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New forces in cell division

An international team led by Nenad Pavin and Iva Tolić, leader and co-leader of the UKF project "The role of microtubule pivoting in formation of complex structures such as microtubule bundles and mitotic spindles", has discovered a novel structure that is important for cell division.

This work, which changes the current paradigm regarding the force balance in mitotic spindles, is published in the prestigious journal Nature Communications. This interdisciplinary study, combines cell biology and genetic with theoretical models, live-cell imaging, laser microsurgery and computer science.
At the onset of division, the cell forms a spindle, a precise self-constructed micro-machine based on microtubules, which divides the chromosomes between the two nascent daughter cells. The attachment of microtubules to chromosomes is mediated by kinetochores, which are protein complexes on the chromosome. The central question is how pulling forces on kinetochores are generated. According to the current paradigm, the forces on kinetochores are produced by k-fibers, bundles of microtubules extending between the spindle pole and the kinetochore. In this study, the authors show that a bundle of non-kinetochore microtubules, termed ‘bridging fiber’, bridges sister k-fibers and balances the tension between sister kinetochores. A strong connection between the bridging fiber and sister k-fibers is shown in elegant experiments, by cutting a k-fiber with a laser, after which the bridging fiber moves together with the k-fibers and kinetochores. By combining a novel model with experiments, the authors have shown a larger relaxation of the inter-kinetochore tension for cuts closer to the kinetochore, implying that the bridging fiber, by linking sister k-fibers, withstands the tension between sister kinetochores. The concepts introduced in this study challenge the current paradigm that the tension on kinetochores is generated by molecular events occurring only at the ends of the k-fiber. Thus, models of forces acting on kinetochores should be extended to include forces exerted along the k-fiber in addition to the forces at the k-fiber ends.
Original paper:
Kajtez, Janko; Solomatina, Anastasia; Novak, Maja; Polak, Bruno; Vukušić, Kruno; Rüdiger, Jonas; Cojoc, Gheorghe; Milas, Ana; Šumanovac Šestak, Ivana; Risteski, Patrik; Tavano, Federica; Klemm, Anna H.; Roscioli, Emanuele; Welburn, Julie; Cimini, Daniela; Glunčić, Matko; Pavin, Nenad; Tolić, Iva M.
Overlap microtubules link sister k-fibers and balance the forces on bioriented kinetochores.
Nat Commun, DOI 10.1038/NCOMMS10298. (IF = 11.470)

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