The Bipolator: A Self-Organized Spindle-like Bipolar Microtubule Assembly Exhibiting Oscillatory Behavior

The mitotic spindle, a self-organized bipolar assembly of microtubules and cross-linking motors, is characterized by a fluxing mixed-polarity nematic center and radially polar poles (aster). This study explores the fundamental components and mechanisms necessary to establish such a structure. Using computer simulations, we reveal that dynamical microtubules growing from persistently microtubule-associated nucleating seeds combined with opposing motors, are sufficient to organize microtubules into bipolar assemblies in the absence of any chromosomes, centrosomes or a nucleation gradient. At steady-state, the microtubules within this structure are predominantly transported by plus motors, switching orientation at the poles before transiting towards the opposing pole, hence demonstrating oscillatory behavior. This behavior depends on microtubule dynamic instability, seed size, and the specific type of minus-end motors used. Intriguingly, the pole-to-pole distance varies with the number of minus-end motors, a phenomenon previously observed in spindles in cells. The control of the length of the Bipolator can be explained considering an equilibrium of microtubule flux across the pole. This study illuminates the fundamental dynamics of a bipolar assembly, thereby deepening our understanding of the bipolarity of a mitotic spindle.