Kinesin-14 motors participate in a force balance at microtubule plus-ends to regulate dynamic instability.

Kinesin-14 molecular motors represent an essential class of proteins that bind microtubules and walk toward their minus-ends. Previous studies have described important roles for Kinesin-14 motors at microtubule minus-ends, but their role in regulating plus-end dynamics remains controversial. Kinesin-14 motors have been shown to bind the EB family of microtubule plus-end binding proteins, suggesting that these minus-end-directed motors could interact with growing microtubule plus-ends. In this work, we explored the role of minus-end-directed Kinesin-14 motor forces in controlling plus-end microtubule dynamics. In cells, a Kinesin-14 mutant with reduced affinity to EB proteins led to increased microtubule lengths. Cell-free biophysical microscopy assays were performed using Kinesin-14 motors and an EB family marker of growing microtubule plus-ends, Mal3, which revealed that when Kinesin-14 motors bound to Mal3 at growing microtubule plus-ends, the motors subsequently walked toward the minus-end, and Mal3 was pulled away from the growing microtubule tip. Strikingly, these interactions resulted in an approximately twofold decrease in the expected postinteraction microtubule lifetime. Furthermore, generic minus-end-directed tension forces, generated by tethering growing plus-ends to the coverslip using λ-DNA, led to an approximately sevenfold decrease in the expected postinteraction microtubule growth length. In contrast, the inhibition of Kinesin-14 minus-end-directed motility led to extended tip interactions and to an increase in the expected postinteraction microtubule lifetime, indicating that plus-ends were stabilized by nonmotile Kinesin-14 motors. Together, we find that Kinesin-14 motors participate in a force balance at microtubule plus-ends to regulate microtubule lengths in cells.