Spindle bipolarity relies on a fine balance of forces exerted by various molecular motors [1-4]. In most animal cells, spindle bipolarity requires sustained outward forces to push the spindle poles apart, an activity that is provided by Eg5, a conserved homotetrameric plus-end-directed kinesin that crosslinks and slides antiparallel microtubules apart [5]. These pushing forces are balanced by inward minus-end-directed forces. Impairing both Eg5 and dynein restores the formation of functional bipolar spindles [4], although the mechanism at play is far from clear. The current model also fails to explain why in some systems Eg5 inhibition does not promote bipolar spindle collapse [6, 7] or why increasing Eg5 levels does not interfere with bipolar spindle assembly [8]. Moreover, the C. elegans Eg5 ortholog is not required for bipolar spindle formation [9]. We show here that the kinesin Hklp2 participates in the assembly and stabilization of the bipolar spindle. Hklp2 localizes to the mitotic microtubules in a TPX2-dependent manner and to the chromosomes through Ki67. Our data indicate that its mechanism of action is clearly distinct from and complementary to that of Eg5, providing an additional understanding of the mechanism driving the formation and maintenance of the bipolar spindle.

The bipolar spindle is a highly dynamic structure that assembles transiently around the chromosomes and provides the mechanical support and the forces required for chromosome segregation. Spindle assembly and chromosome movements rely on the regulation of microtubule dynamics and a fine balance of forces exerted by various molecular motors. Chromosomes are themselves central players in spindle assembly. They generate a RanGTP gradient that triggers microtubule nucleation and stabilization locally and they interact dynamically with the microtubules through motors targeted to the chromatin. We have previously identified and characterized two of these so-called chromokinesins: Xkid (kinesin 10) and Xklp1 (kinesin 4). More recently, we found that Hklp2/kif15 (kinesin 12) is targeted to the chromosomes through an interaction with Ki-67 in human cells and is therefore a novel chromokinesin. Hklp2 also associates with the microtubules specifically during mitosis, in a TPX2 (targeting protein for Xklp2)-dependent manner. We have shown that Hklp2 participates in spindle pole separation and in the maintenance of spindle bipolarity in metaphase. To better understand the function of Hklp2, we have performed a detailed domain analysis. Interestingly, from its positioning on the chromosome arms, Hklp2 seems to restrict spindle pole separation. In the present review, we summarize the current knowledge of the function and regulation of the different kinesins associated with chromosome arms during cell division, including Hklp2 as a novel member of this so-called chromokinesin family.