Insights into the assembly and activation of the microtubule nucleator γ-TuRC

Peng Liu, Erik Zupa, Annett Neuner, Anna Böhler, Justus Loerke, Dirk Flemming, Thomas Ruppert, Till Rudack, Christoph Peter, Christian Spahn, Oliver J. Gruss, Stefan Pfeffer () and Elmar Schiebel ()
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Peng Liu: Universität Heidelberg, DKFZ-ZMBH Allianz
Erik Zupa: Universität Heidelberg, DKFZ-ZMBH Allianz
Annett Neuner: Universität Heidelberg, DKFZ-ZMBH Allianz
Anna Böhler: Universität Heidelberg, DKFZ-ZMBH Allianz
Justus Loerke: Institute for Medical Physics and Biophysics
Dirk Flemming: Heidelberg University Biochemistry Center
Thomas Ruppert: Universität Heidelberg, DKFZ-ZMBH Allianz
Till Rudack: Ruhr-University Bochum
Christoph Peter: Institut für Genetik Universität Bonn
Christian Spahn: Institute for Medical Physics and Biophysics
Oliver J. Gruss: Institut für Genetik Universität Bonn
Stefan Pfeffer: Universität Heidelberg, DKFZ-ZMBH Allianz
Elmar Schiebel: Universität Heidelberg, DKFZ-ZMBH Allianz

Nature, 2020, vol. 578, issue 7795, 467-471

Abstract: Abstract Microtubules are dynamic polymers of α- and β-tubulin and have crucial roles in cell signalling, cell migration, intracellular transport and chromosome segregation1. They assemble de novo from αβ-tubulin dimers in an essential process termed microtubule nucleation. Complexes that contain the protein γ-tubulin serve as structural templates for the microtubule nucleation reaction2. In vertebrates, microtubules are nucleated by the 2.2-megadalton γ-tubulin ring complex (γ-TuRC), which comprises γ-tubulin, five related γ-tubulin complex proteins (GCP2–GCP6) and additional factors3. GCP6 is unique among the GCP proteins because it carries an extended insertion domain of unknown function. Our understanding of microtubule formation in cells and tissues is limited by a lack of high-resolution structural information on the γ-TuRC. Here we present the cryo-electron microscopy structure of γ-TuRC from Xenopus laevis at 4.8 Å global resolution, and identify a 14-spoked arrangement of GCP proteins and γ-tubulins in a partially flexible open left-handed spiral with a uniform sequence of GCP variants. By forming specific interactions with other GCP proteins, the GCP6-specific insertion domain acts as a scaffold for the assembly of the γ-TuRC. Unexpectedly, we identify actin as a bona fide structural component of the γ-TuRC with functional relevance in microtubule nucleation. The spiral geometry of γ-TuRC is suboptimal for microtubule nucleation and a controlled conformational rearrangement of the γ-TuRC is required for its activation. Collectively, our cryo-electron microscopy reconstructions provide detailed insights into the molecular organization, assembly and activation mechanism of vertebrate γ-TuRC, and will serve as a framework for the mechanistic understanding of fundamental biological processes associated with microtubule nucleation, such as meiotic and mitotic spindle formation and centriole biogenesis4.

Date: 2020
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DOI: 10.1038/s41586-019-1896-6

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