- The bipolar kinesin-5 Cin8 switches from minus- to plus-end-directed motility under various conditions in vitro The mechanism and physiological significance of this switch remain unknown. Here we show that under high ionic strength conditions, Cin8 moves towards and concentrates in clusters at the minus ends of stable and dynamic microtubules. Clustering of Cin8 induces a switch from fast minus- to slow plus-end-directed motility and forms sites that capture antiparallel MTs and induces their sliding-apart by plus-end-directed motility. In early mitotic cells with monopolar spindles, Cin8 localizes near the spindle poles at microtubule minus-ends. This localization is dependent on the minus-end-directed motility of Cin8. In cells with assembled bipolar spindles, Cin8 is distributed along the spindle microtubules. We propose that minus-end-directed motility is required for Cin8 clustering near the spindle poles prior to spindle assembly. Cin8 clusters promote capturing of microtubules emanating from the neighboring spindle poles and mediate their antiparallel sliding. This activity is essential to maximize microtubule crosslinking prior to bipolar spindle assembly and to provide the initial separation of the spindle poles.