The kinesin Kip2 promotes microtubule growth using a bipartite polymerase module to deliver tubulin to microtubule plus ends

Abstract

Kinesin molecular motors are essential for processes such as chromosome segregation and vesicular transport. To fulfill their function, many kinesins promote microtubule growth, but the underlying molecular mechanism 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 Kip2’s ability to polymerize microtubules is coupled to binding and transport of free tubulin. The unstructured, basic N terminus of Kip2 is essential for microtubule elongation in vitro and in vivo, binding free tubulin and transporting it to microtubule plus ends. In addition to the N terminus, ATP hydrolysis and motor activity are required for polymerization. Notably, transferring the Kip2 N terminus to kinesin-1, which lacks polymerase activity, converts kinesin-1 into a tubulin-transporting polymerase. We propose that motor-driven tubulin delivery to microtubule plus ends is an efficient mechanism used by kinesins to promote microtubule polymerization.