REPORTS Stimulation of Microtubule Aster Formation and Spindle Assembly by the Small GTPase Ran Andrew Wilde and Yixian Zheng* Ran, a small guanosine triphosphatase, is suggested to have additional functions beyond its well-characterized role in nuclear trafficking. Cuanosine triphos- phate-bound Ran, but not guanosine diphosphate-bound Ran, stimulated po- lymerization 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 yTuRC (y-tubulin ring complex)- and XMAP215 (Xenopus microtubule associated protein)-dependent manner. Therefore, Ran could be a key signaling molecule replating microtubule polymerization during mitosis. In animal cells, the transition from interphase to mitosis is accompanied by pronounced changes in cellular architecture. One of the biggest changes is the conversion of the in- Carnegie Institution of Washington, Department of Embryology, Baltimore, MD 21210, USA. *To whom correspondence should be addressed. E- mail: zheng@maill.ciwemb.edu terphase microtubule array into a highly dy- namic mitotic spindle. This requires more than the presence of microtubule nucleating centers (called centrosomes in animal cells) and the conversion of cytosol into a mitotic state (1, 2). Nuclear signals released into the cytoplasm upon nuclear envelope breakdown (NEB) influence the organization of the mi- crotubule arrays. For example, premature rupture of the nuclear envelope in grasshop- per spermatocytes during prophase causes the formation of a mitotic spindle next to the chromosomes (3). Chromosomes and nuclei . , themselves can initiate large microtubule as- ters from nearby centrosomes (4, 5). Even in the absence of centrosomes, for example, during meiosis in female Drosophila, micro- tubule assembly is promoted near chromo- somes (6). Chromosomes can also polarize existing microtubule arrays into a spindle- like structure in mitosis (7). Furthermore, artificial chromosomes tethered to beads stimulate mitotic spindle formation in the absence of centrosomes and kinetochores in Xenopus egg extracts (8). These studies indi- cate that nuclear signals, including those as- sociated with chromosomes, influence the formation and rearrangement of microtubule structures during mitosis after NEB. Ran-GTP, the predominant form of Ran in the nucleus (9), is a good candidate for such a signal (10-13). In the yeast Saccharomyces cerevisiae, overexpression of RanGEF1, the guanine nucleotide exchange factor for Ran, specifically suppresses a class of a-tubulin mutations that arrest with excess microtu- bules as large, budded cells. This suggests a link between the Ran pathway and microtu- bule polymerization (12). Moreover, cells with mutations in RanGEFl and the Ran- binding protein, RanBP1, arrest with a mis- www.sciencemag.org SCIENCE VOL 284 21 MAY 1999