Molecular Requirements for the Transition from Lateral to End-on Microtubule Binding and Dynamic Coupling

Accurate chromosome segregation relies on microtubule end conversion, the ill understood ability of kinetochores to transit from lateral microtubule attachment to durable association with dynamic microtubule plus ends. The molecular requirements for this conversion and the underlying biophysical mechanisms are ill understood. We reconstituted end conversion in vitro using two kinetochore components: the plus end directed kinesin CENP-E and microtubule binding Ndc80 complex, combined on the surface of a microbead. The primary role of CENPE is to ensure close proximity between Ndc80 complexes and the microtubule plus end, whereas Ndc80 complexes provide lasting microtubule association by diffusing on the microtubule wall near its tip. Together, these proteins mediate robust plus end coupling during several rounds of microtubule dynamics, in the absence of any specialized tip binding or regulatory proteins. Using a Brownian dynamics model, we show that end conversion is an emergent property of multimolecular ensembles of microtubule wall binding proteins with finely tuned force dependent motility characteristics.