XMAP215 is a microtubule nucleation factor that functions synergistically with the gamma-tubulin ring complex

How microtubules (MT) are generated in the cell is a major question in understanding how the cytoskeleton is assembled. For several decades, γ-tubulin has been accepted as the cell’s universal MT nucleator. Although there is evidence that γ-tubulin complexes are not the sole MT nucleators, identification of other nucleation factors has proven difficult. Here, we report that the well-characterized MT polymerase XMAP215 (chTOG/Msps/Stu2p/Alp14/Dis1 homologue) is essential for MT nucleation in Xenopus egg extracts. The concentration of XMAP215 determines the extent of MT nucleation. Even though XMAP215 and γ-tubulin ring complex (γ-TuRC) possess minimal nucleation activity individually, together these factors synergistically stimulate MT nucleation in vitro. The N-terminal TOG domains 1–5 of XMAP215 bind αβ-tubulin and promote MT polymerization, while the conserved C-terminus is required for efficient MT nucleation and directly binds γ-tubulin. In sum, XMAP215 and γ-TuRC together function as the principal nucleation module that generates MTs in cells.