Transition of human γ-tubulin ring complex into a closed conformation during microtubule nucleation.

Microtubules are essential for intracellular organization and chromosome segregation. They are nucleated by the γ-tubulin ring complex (γTuRC). However, isolated vertebrate γTuRC adopts an open conformation that deviates from the microtubule structure, raising the question of the nucleation mechanism. Here we determine cryo-electron microscopy structures of human γTuRC bound to a nascent microtubule. Structural changes of the complex into a closed conformation ensure that γTuRC templates the 13-protofilament microtubules that exist in human cells. Closure is mediated by a latch that interacts with incorporating tubulin, making it part of the closing mechanism. Further rearrangements involve all γ-tubulin ring complex subunits and the removal of the actin-containing luminal bridge. Our proposed mechanism of microtubule nucleation by human γTuRC relies on large-scale structural changes that are likely the target of regulation in cells.