The bipolar mitotic spindle is organised through microtubule-associated proteins, including motor proteins. The kinesin-5 motor family is required for the separation of centrosomes or spindle pole bodies to set up and maintain spindle bipolarity. Kinesin-5 crosslinks and slides apart antiparallel microtubules through its plus-end-directed motility. However, the Saccharomyces cerevisiae kinesin-5 Cin8 displays minus-end motility on single microtubules and has the capacity to switch to a plus-end-directed mode; but neither the mechanism of minus-end motility for Cin8 nor that of the switch are currently understood. In this issue (p. 725), Larisa Gheber, Jawdat Al-Bassam and colleagues examine the physiological functions of Cin8 motility in spindle assembly. They show that the movement and subsequent clustering of Cin8 at minus ends of stable and dynamic microtubules induces a switch of directionality together with a change from a fast minus-end to a slow plus-end motility. This now allows Cin8 to capture antiparallel microtubules and slide them apart, thereby promoting separation of spindle pole bodies and set-up of bipolarity of the mitotic spindle. Accordingly, the authors find Cin8 at spindle poles on microtubule minus-ends in early mitosis as well as along spindle microtubules once bipolar spindles are assembled. Overall, this work provides evidence that Cin8 clustering influences its motile properties and directionality, which underlie kinesin-5 function in bipolar mitotic spindle assembly.
- © 2017. Published by The Company of Biologists Ltd