How microtubules (MTs) 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 universal MT nucleator of the cell. 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 the γ-tubulin ring complex (γ-TuRC) possess minimal nucleation activity individually， toget... More
How microtubules (MTs) 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 universal MT nucleator of the cell. 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 the γ-tubulin ring complex (γ-TuRC) possess minimal nucleation activity individually， together， these factors synergistically stimulate MT nucleation in vitro. The amino-terminal TOG domains 1-5 of XMAP215 bind to αβ-tubulin and promote MT polymerization， whereas the conserved carboxy terminus is required for efficient MT nucleation and directly binds to γ-tubulin. In summary， XMAP215 and γ-TuRC together function as the principal nucleation module that generates MTs in cells.
