The Kinesin-12 Kif15 is a processive track-switching tetramer.

Kinesin-12 motors are a little studied branch of the kinesin superfamily with the human protein (Kif15) implicated in spindle mechanics and chromosome movement. In this study, we reconstitute full-length hKif15 and its microtubule-targeting factor hTpx2 in vitro to gain insight into the motors mode of operation. We reveal that hKif15 is a plus-end-directed processive homotetramer that can step against loads of up to 3.5 pN. We further show that hKif15 is the first kinesin that effectively switches microtubule tracks at intersections, enabling it to navigate microtubule networks, such as the spindle. hKif15 tetramers are also capable of cross-linking microtubules, but unexpectedly, this does not depend on hTpx2. Instead, we find that hTpx2 inhibits hKif15 stepping when microtubule-bound. Our data reveal that hKif15 is a second tetrameric spindle motor in addition to the kinesin-5 Eg5 and provides insight into the mechanisms by which hKif15 and its inhibitor hTpx2 modulate spindle microtubule architecture. DOI: http://dx.doi.org/10.7554/eLife.01724.001.