Computer simulations show how two types of motors organize a bipolar spindle

The mitotic spindle is a complex microtubule network that segregates the genetic material during cell division. Several types of microtubule-crosslinking motor proteins are essential to organize the filamentous microtubules into a network with the correct bipolar spindle shape. Using stochastic computer simulations, we explored which types of motors are sufficient for this organizational task and found that motors with opposite directionality are sufficient, provided they have the correct properties. These turn out to be exactly the properties of the two main spindle motors kinesin-5 and dynein in human cells. This result reveals a core physical design principle of the dynamic process of spindle assembly and maintenance. It explains why the known main spindle motors have evolved the properties that they have. This study at the interface between cell biology and active matter physics provides new mechanistic insight into how the activities of dynamic filaments and molecular motors need to work together to ensure that one of the most important macromolecular assemblies in human cells is correctly built.