Yeast Kinesin-5 Motor Protein CIN8 Promotes Accurate Chromosome Segregation.

Accurate chromosome segregation depends on bipolar chromosome-microtubule attachment and tension generation on chromosomes. Incorrect chromosome attachment results in chromosome missegregation, which contributes to genome instability. The kinetochore is a protein complex that localizes at the centromere region of a chromosome and mediates chromosome-microtubule interaction. Incorrect chromosome attachment leads to checkpoint activation to prevent anaphase onset. Kinetochore detachment activates the spindle assembly checkpoint (SAC), while tensionless kinetochore attachment relies on both the SAC and tension checkpoint. In budding yeast , kinesin-5 motor proteins Cin8 and Kip1 are needed to separate spindle pole bodies for spindle assembly, and deletion of causes lethality in the absence of SAC. To study the function of Cin8 and Kip1 in chromosome segregation, we constructed an auxin-inducible degron (AID) mutant, . With this conditional mutant, we first confirmed that double mutants were lethal when Cin8 is depleted in the presence of auxin. These cells arrested in metaphase with unseparated spindle pole bodies and kinetochores. We further showed that the absence of either the SAC or tension checkpoint was sufficient to abolish the cell-cycle delay in mutants, causing chromosome missegregation and viability loss. The tension checkpoint-dependent phenotype in cells with depleted Cin8 suggests the presence of tensionless chromosome attachment. We speculate that the failed spindle pole body separation in mutants could increase the chance of tensionless syntelic chromosome attachments, which depends on functional tension checkpoint for survival.