Inside cells there are tiny motor engines that ride on thin rods called microtubules. Without this, cells would not be able to grow, reproduce or communicate.
We are interested in how motor proteins and microtubules self-organize to produce fascinating assemblies such as the mitotic spindle with its orchestrated movements of chromosomes. In our interdisciplinary approach, we combine cell and molecular biology, molecular genetics, biophysical tools including laser microsurgery, optical engineering, computer science and theoretical physics. We aim at quantitative descriptions and development of theoretical models that help us understand the self-organization processes in the cell.
The main task of the mitotic spindle is to generate forces that position the chromosomes at the metaphase plate and subsequently pull them apart towards the opposite spindle poles. These forces in living cells are, unfortunately, not easily accessible by current experimental techniques. However, much about the forces can be inferred from the shape of the spindle because the shape is an outcome of forces. K-fibers, which are bundles of microtubules ending at the kinetochore, are typically curved, suggesting that they are under compression. This inference contradicts the fact that sister kinetochores and thus also sister k-fibers are under tension, leaving us with a paradox about the origin of the curved shape of the spindle.
To illustrate the origin of the curved shape of the spindle, we have constructed two macroscopic models made of wooden rods and a rope. We chose the rods to represent microtubule bundles because they are stiff, and the rope to symbolize the flexible chromatin. The current paradigm, where sister k-fibers are connected by chromatin, is illustrated by a model in which two rods are connected by a rope. However, such a structure cannot take on a curved shape. If, on the other hand, we add a third rod as a link between these two rods, the entire structure can curve under compression (Tolić and Pavin, Cell Cycle, 2016).
We have indeed observed a third microtubule bundle, which forms a bridge between sister k-fibers and thus we called it a bridging fiber (Kajtez et al., Nat Commun 2016). By severing a k-fiber with a laser, we have shown that the bridging fiber moves together with sister k-fibers, revealing that these three fibers are strongly linked and thus able to survive large physical perturbations such as cutting of k-fibers. Severing of a k-fiber at different locations also revealed that the bridging fiber is linked laterally to the k-fibers in the region away from the kinetochore, while these fibers separate from each other close to the kinetochore (Kajtez et al., Nat Commun 2016; Milas and Tolić, Matters Select 2016). The bridging fiber consists of 10-15 microtubules arranged in an anti-parallel manner, where the anti-parallel overlap, measured by PRC1-GFP, extends over 5 μm: 1 μm between sister kinetochores and 2 μm along each sister k-fiber. Based on our experiments and theory, we conclude that the bridging fiber, by linking sister k-fibres, withstands the tension between sister kinetochores and enables the spindle to obtain a curved shape.