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 ΔPP1^dis2 genetic background in Schizosaccharomyces pombe identified phosphorylation of T73 or T75 in the regulatory B56^Par1 subunit. The B56^Par1.T73T75 phosphorylation is directly mediated by CDK1-Cyclin B, and a phospho-mimetic mutation increased while a non-phosphorylatable mutation reduced PP2A-B56^Par1 phosphatase activity. Blocking B56^Par1.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.