis. Protein phosphatases 2A are heterotrimeric complexes that play essential roles in cell growth, proliferation, and regulation of the cell cycle. Here, we review the function of the protein phosphatase 2A family as the counteracting force for the mitotic kinases. We focus on recent findings in the regulation of mitotic exit and cytokinesis by PP2A phosphatases in S. cerevisiae and other fungal species.

Eukaryotic cells divide and separate all their components after chromosome segregation by a process called cytokinesis to complete cell division. Cytokinesis is highly regulated by the recruitment of the components to the division site and through post-translational modifications such as phosphorylations. The budding yeast mitotic kinases Cdc28-Clb2, Cdc5, and Dbf2-Mob1 phosphorylate several cytokinetic proteins contributing to the regulation of cytokinesis. The PP2A-Cdc55 phosphatase regulates mitosis counteracting Cdk1- and Cdc5-dependent phosphorylation. This prompted us to propose that PP2A-Cdc55 could also be counteracting the mitotic kinases during cytokinesis. Here we show that in the absence of Cdc55, AMR contraction and the primary septum formation occur asymmetrically to one side of the bud neck supporting a role for PP2A-Cdc55 in cytokinesis regulation. In addition, by in vivo and in vitro assays, we show that PP2A-Cdc55 dephosphorylates the chitin synthase II (Chs2 in budding yeast) a component of the Ingression Progression Complexes (IPCs) involved in cytokinesis. Interestingly, the non-phosphorylable version of Chs2 rescues the asymmetric AMR contraction and the defective septa formation observed in cdc55∆ mutant cells. Therefore, timely dephosphorylation of the Chs2 by PP2A-Cdc55 is crucial for proper actomyosin ring contraction. These findings reveal a new mechanism of cytokinesis regulation by the PP2A-Cdc55 phosphatase and extend our knowledge of the involvement of multiple phosphatases during cytokinesis.

SummaryEukaryotic cells divide and separate all their components after chromosome segregation by a process called cytokinesis to complete cell division. Cytokinesis is regulated by exclusive elements of the process, and by some mitotic exit regulators. The mitotic kinases Cdc28-Clb2, Cdc5, and Dbf2-Mob1 phosphorylate cytokinetic proteins in budding yeast, but very little is known about the phosphatases regulating cytokinesis. The PP2A-Cdc55 phosphatase regulates mitosis counteracting Cdk1- and Cdc5-dependent phosphorylations. This prompted us to propose that PP2A-Cdc55 could also regulate cytokinesis by counteracting the mitotic kinases. Here, we demonstrate by in vivo and in vitro assays that PP2A-Cdc55 dephosphorylates the F-BAR protein Hof1 and the chitin synthase Chs2, two components of the Ingression Progression Complexes (IPC) involved in cytokinesis regulation. Primary septum formation and actomyosin ring contraction are impaired in absence of PP2A-Cdc55. Interestingly, the non-phosphorylable version of Chs2 rescue the asymmetric AMR contraction observed in absence of Cdc55, indicating that timely dephosphorylation of the IPC proteins by PP2A-Cdc55 is crucial for proper actomyosin ring contraction and septum formation. These findings reveal a new mechanism of cytokinesis regulation by the PP2A-Cdc55 phosphatase and extend our knowledge in the involvement of multiple phosphatases during cytokinesis.