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Insights into microtubule nucleation from the crystal structure of human γ-tubulin

Nature volume 435, pages 523–527 (2005)Cite this article

Abstract

Microtubules are hollow polymers of αβ-tubulin that show GTP-dependent assembly dynamics and comprise a critical part of the eukaryotic cytoskeleton. Initiation of new microtubules in vivo requires γ-tubulin, organized as an oligomer within the 2.2-MDa γ-tubulin ring complex (γ-TuRC) of higher eukaryotes1,2,3. Structural insight is lacking regarding γ-tubulin, its oligomerization and how it promotes microtubule assembly. Here we report the 2.7-Å crystal structure of human γ-tubulin bound to GTP-γS (a non-hydrolysable GTP analogue). We observe a ‘curved’ conformation for γ-tubulin–GTPγS, similar to that seen for GDP-bound, unpolymerized αβ-tubulin4. Tubulins are thought to represent a distinct class of GTP-binding proteins, and conformational switching in γ-tubulin might differ from the nucleotide-dependent switching of signalling GTPases. A crystal packing interaction replicates the lateral contacts between α- and β-tubulins in the microtubule5, and this association probably forms the basis for γ-tubulin oligomerization within the γ-TuRC. Laterally associated γ-tubulins in the γ-TuRC might promote microtubule nucleation by providing a template that enhances the intrinsically weak lateral interaction between αβ-tubulin heterodimers. Because they are dimeric, αβ-tubulins cannot form microtubule-like lateral associations in the curved conformation5. The lateral array of γ-tubulins we observe in the crystal reveals a unique functional property of a monomeric tubulin.

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Figure 1: Structure and nucleotide binding-properties of γ-tubulin.
Figure 2: γ-tubulin adopts a curved conformation.
Figure 3: Lateral interactions between γ-tubulins resemble lateral interactions in the microtubule lattice.

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Acknowledgements

We thank S. Murphy for protein purification advice early in the project, the Mullins laboratory for the use of their ultraviolet illuminator, and the Agard laboratory centrosome group for discussions and input. We acknowledge the support of this work by grants from the National Institutes of Health (D.A.A. and T.S.) and the Howard Hughes Medical Institute. H.A. acknowledges support from an NIGMS predoctoral fellowship, and L.M.R. was a Paul Sigler/Agouron Institute fellow of the Helen Hay Whitney Foundation. D.A.A. is a Howard Hughes Medical Institute Investigator.

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Author notes

  1. Hector Aldaz

    Present address: Department of Molecular and Cell Biology, University of California, Berkeley, 16 Barker Hall, Berkeley, California, 94720-3202, USA

  2. Hector Aldaz and Luke M. Rice: *These authors contributed equally to this work

Authors and Affiliations

  1. Howard Hughes Medical Institute and Department of Biochemistry and Biophysics, University of California, San Francisco, 600 16th Street, San Francisco, California, 94143, USA

    Hector Aldaz, Luke M. Rice & David A. Agard

  2. Department of Biological Sciences, Stanford University, Stanford, California, 94305-5020, USA

    Tim Stearns

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  1. Hector Aldaz
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Corresponding author

Correspondence to David A. Agard.

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Competing interests

Coordinates and structure factors have been deposited in the Protein Data Bank under accession numbers 1Z5V and 1Z5W. Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.

Supplementary information

Supplementary Methods

Methods used, including protein preparation and nucleotide binding experiments. (DOC 30 kb)

Supplementary Figure Legends

Legends for Supplementary Figures S1 and S2. (DOC 22 kb)

Supplementary Figure S1

Unique surface features of γ-tubulin. (PPT 1185 kb)

Supplementary Figure S2

Structural comparisons between the γ-tubulin structure and the curved and straight γ-tubulin structures. (PPT 1951 kb)

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Aldaz, H., Rice, L., Stearns, T. et al. Insights into microtubule nucleation from the crystal structure of human γ-tubulin. Nature 435, 523–527 (2005). https://doi.org/10.1038/nature03586

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