RanGTP regulates the augmin complex

Abstract

Spindles are composed of microtubules that must nucleate at the right place and time during mitosis. Spindle microtubule nucleation is regulated by the GTPase Ran, which, through importin-{beta}, releases a gradient of spindle assembly factors (SAFs) centered at chromosomes. Branching MT nucleation generates most spindle MTs and requires the augmin complex. In Xenopus laevis, Ran can control branching through the SAF TPX2, TPX2 is non-essential in other organisms. Thus, how Ran regulates branching MT nucleation in the absence of TPX2 is unknown. Here, we use in vitro pulldowns and TIRF microscopy to show that augmin is itself a SAF. Augmin directly interacts with both importins through two nuclear localization sequences on the Haus8 subunit, which overlap the MT binding site. Moreover, Ran controls localization of augmin to MTs in both Xenopus egg extract and in vitro. By uncovering that RanGTP directly regulates augmin, we demonstrate how Ran controls branching MT nucleation and, thereby, spindle assembly and cell division.