The Molecular Function of the Yeast Polo-like Kinase Cdc5 in Cdc14 Release during Early Anaphase

In the budding yeast Saccharomyces cerevisiae, Cdc14 is sequestered within the nucleolus before anaphase entry through its association with Net1/Cfi1, a nucleolar protein. Protein phosphatase PP2ACdc55 dephosphorylates Net1 and keeps it as a hypophosphorylated form before anaphase. Activation of the Cdc fourteen early anaphase release (FEAR) pathway after anaphase entry induces a brief Cdc14 release from the nucleolus. Some of the components in the FEAR pathway, including Esp1, Slk19, and Spo12, inactivate PP2ACdc55, allowing the phosphorylation of Net1 by mitotic cyclin-dependent kinase (Cdk) (Clb2-Cdk1). However, the function of another FEAR component, the Polo-like kinase Cdc5, remains elusive. Here, we show evidence indicating that Cdc5 promotes Cdc14 release primarily by stimulating the degradation of Swe1, the inhibitory kinase for mitotic Cdk. First, we found that deletion of SWE1 partially suppresses the FEAR defects in cdc5 mutants. In contrast, high levels of Swe1 impair FEAR activation. We also demonstrated that the accumulation of Swe1 in cdc5 mutants is responsible for the decreased Net1 phosphorylation. Therefore, we conclude that the down-regulation of Swe1 protein levels by Cdc5 promotes FEAR activation by relieving the inhibition on Clb2-Cdk1, the kinase for Net1 protein.