CDC14 Coordinates Cyclin Destruction with the Onset of Cytokinesis
Bembenek, Joshua Nathaniel
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The Cdc14 family of protein phosphatases operate during the final stages of mitosis in various organisms. The Cdc14 phosphatases are downstream components of two homologous signaling pathways: the mitotic exit network (MEN) of S. cerevisiae and septation initiation network (SIN) of S. pombe. Studies of these pathways have revealed divergent roles of Cdc14. In the MEN pathway, Cdc14 is required for cyclin degradation by dephosphorylating Cdh1. The dephosphorylated form of Cdh1 binds to and activates a ubiquitin ligase known as the anaphase-promoting complex (APC/C), which then ubiquitinates mitotic cyclins, targeting them for degradation by the 26S proteosome. In contrast, Cdc14 of the SIN is dispensable for cyclin degradation, but plays an important role during cytokinesis. Two Cdc14 homologues are found in vertebrates, hCdc14A and hCdc14B. I have investigated the regulation of Cdc14 phosphatases to obtain insights into the mechanisms of mitotic exit in higher eukaryotes. Biochemical studies demonstrate that recombinant hCdc14A and hCdc14B can dephosphorylate human Cdh1 and stimulate APC/CCdh1 ligase activity in vitro. Since both the MEN and SIN pathways control Cdc14 localization, I have examined the regulation of the subcellular localization of hCdc14A, hCdc14B and the budding yeast Cdc14. In HeLa cells, hCdc14A localizes to the centrosome whereas hCdc14B is nucleolar during interphase. Both hCdc14 homologues localize to the centrosome and midbody during mitosis. In budding yeast, Cdc14p localizes to the nucleolus during most of the cell cycle and is released in late anaphase when it localizes to the centrosome and the bud neck. The subcellular localization the Cdc14 homologues in HeLa cells is regulated by a nuclear export signal. S. cerevisiae strains carrying only NES mutant CDC14 alleles are capable of degrading mitotic cyclins and escaping mitosis. However, they exhibit a temperature-sensitive phenotype at 37°C because they fail to complete cytokinesis and lack centrosome and bud neck localization of Cdc14. This demonstrates that the Cdc14 phosphatases are regulated by nucleocytoplasmic shuttling. Collectively, my work strongly suggests that the Cdc14 phosphatases play a conserved role in coordinating the destruction of mitotic cyclins with the execution of cytokinesis.