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© The Rockefeller University Press, 0021-9525/2000/6/1405/ $5.00
The Journal of Cell Biology, Volume 149, Number 7, June 26, 2000 1405-1418

Original Article

TPX2, A Novel Xenopus MAP Involved in Spindle Pole Organization

Torsten Wittmanna, Matthias Wilmb, Eric Karsentia, and Isabelle Vernosa
a Cell Biology and Cell Biophysics Program, European Molecular Biology Laboratory, D-69117 Heidelberg, Germany
b Biochemical Instrumentation Program, European Molecular Biology Laboratory, D-69117 Heidelberg, Germany

Correspondence to: Isabelle Vernos, European Molecular Biology Laboratory, Cell Biology and Cell Biophysics Program, D-69117 Heidelberg, Germany. Tel:49 6221 387 342 Fax:49 6221 387 512 E-mail:vernos{at}embl-heidelberg.de.

TPX2, the targeting protein for Xenopus kinesin-like protein 2 (Xklp2), was identified as a microtubule-associated protein that mediates the binding of the COOH-terminal domain of Xklp2 to microtubules (Wittmann, T., H. Boleti, C. Antony, E. Karsenti, and I. Vernos. 1998. J. Cell Biol. 143:673–685). Here, we report the cloning and functional characterization of Xenopus TPX2. TPX2 is a novel, basic 82.4-kD protein that is phosphorylated during mitosis in a microtubule-dependent way. TPX2 is nuclear during interphase and becomes localized to spindle poles in mitosis. Spindle pole localization of TPX2 requires the activity of the dynein–dynactin complex. In late anaphase TPX2 becomes relocalized from the spindle poles to the midbody. TPX2 is highly homologous to a human protein of unknown function and thus defines a new family of vertebrate spindle pole components. We investigated the function of TPX2 using spindle assembly in Xenopus egg extracts. Immunodepletion of TPX2 from mitotic egg extracts resulted in bipolar structures with disintegrating poles and a decreased microtubule density. Addition of an excess of TPX2 to spindle assembly reactions gave rise to monopolar structures with abnormally enlarged poles. We conclude that, in addition to its function in targeting Xklp2 to microtubule minus ends during mitosis, TPX2 also participates in the organization of spindle poles.

Key Words: spindle pole, microtubules, dynein–, dynactin, TPX2, Xklp2


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