The GTPase Ran has recently been shown to stimulate microtubule polymerizat
ion in mitotic extracts, but its mode of action is not understood. Here we
show that the mitotic role of Ran is largely mediated by the nuclear transp
ort factor importin beta. Importin beta inhibits spindle formation in vitro
and in vivo and sequesters an aster promoting activity (APA) that consists
of multiple, independent factors. One component of APA is the microtubule-
associated protein NuMA. NuMA and other APA components are discharged from
importin beta by RanGTP and induce spindle-like structures in the absence o
f centrosomes, chromatin, or Ran. We propose that RanGTP functions in mitos
is as in interphase by locally releasing cargoes from transport factors. In
mitosis, this promotes spindle assembly by organizing microtubules in the
vicinity of chromosomes.