End binding 1 (EB1) is a plus-end-tracking protein (+TIP) that localizes to microtubule plus ends where it modulates their dynamics and interactions with intracellular organelles^{1, 2}. Although the regulating activity of EB1 on microtubule dynamics has been studied in cells^{3, 4, 5, 6} and purified systems^{7, 8, 9, 10, 11, 12, 13}, the molecular mechanisms involved in its specific activity are still unclear. Here, we describe how EB1 regulates the dynamics and structure of microtubules assembled from pure tubulin. We found that EB1 stimulates spontaneous nucleation and growth of microtubules, and promotes both catastrophes (transitions from growth to shrinkage) and rescues (reverse events). Electron cryomicroscopy showed that EB1 induces the initial formation of tubulin sheets, which rapidly close into the common 13-protofilament-microtubule architecture. Our results suggest that EB1 favours the lateral association of free tubulin at microtubule-sheet edges, thereby stimulating nucleation, sheet growth and closure. The reduction of sheet length at microtubule growing-ends together with the elimination of stressed microtubule lattices may account for catastrophes. Conversely, occasional binding of EB1 to the microtubule lattice may induce rescues.

1. Université de Rennes 1, Interactions Cellulaires et Moléculaires, Centre Nationale de la Recherche Scientifique, Unité Mixte de Recherche 6026, Institut Fédératif de Recherche 140, Génétique Fonctionnelle Agronomie et Santé, Campus de Beaulieu, 263, Avenue du Général Leclerc, 35042 Rennes, France.

Correspondence to: Isabelle Arnal¹ e-mail: isabelle.arnal@univ-rennes1.fr;

Correspondence to: Denis Chrétien¹ e-mail: denis.chretien@univ-rennes1.fr

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