| Summary: | In Schizosaccharomyces pombe cytokinesis requires assembly and constriction of an actomyosin-based contractile ring (CR). Nucleation of F-actin for the CR requires a single essential formin, Cdc12, that localizes to the cell middle upon mitotic onset. The molecular mechanisms dictating its divison site targeting during cytokinesis are unknown. We defined that a Cdc12 N-terminal motif directly binds the F-BAR domain of the scaffolding protein Cdc15 and this interaction is controlled by Cdk1 phosphorylation of Cdc12. Phosphorylation of Cdc12 inhibits binding to the F-BAR Cdc15. cdc12 alleles that cannot bind Cdc15 or with all six Cdk1 sites mutated to phospho-mimetic residues show reduced Cdc12 cell division site accumulation and delayed CR formation. Thus Cdk1 phosphorylation of Cdc12 antagonizes its interaction with Cdc15 and its division site localization, consistent with a general role for Cdk1 in inhibiting cytokinesis until chromosome segregation is complete.
The CR is physically linked to the plasma membrane (PM). Cells lacking efr3, which encodes a conserved PM scaffold for the phosphatidylinositol 4-kinase Stt4, build CRs that can slide away from the cell middle in a myosin-V-dependent manner. The Efr3-dependent CR anchoring mechanism is distinct from previously reported pathways dependent on the F-BAR protein Cdc15 and paxillin. In efr3â, the concentrations of several membrane-binding proteins were reduced in the CR and/or on the PM. Our results suggest that proper PM lipid composition is important to stabilize the central position of the CR and resist myosin V-based forces to promote the fidelity of cell division.
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