The mitotic spindle is chiral due to torques generated by motor proteins

Mitosis relies on forces generated in the spindle, a micro-machine composed of microtubules and associated proteins1,2. Forces are required for the congression of chromosomes to the metaphase plate and separation of chromatids in anaphase3-6. However, torques may also exist in the spindle, yet they have not been investigated. Here we show that the spindle is chiral. Chirality is evident from the finding that microtubule bundles follow a left-handed helical path, which cannot be explained by forces but rather by torques acting in the bundles. STED super-resolution microscopy, as well as confocal microscopy, of human spindles shows that the bundles have complex curved shapes. The average helicity of the bundles with respect to the spindle axis is 1.2{degrees}/m. Inactivation of kinesin-5 (Eg5/Kif11) abolished the chirality of the spindle, suggesting that this motor generates the helical shape of microtubule bundles. To explain the observed shapes, we introduce a theoretical model for the balance of forces and torques acting in the spindle, and show that torque is required to generate the helical shapes. We conclude that torques generated by motor proteins, in addition to forces, exist in the spindle and determine its architecture.