Microtubule-organizing centers (MTOCs) nucleate microtubules that can grow autonomously in any direction. To generate bundles of parallel microtubules originating from a single MTOC, the growth of multiple microtubules needs to coordinated, but the underlying mechanism is unknown. Here, we show that a conserved two-component system consisting of the plus-end tracker EB1 and the minus-end-directed molecular motor Kinesin-14 is sufficient to promote parallel microtubule growth. The underlying mechanism relies on the ability of Kinesin-14 to guide growing plus ends along existing microtubules. The generality of this finding is supported by yeast, Drosophila, and human EB1/Kinesin-14 pairs. We demonstrate that plus-end guiding involves a directional switch of the motor due to a force applied via a growing microtubule end. The described mechanism can account for the generation of parallel microtubule networks required for a broad range of cellular functions such as spindle assembly or cell polarization.
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