Abstract
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
Before a cell can divide, it produces an extra copy of all its chromosomes, and it must then ensure that each daughter cell ends up with one copy of each chromosome. During the division process, a structure called the spindle forms in the cell. This spindle is made of thread-like extensions called microtubules that grow from two poles at opposite ends of the cell. These microtubules are responsible for getting the chromosomes to line up in the middle of the cell, and then pulling half of the chromosomes to one end of the cell, and half to the other end. The cell then divides into two daughter cells.
Two motor proteins—so-called because they consume chemical energy to ‘walk’ along the microtubules—have important roles in this process: Kif11 motor proteins mainly drive the formation of the spindle and thus division of the chromosomes. A cell that does not contain Kif11 can only divide if it contains extra copies of a second motor protein called Kif15: this suggests that Kif15 can serve as some sort of back up for Kif11.
Normal cells only divide when new cells are needed for growth or to replace old cells that have died. Cancer cells, on the other hand, divide in a way that is not controlled. Drugs that interfere with Kif11 have been developed in the hope that they will stop cancer cells dividing, but these drugs have not been very effective in clinical tests, possibly due to the Kif15 back up. Scientists hope, therefore, that a better understanding of the role of Kif15 may lead to improved cancer treatments.
Drechsler et al. have isolated individual Kif15 motor proteins and used advanced microscopy techniques to study them in action. These experiments showed that Kif15 motor proteins can travel long distances along a single microtubule, and can also switch to a different microtubule at intersections. This movement of Kif15 is stopped when they bump into Tpx2 proteins, which are sitting on the microtubules. Together, these proteins can also form links between microtubules that can withstand high forces. These properties provide a starting point to understand how Kif15 can act as a back up for Kif11 in cells. In the future, it will be important to work out how Kif11 and Kif15 motor proteins work together in teams to build the spindle.
DOI:http://dx.doi.org/10.7554/eLife.01724.002
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