Kinesin-5/Cut7 moves bidirectionally on fission-yeast spindles with activity that increases in anaphase

Kinesin-5 motors are essential to separate mitotic spindle poles and assemble a bipolar spindle in many organisms. These motors crosslink and slide apart antiparallel microtubules via microtubule plus-end-directed motility. However, kinesin-5 localization is enhanced away from antiparallel overlaps. Increasing evidence suggests this localization occurs due to bidirectional motility or trafficking. Purified fission-yeast kinesin-5 Cut7p moves bidirectionally, but bidirectionality has not been shown in cells and the function of the minus-end-directed movement is unknown. We characterized the motility of Cut7p on bipolar and monopolar spindles and observed movement toward both plus and minus ends of microtubules. Notably, the activity of the motor increases at anaphase B onset. Perturbations to microtubule dynamics only modestly changed Cut7p movement, while Cut7p mutation reduced movement. These results suggest that the directed motility of Cut7p contributed to the movement of the motor. Comparison of Cut7 mutant and human Eg5 localization suggest a new hypothesis for the function of minus-end-directed motility and spindle-pole localization of kinesin-5s.