During metaphase, forces on kinetochores are exerted by k-fibres, bundles of microtubules that end at the kinetochore. Interestingly, non-kinetochore microtubules have been observed between sister kinetochores, but their function is unknown. Here we show by laser-cutting of a k-fibre in HeLa and PtK1 cells that a bundle of non-kinetochore microtubules, which we term ‘bridging fibre', bridges sister k-fibres and balances the interkinetochore tension. We found PRC1 and EB3 in the bridging fibre, suggesting that it consists of antiparallel dynamic microtubules. By using a theoretical model that includes a bridging fibre, we show that the forces at the pole and at the kinetochore depend on the bridging fibre thickness. Moreover, our theory and experiments show larger relaxation of the interkinetochore distance for cuts closer to kinetochores. We conclude that the bridging fibre, by linking sister k-fibres, withstands the tension between sister kinetochores and enables the spindle to obtain a curved shape.

SummaryDuring cell division, mitotic spindle microtubules segregate chromosomes by exerting forces on kinetochores. What forces drive chromosome segregation in anaphase remains a central question. The current model for anaphase in human cells includes shortening of kinetochore fibers and separation of spindle poles. Both processes require kinetochores to be linked with the poles. Here we show, by combining laser ablation, photoactivation, and theoretical modeling, that kinetochores can separate without any attachment to one spindle pole. This separation requires the bridging fiber, a microtubule bundle that connects sister kinetochore fibers. Bridging fiber microtubules in intact spindles slide apart with kinetochore fibers, indicating strong crosslinks between them. We conclude that sliding of microtubules within the bridging fibers drives pole separation and pushes kinetochore fibers poleward by the friction of passive crosslinks between these fibers. Thus, sliding within the bridging fiber works together with the shortening of kinetochore fibers to segregate chromosomes.

During metaphase, chromosome position at the spindle equator is regulated by the forces exerted by kinetochore microtubules and polar ejection forces. However, the role of forces arising from mechanical coupling of sister kinetochore fibers with bridging fibers in chromosome alignment is unknown. Here we develop an optogenetic approach for acute removal of PRC1 to partially disassemble bridging fibers and show that they promote chromosome alignment. Tracking of the plus-end protein EB3 revealed longer antiparallel overlaps of bridging microtubules upon PRC1 removal, which was accompanied by misaligned and lagging kinetochores. Kif4A/kinesin-4 and Kif18A/kinesin-8 were found within the bridging fiber and largely lost upon PRC1 removal, suggesting that these proteins regulate the overlap length of bridging microtubules. We propose that PRC1-mediated crosslinking of bridging microtubules and recruitment of kinesins to the bridging fiber promotes chromosome alignment by overlap length-dependent forces transmitted to the associated kinetochore fibers.

During mitosis, tension forces acting on kinetochores are required for passage through the spindle assembly checkpoint and for chromatid segregation. It is generally thought that the interkinetochore tension is generated by molecular events occurring at the ends of k-fibers, bundles of microtubules bound to kinetochores. However, recent work has shown that bridging microtubules, which interact laterally with sister k-fibers and act as a bridge between sister kinetochores, contribute to interkinetochore tension. We set out to test the role of the bridging fibers in the origin of interkinetochore tension during metaphase in human U2OS and rat-kangaroo PtK1 cells. We severed k-fibers at different distances from the kinetochore and measured the change in the interkinetochore distance, which we use as a readout of the change in interkinetochore tension. We found that severing of a k-fiber closer to the kinetochore results in a larger relaxation of the interkinetochore tension than severing far from the kinetochore. These findings imply that a long k-fiber stub, which remains connected to the bridging fiber, withstands the interkinetochore tension and prevents relaxation of the interkinetochore distance, whereas a short stub does not. Thus, our results are consistent with a role of bridging microtubules in the origin of interkinetochore tension.

During metaphase, chromosome position at the spindle equator is mainly regulated by the forces exerted by kinetochore microtubules. However, the role of forces arising from mechanical coupling between sister kinetochore fibers and bridging fibers, whose antiparallel microtubules are crosslinked by protein regulator of cytokinesis 1 (PRC1), in chromosome alignment is unknown. Here we develop an optogenetic approach for acute removal of PRC1 and show that PRC1 promotes kinetochore alignment. PRC1 removal resulted in reduction of bridging fibers and straightening of outermost kinetochore fibers. The inter-kinetochore distance decreased, the metaphase plate widened, and lagging kinetochores appeared, suggesting a role of PRC1 in regulating forces on kinetochores. MKLP1/kinesin-6 was lost from the spindle together with PRC1, whereas Kif4A/kinesin-4 remained on chromosomes and CLASP1, Kif18A/kinesin-8, and CENP-E/kinesin-7 on kinetochore fiber tips. We conclude that in metaphase PRC1, by mechanically coupling bridging and kinetochore fibers, regulates spindle mechanics and buffers kinetochore movements, promoting chromosome alignment.