The Kin1 kinase and the calcineurin phosphatase cooperate to link actin ring assembly and septum synthesis in fission yeast

Background information The Kin1 protein kinase of fission yeast, which regulates cell surface cohesiveness during interphase cell growth, is also present at the cell division site during mitosis; however, its function in cell division has remained elusive. Results In FK506‐mediated calcineurin deficient cells, mitosis is extended and ring formation is transiently compromised but septation remains normal. Here, we show that Kin1 inhibition in these cells leads to polyseptation and defects in membrane closure. Actomyosin ring disassembly is prevented and ultimately the daughter cells fail to separate. We show that the Pmk1 MAP kinase pathway and the type V myosin Myo4 act downstream of the cytokinetic function of Kin1. Kin1 inhibition also promotes polyseptation in myo3Δ, a type II myosin heavy‐chain mutant defective in ring assembly. In contrast, Kin1 inactivation rescues septation in a myosin light‐chain cdc4–8 thermosensitive mutant. A structure/function analysis of the Kin1 protein sequence identified a novel motif outside the kinase domain that is important for its polarised localisation and its catalytic activity. This motif is remarkably conserved in all fungal Kin1 homologues but is absent in related kinases of metazoans. Conclusions We conclude that calcineurin and Kin1 activities must be tightly coordinated to link actomyosin ring assembly with septum synthesis and membrane closure and to ensure separation of the daughter cells. In fission yeast, the function of the Kin1 protein kinase in cell division has remained elusive. In calcineurin phosphatase deficient cells, mitosis is extended and ring formation is transiently compromised but ultimately cell division is completed. Inhibition of Kin1 in these cells impinges on cytokinesis, preventing separation of the daughter cells. Thus, calcineurin and Kin1 activities must be tightly coordinated to link actin ring structure to septation at the end of the cell cycle in fission yeast.