CDK1-Cyclin B mediated phosphorylation of PP2A-B56 on B56Par1.T73T75 controls mitotic entry timing and Schizosaccharomyces pombe cell size

Mitosis is triggered when the rising activity of CDK1-Cyclin B, amplified by the CDK1/Cdc25/Wee1 feedback loop, overcomes inhibitory signalling from Wee1 and counteracting phosphatases. CDK-opposing phosphatases PP1, PP2A-B55 and PP2A-B56 are regulators of mitosis. A screen for differentially phosphorylated sites in a ΔPP1dis2 genetic background in Schizosaccharomyces pombe identified phosphorylation of T73 or T75 in the regulatory B56Par1 subunit. The B56Par1.T73T75 phosphorylation is directly mediated by CDK1-Cyclin B, and a phospho-mimetic mutation increased while a non-phosphorylatable mutation reduced PP2A-B56Par1 phosphatase activity. Blocking B56Par1.T73T75 phosphorylation reduced cell length in unperturbed divisions from 14 to 12 µm, without causing pleiotropic defects. Therefore, blocking phosphorylation at T73T75 alone prematurely unlocked amplification of the CDK1/Cdc25/Wee1 feedback loop, advancing cells into mitosis. Signalling from T73T75 reveals for the first time that timely mitotic commitment in unperturbed cycles is mediated by PP2A-B56.