Stimulation of microtubule aster formation and spindle assembly by the small GTPase Ran

Ran, a small guanosine triphosphatase, is suggested to have additional func tions beyond its well-characterized role in nuclear trafficking, Guanosine triphosphate-bound Ran, but not guanosine biphosphate-bound Ran, stimulated polymerization of astral microtubules from centrosomes assembled on Xenopu s sperm, Moreover, a Ran allele with a mutation in the effector domain (Ran L43E) induced the formation of microtubule asters and spindle assembly, in the absence of sperm nuclei, in a gamma TuRC (gamma-tubulin ring complex)- and XMAP215 (Xenopus microtubule associated protein)-dependent manner. Ther efore, Ran could be a key signaling molecule regulating microtubule polymer ization during mitosis.