REGULATION OF CHROMOSOME SEGREGATION BY CONSERVED PHOSPHATASE CDC14 AND KINASE CDC5

The faithful transmission of the replicated genome from the mother to the daughter cell requires the correct establishment of linkages between the duplicated chromosomes (sister chromatids) and their bi-orientation on the mitotic spindle. Chromosome segregation initiates only after each sisters pair is correctly aligned onto the microtubules emanating from the spindle poles. Next, Esp1-mediated cleavage of cohesin is required to trigger anaphase onset while the physical segregation of the separated sisters is next driven by spindle activity. However, this scenario appears to be more complicated involving additional factors driving the sister chromatid segregation process (i.e. the Top2-mediated resolution of replication catenates). In budding yeast, anaphase progression and exit from mitosis require the protein phosphatase Cdc14 whose activation relies on two consecutive protein pathways, the FEAR network and the MEN. As the polo-like kinase Cdc5 is a component of both pathways its activity is essential to Cdc14 release and in its absence Cdc14 is never released. By combining loss-of-function alleles of Cdc5 and Cdc14 we obtained double mutant cells that had cohesin cleaved but still arrested with undivided nuclei and short bipolar spindles. Anaphase spindle elongation initiates quickly after cohesin removal (anaphase A) and then switches to a slower elongation rate (anaphase B) due to changes in spindle behaviour mediated by motor proteins and microtubule-associated enzymes. Although some residual cohesion between sister chromatids seems to contribute to the terminal phenotype of cdc14 cdc5 cells, our data indicate that anaphase B is the main mitotic defect of these cells. We conclude that Cdc5 and Cdc14 are redundantly involved in activating spindle activity following cohesion resolution, suggesting the existence of a regulatory network that coordinates sister chromatid separation with spindle elongation after cohesin cleavage. Importantly, we identified the motor protein Cin8 as a (direct or indirect) target of Cdc5 in the regulation of spindle elongation.