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J. Biol. Chem., Vol. 277, Issue 52, 50973-50979, December 27, 2002

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Microtubule Nucleation from Stable Tubulin Oligomers^*

Nicolas Caudron, Isabelle Arnal^§, Eric Buhler^¶, Didier Job, and Odile Valiron

From  INSERM Unité 366, Département Réponse et Dynamique Cellulaires/Cytosquelette, Commissariat à l'Energie Atomique/Grenoble, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France, ^§ Equipe "Structure et Dynamique du Cytosquelette," UMR 6026 CNRS, Université de Rennes 1, Campus Beaulieu Bâtiment 13, Rennes, 35042, France, and the ^¶ Centre de Recherches sur les Macromolécules Végétales, CNRS-UPR 5301, BP 53, 38041 Grenoble Cedex 9, France

Microtubule assembly from purified tubulin preparations involves both microtubule nucleation and elongation. Whereas elongation is well documented, microtubule nucleation remains poorly understood because of difficulties in isolating molecular intermediates between tubulin dimers and microtubules. Based on kinetic studies, we have previously proposed that the basic building blocks of microtubule nuclei are persistent tubulin oligomers, present at the onset of tubulin assembly. Here we have tested this model directly by isolating nucleation-competent cross-linked tubulin oligomers. We show that such oligomers are composed of 10-15 laterally associated tubulin dimers. In the presence of added free tubulin dimers, several oligomers combine to form microtubule nuclei competent for elongation. We provide evidence that these nuclei have heterogeneous structures, indicating unexpected flexibility in nucleation pathways. Our results suggest that microtubule nucleation in purified tubulin solution is mechanistically similar to that templated by -tubulin ring complexes with the exception that in the absence of -tubulin complexes the production of productive microtubule seeds from tubulin oligomers involves trial and error and a selection process.

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