During mitosis both kinesin-12 (Kif15) and kinesin-5 (Eg5) protein motors play important roles in assembling the bipolar spindle, a structure made up of microtubules (MTs) that drives chromosome separation. In mammals, the predominant motor Eg5 functions by sliding two antiparallel MTs apart (1, 2). However, previous research has revealed the presence of cell cycle mechanisms that do not require Eg5 (1). In both Dictyostelium and C. elegans, for instance, spindle assembly does not involve any motor from the kinesin-5 family; instead, a kinesin-12 acts predominantly (1). Furthermore, when Eg5 is inhibited, Kif15 is able to take over all of the necessary functions to drive centrosome separation (1, 2). These data have led some researchers to hypothesize that Kif15 acts redundantly with Eg5 (1). Yet in a recent study Sturgill and Ohi demonstrate how these two motors are different in both localization and function during bipolar spindle assembly. As the exact mechanism of Kif15 has yet to be reported, this study is significant in that it not only proposes a possible mechanism by which Kif15 produces force and mediates spindle assembly, but it also demonstrates how the functionality of Kif15 differs from that of Eg5.
To first determine whether Kif15 and Eg5 differ in localization, Sturgill and Ohi generated antibodies against the C-terminus of Kif15 and used immunostaining to monitor localization at various mitosis stages in HeLa cells (1). Kif15 localized to kinetochore microtubules (K-MTs) and levels increased as mitosis proceeded (1). Eg5, though, localized to centrosome-nucleated MTs prior to nuclear envelope breakdown (NEB). Together, these data suggest the importance of Eg5 during early spindle assembly and that Kif15 becomes necessary after NEB. In addition to temporal differences, spatial differences between the motors also exist. While Kif15 is distributed in a uniform manner along spindle MTs during metaphase, Eg5 is abundant on the spindle poles (1).
The researchers next determined which MTs Kif15 and Eg5 act upon. Essential K-MT protein Nuf2 was depleted to impair motor function (1). Indicated by a low monopolar index, bipolar spindle assembly was greatly reduced (1). When both Kif15 and Nuf2 were depleted, similar results were obtained—demonstrating that Kif15 functions specifically on K-MTs. Yet when Eg5 was inhibited with S-trityl-L-cysteine, Nuf2-depleted spindles collapsed (1). Therefore, Eg5 activity on non-K-MT spindles is important in spindle bipolarity. Preformed bipolar spindles were retained, however, which reveals that mature kinetochore fibers (K-fibers) do not require Eg5 to maintain bipolarity (1).
In order to challenge the notion that Kif15 is redundant, specific functions of Kif15 must be determined. Accordingly, the researchers measured spindle length in cells depleted of Kif15, Nuf2, or both to determine Kif15’s interaction with K-fibers (1). Results indicate that Kif15 influences K-fiber length in a...