Mitotic entry is controlled by the plant-specific phosphatase BSL1 and cyclin-dependent kinase B.

Cell cycle regulation is well understood in opisthokonts (fungi and metazoans) but not in plants or Apicomplexa, as some cell cycle regulators are not conserved. In opisthokonts, cell cycle progression requires the dephosphorylation of cyclin-dependent kinase (CDK) by the CDC25 phosphatase. Plants have no CDC25, and thus their mechanisms of cell cycle regulation remain elusive. Here we show that the BSL1 phosphatase dephosphorylates CDKB1 to promote mitotic entry in Chlamydomonas. Alterations of BSL1 or CDKB1 block mitotic entry after DNA replication. BSL1 shows dynamic localization through the cell cycle at the basal bodies, spindle poles and cleavage furrow. CDKB1 is hyperphosphorylated at the Thr14 and Tyr15 residues in the bsl1 mutant and in wild-type cells treated with DNA replication inhibitors. BSL1 binds to CDKB1 and dephosphorylates CDKB1 pThr14/pTyr15 in vitro. Phospho-mimicking alterations of Thr14/Tyr15 inactivate CDKB1 function, whereas phospho-blocking alterations cause sensitivity to DNA replication inhibitors, which delay cytokinesis in wild-type cells more than in cells expressing unphosphorylatable mutant CDKB1. These results indicate that CDKB1 Thr14 and Tyr15 are phosphorylated to block mitotic entry before DNA replication is complete, and BSL1 dephosphorylates CDKB1 to promote mitosis. Our study demonstrates that BSL1, a phosphatase conserved in plants and Apicomplexa but absent in fungi and animals, is a CDKB1-activating mitosis-promoting factor that has evolved additional signalling functions in receptor kinase pathways in higher plants.