Ran–GTP coordinates regulation of microtubule nucleation and dynamics during mitotic-spindle assembly

It was recently reported that GTP-bound Ran induces microtubule and pseudo-spindle assembly in mitotic egg extracts in the absence of chromosomes and centrosomes, and that chromosomes induce the assembly of spindle microtubules in these extracts through generation of Ran–GTP. Here we examine the effects of Ran–GTP on microtubule nucleation and dynamics and show that Ran–GTP has independent effects on both the nucleation activity of centrosomes and the stability of centrosomal microtubules. We also show that inhibition of Ran–GTP production, even in the presence of duplicated centrosomes and kinetochores, prevents assembly of a bipolar spindle in M-phase extracts.