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Yeast kinetochore protein Slk19 is required for kinetochore clustering, and nocodazole exposure to slk19 mutant cells causes impaired kinetochore capture and delayed chromosome bipolar attachment after nocodazole washout.

The spindle assembly checkpoint (SAC) monitors chromosome attachment defects and the assembly of SAC proteins at kinetochores is essential for its activation, but the SAC disassembly process remains unknown. We found that deletion of a 14-3-3 protein, Bmh1, or hyper-activation of FEAR (Cdc14 Early Anaphase Release) allows premature SAC silencing in budding yeast, which depends on a kinetochore protein Fin1 that forms a complex with protein phosphatase PP1. Previous works suggest that FEAR-dependent Fin1 dephosphorylation promotes Bmh1-Fin1 dissociation, which enables kinetochore recruitment of Fin1-PP1. We found persistent kinetochore association of SAC protein Bub1 in fin1Δ mutants after anaphase entry. Therefore, we revealed a mechanism that clears SAC proteins from kinetochores. After anaphase entry, FEAR activation promotes kinetochore enrichment of Fin1-PP1, resulting in SAC disassembly at kinetochores. This mechanism is required for efficient SAC silencing after SAC being challenged, and untimely Fin1-kinetochore association causes premature SAC silencing and chromosome missegregation.

Because cohesion prevents sister-chromatid separation and spindle elongation, cohesion dissolution may trigger these two events simultaneously. However, the relatively normal spindle elongation kinetics in yeast cohesin mutants indicates an additional mechanism for the temporal control of spindle elongation. Here we show evidence indicating that S-phase CDK (cyclin dependent kinase) negatively regulates spindle elongation. In contrast, mitotic CDK promotes spindle elongation by activating Cdc14 phosphatase, which reverses the protein phosphorylation imposed by S-phase CDK. Our data suggest that S-phase CDK negatively regulates spindle elongation partly through its phosphorylation of a spindle pole body (SPB) protein Spc110. We also show that hyperactive S-phase CDK compromises the microtubule localization of Stu2, a processive microtubule polymerase essential for spindle elongation. Strikingly, we found that hyperactive mitotic CDK induces uncoupled spindle elongation and sister-chromatid separation in securin mutants (pds1Δ), and we speculate that asynchronous chromosome segregation in pds1Δ cells contributes to this phenotype. Therefore, the tight temporal control of spindle elongation and cohesin cleavage assure orchestrated chromosome separation and spindle elongation.

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