Abstract
The mitotic spindle is a micro-machine made of bundles of microtubules, which divide the chromosomes by pulling on kinetochores, protein complexes on the chromosome. During metaphase, forces on kinetochores are exerted by k-fibers, 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-fiber in HeLa and PtK1 cells that a bundle of nonkinetochore microtubules, which we term ‘bridging fiber', bridges sister k-fibers and balances the tension between sister kinetochores. We found PRC1 and EB3 in the bridging fiber, suggesting that this bundle consists of anti-parallel dynamic microtubules. By introducing a theoretical model, we predict that thicker bridging fibers are under larger compressive forces, and confirmed the prediction experimentally. Moreover, our theory and experiments show a larger relaxation of the inter-kinetochore distance for cuts closer to the kinetochore. We conclude that the bridging fiber, by linking sister k-fibers, withstands the tension between sister kinetochores and enables the spindle to obtain a curved shape. During my talk, I will also discuss how cross-linking proteins facilitate formation of microtubule bundles, by introducing a theoretical model which describes Brownian angular movement of microtubules and elastic forces exerted by proteins that cross-link microtubules extending from the same spindle pole.

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