Centering of kinetochores is promoted by length-dependent pulling forces exerted by bridging microtubules*

The mitotic spindle, by exerting forces, segregates chromosomes into two daughter cells during cell division. During metaphase, chromosomes are positioned in the spindle equatorial plane, which is necessary to prevent lagging chromosomes and abnormal nuclear envelope reformation. It has been proposed that two centering mechanisms play a key role here, microtubule catastrophe promoted by kinesin-8 motors and pushing forces exerted by chromokinesins. Here we show, by combining a theoretical model and quantitative experiments, that kinetochore microtubules cross-linked by bridging microtubules exert length-dependent centering pulling forces. The signature of this centering mechanism is larger poleward flux velocities of bridging fiber as compared to the velocities of kinetochore fiber, which we confirmed in a preliminary experiment. In conclusion, we propose that antiparallel overlaps exert length-dependent forces on kinetochores to navigate their positioning in the center of the metaphase plate.