A Highlights from MBoC Selection: Cdk1 promotes cytokinesis in fission yeast through activation of the septation initiation network

In Schizosaccharomyces pombe, late mitotic events are coordinated withcytokinesis by the septation initiation network (SIN), an essential spindle pole body(SPB)–associated kinase cascade, which controls the formation, maintenance, and constrictionof the cytokinetic ring. It is not fully understood how SIN initiation is temporally regulated, butit depends on the activation of the GTPase Spg1, which is inhibited during interphase by theessential bipartite GTPase-activating protein Byr4-Cdc16. Cells are particularly sensitive to themodulation of Byr4, which undergoes cell cycle–dependent phosphorylation presumed toregulate its function. Polo-like kinase, which promotes SIN activation, is partially responsible forByr4 phosphorylation. Here we show that Byr4 is also controlled by cyclin-dependent kinase(Cdk1)–mediated phosphorylation. A Cdk1 nonphosphorylatable Byr4 phosphomutant displayssevere cell division defects, including the formation of elongated, multinucleate cells, failure tomaintain the cytokinetic ring, and compromised SPB association of the SIN kinase Cdc7. Our analysesshow that Cdk1-mediated phosphoregulation of Byr4 facilitates complete removal of Byr4 frommetaphase SPBs in concert with Plo1, revealing an unexpected role for Cdk1 in promoting cytokinesisthrough activation of the SIN pathway.