The 14-3-3 protein Rad24p modulates function of the Cdcl4p family phosphatase Clp1p/Flp1p in fission yeast

Schizosaccharomyces pombe cells divide through the use of an actomyosin-based contractile ring. In response to perturbation of the actomyosin ring, S. pombe cells delay in a "cytokinesis-competent" state characterized by continuous repair and maintenance of the actomyosin ring [1] and a G2 delay [2, 3]. This checkpoint mechanism requires the function of the Cdc14p-family phosphatase Clp1p/Flp1p [4, 5] and the septation initiation network (SIN) [1-3]. In response to cytokinetic defects, Clp1p, normally nucleolar in interphase, is retained in the cytoplasm until completion of cell division in a SIN-dependent manner [1, 4, 5]. Here, we show that a phosphorylated form of Clp1p binds the 14-3-3 protein Rad24p and is retained in the cytoplasm in a Rad24p-dependent manner in response to cytokinesis defects. This physical interaction depends on the function of the SIN component, Sid2p. In the absence of Rad24p, cells are unable to maintain SIN signaling and lose viability upon mild cytokinetic stress. The requirement of Rad24p in this checkpoint is bypassed by ectopic activation of the SIN. Furthermore, SIN-dependent nuclear exclusion of Clp1p is dependent on Rad24p function. We conclude that Rad24p-mediated cytoplasmic retention of Clp1p/Flp1p is important for cell viability upon stress to the division apparatus.