Model of mitotic spindle self-assembly without motor proteins

The microtubule-based mitotic spindle segregates chromosomes during eukaryotic cell division. Assembly of the mitotic spindle is an out-of-equilibrium process typically requiring interaction between dynamic microtubules (MTs), MT-associated proteins, and MT organizing centers. We study mitotic spindle formation in the fission yeast S. Pombe because of its relatively small number of components. Surprisingly, spindle formation in S. Pombe requires no force generation from motor proteins; in the absence of motors, the crosslinking molecule Ase1/PRC1 drives spindle assembly. However, it is not clear what physical properties of Ase1 are necessary for spindle bipolarity. Our coarse-grained model demonstrates formation of stable bipolar spindles with only crosslinkers present. By varying properties of our simulated Ase1 molecules, we find specific molecular features required for spindle assembly. In particular, Ase1 length, binding kinetics, and MT-bound diffusion coefficient control its ability to trap MT antiparallel overlaps and form a bipolar spindle. Our results illustrate how molecular features of a MT bundler promote self-organization and stability of the mitotic spindle.