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Abstract:
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The Sli15 -Ipl1 -Bir1 chromosomal passenger complex is essential for proper kinetochore -microtubule attachment and spindle stability in the budding yeast Saccharomyces cerevisiae . Subcellular localization of this complex during anaphase is regulated by the Cdc14 protein phosphatase , which is kept inactive in the nucleolus until anaphase onset . I show here that the predominantly nucleolar ribosome biogenesis protein Utp7 is also present at kinetochores and is required for normal organization of kinetochore proteins and proper chromosome segregation . Utp7 associates with and regulates the localization of Sli15 and Cdc14 . It prevents the abnormal localization of Sli15 on cytoplasmic microtubules , the premature concentration of Sli15 on the pre -anaphase spindle , and the premature nucleolar release of Cdc14 before anaphase onset . Utp7 regulates Sli15 localization not entirely through its effect on Cdc14 . Furthermore , the mitotic exit block caused by Cdc14 inactivation is relieved partially by the simultaneous inactivation of Utp7 . Thus , Utp7 is a multifunctional protein that plays essential roles in the vital cellular processes of ribosome biogenesis , chromosome segregation and cell cycle control . Protein phosphatase 1 , Glc7 opposes in vivo functions of the Ipl1 -Sli15 -Bir1 kinase complex in budding yeast . I show here Scd5 - a targeting subunit of Glc7 that regulates endocytosis /cortical actin organization and undergoes nuclear -cytoplasmic shuttling - is present at kinetochores . Ipl1 associates with both Glc7 and Scd5 . The scd5 -PP1[Delta]2 mutation , which disrupts the association between Glc7 and Scd5 , also disrupts the association between Ipl1 and Scd5 -Glc7 without affecting the kinetochore localization of these proteins . Genetic studies suggest that Scd5 may positively regulate both Glc7 phosphatase and the Ipl1 kinase complex . In accordance , Scd5 stimulates in vitro kinase activity of Ipl1 . scd5 -PP1[Delta]2 cells missegregate chromosomes severely due to several defects : i ) at least one of sister kinetochores appears not attached to microtubule . ii ) sister chromatids are persistently cohesed through anaphase . iii ) Sli15 is hyperphosphorylated and less abundant on the anaphase spindle resulting in unstable mitotic spindle . These results together suggest that Scd5 functions in diverse processes that are essential for faithful chromosome segregation . How Scd5 coordinately regulates two apparently antagonistic enzymatic activities of Ipl1 and Glc7 remains to be determined . |