Importin α-regulated nucleation of microtubules by TPX2

• Published in: The EMBO journal 2003-05, Vol.22 (9), p.2060-2070
• Main Authors: Mattaj, Iain W, Schatz, Christoph A, Santarella, Rachel, Hoenger, Andreas, Karsenti, Eric, Gruss, Oliver J, Carazo-Salas, Rafael E
• Format: Article
• Language: English
• Subjects: alpha Karyopherins - metabolism; Animals; Binding Sites; EMBO20; importin alpha; importin α; Microscopy, Electron; Microtubules - metabolism; Microtubules - ultrastructure; nuclear localization sequence; Ran; Recombinant Proteins - metabolism; Recombinant Proteins - ultrastructure; spindle assembly; TPX2; TPX2 protein; Xenopus
• ISSN: 0261-4189; 1460-2075; 1460-2075
• DOI: 10.1093/emboj/cdg195

The importin α‐regulated microtubule‐associated protein TPX2 is known to be critical for meiotic and mitotic spindle formation in vertebrates, but its detailed mechanism of action and regulation is not understood. Here, the site of interaction on TPX2 for importin α is mapped. A TPX2 mutant that cannot bind importin α is constitutively active in the induction of microtubule‐containing aster‐like structures in Xenopus egg extract, demonstrating that no other importin α or RanGTPase target is required to mediate microtubule assembly in this system. Further, recombinant TPX2 is shown to induce the formation and bundling of microtubules in dilute solutions of pure tubulin. In this purified system, importin α prevents TPX2‐induced microtubule formation, but not TPX2–tubulin interaction or microtubule bundling. This demonstrates that TPX2 has more than one mode of interaction with tubulin and that only one of these types of interaction is abolished by importin α. The data suggest that the critical early function in spindle formation regulated by importin α is TPX2‐mediated microtubule nucleation.