Mechanism of how the universal module XMAP215 γ-TuRC nucleates microtubules
It has become increasingly evident in recent years that nucleation of microtubules from a diverse set of MTOCs requires both the {gamma}-tubulin ring complex ({gamma}-TuRC) and the microtubule polymerase XMAP215. Despite their essentiality, little is known about how these nucleation factors interact and work together to generate microtubules. Using biochemical domain analysis of XMAP215 and structural approaches, we find that a sixth TOG domain in XMAP215 binds {gamma}-TuRC via {gamma}-tubulin as part of a broader interaction involving the C-terminal region. Moreover, TOG6 is required for XMAP215 to promote nucleation from {gamma}-TuRC to its full extent. Interestingly, we find that XMAP215 also depends strongly on TOG5 for microtubule lattice binding and nucleation. Accordingly, we report a cryo-EM structure of TOG5 bound to the microtubule lattice that reveals promotion of lateral interactions between tubulin dimers. Finally, we find that while XMAP215 constructs effects on nucleation are generally proportional to their effects on polymerization, formation of a direct complex with {gamma}-TuRC allows cooperative nucleation activity. Thus, we propose that XMAP215s C-terminal TOGs 5 and 6 play key roles in promoting nucleation by promoting formation of longitudinal and lateral bonds in {gamma}-TuRC templated nascent microtubules at cellular MTOCs.