Multi-kinesin clusters impart mechanical stress that reveals mechanisms of microtubule breakage in cells.

Microtubules are cytoskeletal filaments that provide structural support for numerous cellular processes. Despite their high rigidity, microtubules can be dramatically bent in cells, and it is unknown how much force a microtubule can withstand before breaking. We find that the kinesin-3 motor KIF1C forms condensates that entangle and break neighboring microtubules. Combining computational simulations and experiments, we show that microtubule breakage is an emergent property that is dependent on a highly processive kinesin motor domain, the cluster properties, cytoplasmic viscosity, and microtubule anchors. We estimate a rupture force for microtubules in cells that is lower than previous estimates based on in vitro studies with taxol-stabilized microtubules. The absence of microtubule breakage under physiological conditions suggests that mechanisms exist to protect microtubule integrity, which may inform about physical constraints on the evolution of motor proteins. We suggest that release of either the motor-cargo or motor-microtubule interaction prevents the accumulation of mechanical stress upon the engagement of multi-motor clusters with microtubules.