Stimulation of Microtubule Aster Formation and Spindle Assembly by the Small GTPase Ran

Abstract

Ran, a small guanosine triphosphatase, is suggested to have additional functions beyond its well-characterized role in nuclear trafficking. Guanosine triphosphate–bound Ran, but not guanosine diphosphate–bound Ran, stimulated polymerization of astral microtubules from centrosomes assembled on Xenopus sperm. Moreover, a Ran allele with a mutation in the effector domain (RanL43E) induced the formation of microtubule asters and spindle assembly, in the absence of sperm nuclei, in a γTuRC (γ-tubulin ring complex)– and XMAP215 (Xenopusmicrotubule associated protein)–dependent manner. Therefore, Ran could be a key signaling molecule regulating microtubule polymerization during mitosis.