Accurate nuclear position is essential for each daughter cell to receive one DNA complement. In budding yeast, a surveillance mechanism known as the spindle position checkpoint ensures that exit from mitosis only occurs when the anaphase nucleus is positioned along the mother-bud axis. We identified the protein kinase Kin4 as a component of the spindle position checkpoint. KIN4 prevents exit from mitosis in cells with mispositioned nuclei by inhibiting the mitotic exit network (MEN), a GTPase signaling cascade that promotes exit from mitosis. Kin4 is active in cells with mispositioned nuclei and predominantly localizes to mother cells, where it is ideally situated to inhibit MEN signaling at spindle pole bodies (SPBs) when anaphase spindle elongation occurs within the mother cell.
Kinetochores of sister chromatids attach to microtubules emanating from the same pole (coorientation) during meiosis I and microtubules emanating from opposite poles (biorientation) during meiosis II. We find that the Aurora B kinase Ipl1 regulates kinetochore-microtubule attachment during both meiotic divisions and that a complex known as the monopolin complex ensures that the protein kinase coorients sister chromatids during meiosis I. Furthermore, the defining of conditions sufficient to induce sister kinetochore coorientation during mitosis provides insight into monopolin complex function. The monopolin complex joins sister kinetochores independently of cohesins, the proteins that hold sister chromatids together. We propose that this function of the monopolin complex helps Aurora B coorient sister chromatids during meiosis I.
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