XMAP215 drives microtubule catastrophe by perturbing the growing microtubule end structure. (A) Microtubules polymerized with low tubulin grow slowly with small GTP-caps. Loss of GTP-tubulin triggers catastrophe. (B) Microtubules polymerized with high tubulin grow fast with large GTP-caps, resisting catastrophe. (C) Microtubules grown with XMAP215 display growth irregularities that trigger catastrophe, despite high GTP-tubulin content.