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  • The EMBO Journal (2003) 22, 2060 - 2070
  • doi:10.1093/emboj/cdg195

Importin alpha-regulated nucleation of microtubules by TPX2

Christoph A. Schatz1, Rachel Santarella1, Andreas Hoenger1, Eric Karsenti1, Iain W. Mattaj1, Oliver J. Gruss1,3 and Rafael E. Carazo-Salas2,3

  1. European Molecular Biology Laboratory, Meyerhofstrasse 1, D-69117 Heidelberg, Germany
  2. Present address: Cancer Research UK, London Research Institute, 44 Lincoln's Inn Fields, London WC2A 3PX, UK
  3. O.J.Gruss and R.E.Carazo-Salas contributed equally to this work

Correspondence to:

Iain W. Mattaj, E-mail: Mattaj@EMBL-Heidelberg.de

Received 19 November 2002; Accepted 4 March 2003; Revised 4 March 2003

The importin alpha-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 alpha is mapped. A TPX2 mutant that cannot bind importin alpha is constitutively active in the induction of microtubule-containing aster-like structures in Xenopus egg extract, demonstrating that no other importin alpha 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 alpha 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 alpha. The data suggest that the critical early function in spindle formation regulated by importin alpha is TPX2-mediated microtubule nucleation.

  • Keywords:

    • importin alpha,
    • nuclear localization sequence,
    • Ran,
    • spindle assembly,
    • TPX2