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 microtubul es in these extracts through generation of Ran-GTP. Here we examine the eff ects of Ran-GTP on microtubule nucleation and dynamics and show that Ran-GT P has independent effects on both the nucleation activity of centrosomes an d the stability of centrosomal microtubules. We also show that inhibition o f Ran-GTP production, even in the presence of duplicated centrosomes and ki netochores, prevents assembly of a bipolar spindle in M-phase extracts.