FIGURE 19-32 The spindle position checkpoint pathway in budding yeast. Cdc14 phosphatase activity is required for exit from mitosis. Top: In S. cerevisiae, during interphase and early mitosis, Cdc14 (red dots) is sequestered and inactivated in the nucleolus. Inactive Tem1-GDP (purple) associates with the spindle pole body (SPB) nearest to the bud as soon as the mitotic spindle forms. If chromosome segregation occurs properly step 1, extension of the spindle microtubules inserts the daughter SPB into the bud, causing Tem1 to be activated by an unknown mechanism. Tem1-GTP activates a protein kinase cascade, which then promotes the release of active Cdc14 from the nucleolus and exit from mitosis. If the spindle apparatus fails to place the daughter SPB in the bud step 2, Kin4 (cyan), an inhibitor of Tem1, is recruited from the parent cell cortex to the parent-cell-located SPB and maintains Tem1 in the GDP-bound form, and mitotic exit does not occur. Lte1 (orange) is an inhibitor of Kin4 and is localized to the bud. Lte1 prevents Kin4 that leaks into the bud from inhibiting Tem1. If the checkpoint fails step 3, cells with mispositioned spindles inappropriately exit mitosis and produce anucleate and multinucleate cells.