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.
Chromosome alignment at the spindle equator during metaphase is the most remarkable feature of mitosis, which promotes proper chromosome segregation and depends on the forces exerted at the plus end of kinetochore microtubules and polar ejection forces. However, forces arising from lateral mechanical coupling of kinetochore fibers with non-kinetochore microtubules play a role in chromosome alignment, but the mechanism is unclear. Here we develop a speckle microscopy assay to measure the poleward flux of individual microtubules in spindles of human cells and show that bridging microtubules slide apart and undergo poleward flux at a higher rate than kinetochore microtubules. Depletion of the microtubule coupler NuMa increased the difference in the flux velocity of kinetochore and bridging microtubules, suggesting that sliding forces from the bridging fiber are transmitted largely through NuMa onto the associated kinetochore fibers. Depletions of Kif18A/kinesin-8, Kif4A/kinesin-4, as well as double depletions of Kif18A together with Kif4A or Kif18A together with the crosslinker of antiparallel microtubules PRC1 increased the flux velocity of kinetochore fibers up to the velocity of bridging fibers, due to the increased coupling resulting from the extended antiparallel overlap regions. We found severe kinetochore misalignment after double Kif18A and Kif4A as well as Kif18A and PRC1 depletions compared to a single Kif18A depletion, suggesting that forces within the bridging fiber have a centering effect on the kinetochores. We propose that lateral length-dependent sliding forces that the bridging fiber exerts onto kinetochore fibers drive the movement of kinetochores towards the spindle center, thereby promoting chromosome alignment.
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.
During metaphase, chromosomes are aligned in a lineup at the equatorial plane of the spindle to ensure synchronous poleward movement of chromatids in anaphase and proper nuclear reformation at the end of mitosis. Chromosome alignment relies on microtubules, several types of motor protein and numerous other microtubule-associated and regulatory proteins. Because of the multitude of players involved, the mechanisms of chromosome alignment are still under debate. Here, we discuss the current models of alignment based on poleward pulling forces exerted onto sister kinetochores by kinetochore microtubules, which show length-dependent dynamics and undergo poleward flux, and polar ejection forces that push the chromosome arms away from the pole. We link these models with the recent ideas based on mechanical coupling between bridging and kinetochore microtubules, where sliding of bridging microtubules promotes overlap length-dependent sliding of kinetochore fibers and thus the alignment of sister kinetochores at the spindle equator. Finally, we discuss theoretical models of forces acting on chromosomes during metaphase. ll
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