Microtubule binding of the human HAUS complex is directly controlled by importins and Ran-GTP

Mitotic spindle assembly during cell division is a highly regulated process. Ran-GTP produced around chromosomes controls the activity of a multitude of spindle assembly factors by releasing them from inhibitory interaction with importins. A major consequence of Ran-GTP regulation is the stimulation of local microtubule nucleation around chromosomes via augmin/HAUS-mediated branched microtubule nucleation, a process that is critically important for correct spindle assembly. However, augmin is not known to be a direct target of the Ran-GTP pathway, raising the question of how its activity is controlled. Here we present the in vitro reconstitution of Ran-GTP-regulated microtubule binding of the human HAUS complex. We demonstrate that importins directly bind to the HAUS complex, which prevents HAUS from binding to microtubules. Ran-GTP relieves this inhibition. Therefore, the HAUS complex is a direct target of the Ran-GTP pathway, suggesting that branching microtubule nucleation is directly regulated by the Ran-GTP gradient around chromosomes in dividing cells.