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

Abstract

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.

γ-TuRCs and the augmin complex are required for the development of highly branched dendritic arbors in Drosophila

“…Thus, microtubule nucleation is promoted by microtubule-associated proteins (MAPs) or protein complexes that locally concentrate tubulin dimers ( Tovey and Conduit, 2018 ). The best-characterized nucleator is the multi-protein γ-tubulin ring complex (γ-TuRC), which templates the lateral assembly of tubulin dimers to form a 13-protofilament microtubule that extends with its plus-end growing away from the γ-TuRC ( Kollman et al, 2011 ; Brito et al, 2024 ). γ-TuRC activity is normally low in the cytosol but increases on recruitment to specific sites called microtubule-organizing centers (MTOCs) ( Sanchez and Feldman, 2016 ; Tillery et al, 2018 ).…”

Mechanism of how the universal module XMAP215 γ-TuRC nucleates microtubules

“…Importantly, γ-TuRC’s overall architecture is splayed open compared to the microtubule such that the template is imperfect with low activity in vitro 10 , 12 , 13 . Nevertheless, computational modeling, nucleation assays using purified components, and structural studies have demonstrated that assembly of tubulin onto the γ-TuRC template provides sufficient lateral bond energy to close the γ-TuRC ring, a conformation in which spoke 14 overlaps and occludes spoke 1 10 , 12 , 14 , 15 . γ-TuRC thus consistently nucleates and caps 13 protofilament microtubules 14 – 16 .…”