Abstract
SUMMARY Kinesin molecular motors are essential for fundamental cellular processes such as chromosome segregation or vesicular transport. To fulfil their function, some kinesins promote microtubule growth, but the molecular mechanism underlying this activity remains unclear. One of the motors with the strongest microtubule growth-promoting activity is Kip2, a kinesin that is required for astral microtubule integrity and spindle positioning in yeast. Here we show that the ability of Kip2 to polymerize microtubules is coupled to binding and transport of free tubulin. We report that the Kip2 N-terminus is required to promote microtubule elongation in vitro and in vivo . Kip2 binds free tubulin through this unstructured, basic domain and delivers it to microtubule plus-ends. In addition to the N-terminus, we find that ATP hydrolysis and motor activity is also required for microtubule polymerisation. Finally, transfer of the Kip2 N-terminus to kinesin-1, a kinesin that lacks polymerase activity, transforms kinesin-1 into a tubulin-transporting microtubule polymerase. We propose that motor-driven tubulin delivery to microtubule plus-ends is an efficient mechanism used by kinesins to promote microtubule polymerization.