The small guanosine triphosphatase Ran is involved in transport of proteins into the nucleus and has been recognized more recently as a regulator of formation of the mitotic spindle. Wiese et al. show that Ran uses the same binding partner, importin-β, to control microtubule formation that it uses to promote protein transport. Importin-β carries cargo proteins to the nucleus and releases them upon interaction with guanosine triphosphate (GTP)-bound Ran. In the case of spindle assembly, importin-β interacts with the NuMA (nuclear-mitotic apparatus) protein, which helps organize microtubules at the spindle poles. Activated Ran causes release of NuMA from importin-β and thus promotes microtubule assembly. Nachury et al. present similar results, and Gruss et al. show that Ran-induced release of TPX2 (a microtubule-associated protein that targets the motor protein Xklp2 to microtubules) from importin-α is also regulated by Ran. The authors conclude that TPX2 is necessary for chromatin-induced microtubule assembly during M phase in Xenopus egg extracts.
M. V. Nachury, T. J. Maresca, W.C. Salmon, C. M. Waterman-Storer, R. Heald, K. Weis, Importin is a mitotic target of the small GTPase Ran in spindle assembly. Cell 104, 95-106 (2001). [Online Journal]
O. J. Gruss, R. E. Carazo-Salas, C. A. Schatz, G. Guarguaglini, J. Kast, M. Wilm, N. Le Bot, I. Vernos, E. Karsenti, I. W. Mattaj, Ran induces spindle assembly by reversing the inhibitory effect of importin on TPX2 activity. Cell 104, 83-93 (2001). [Online Journal]