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Microtubule nucleating γ-TuSC assembles structures with 13-fold microtubule-like symmetry


Microtubules are nucleated in vivo by γ-tubulin complexes. The 300-kDa γ-tubulin small complex (γ-TuSC), consisting of two molecules of γ-tubulin and one copy each of the accessory proteins Spc97 and Spc98, is the conserved, essential core of the microtubule nucleating machinery1,2. In metazoa multiple γ-TuSCs assemble with other proteins into γ-tubulin ring complexes (γ-TuRCs). The structure of γ-TuRC indicated that it functions as a microtubule template2,3,4,5. Because each γ-TuSC contains two molecules of γ-tubulin, it was assumed that the γ-TuRC-specific proteins are required to organize γ-TuSCs to match 13-fold microtubule symmetry. Here we show that Saccharomyces cerevisiae γ-TuSC forms rings even in the absence of other γ-TuRC components. The yeast adaptor protein Spc110 stabilizes the rings into extended filaments and is required for oligomer formation under physiological buffer conditions. The 8-Å cryo-electron microscopic reconstruction of the filament reveals 13 γ-tubulins per turn, matching microtubule symmetry, with plus ends exposed for interaction with microtubules, implying that one turn of the filament constitutes a microtubule template. The domain structures of Spc97 and Spc98 suggest functions for conserved sequence motifs, with implications for the γ-TuRC-specific proteins. The γ-TuSC filaments nucleate microtubules at a low level, and the structure provides a strong hypothesis for how nucleation is regulated, converting this less active form to a potent nucleator.

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Figure 1: γ-TuSC oligomers form spontaneously and are stabilized by Spc110.
Figure 2: γ-TuSC filament structure.
Figure 3: γ-Tubulin in the γ-TuSC filament has a geometry similar to 13-protofilament microtubules.
Figure 4: γ-TuSC oligomers nucleate microtubules at low levels.
Figure 5: Models of nucleation complex attachment and activation.

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Data deposits

The cryo-electron microscopic reconstruction is deposited with the Electron Microscopy Database with the accession code 1731.


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We thank M. Braunfeld and A. Avila-Sakar for microscopy assistance; Y. Cheng, E. Muller, M. Moritz and K. Huang for helpful discussions; and B. Carragher, C. Potter and J. Quispe for the use of their electron microscopy facilities and technical assistance with data collection. Some of the work presented here was conducted at the National Resource for Automated Molecular Microscopy, which is supported by the National Institutes of Health (NIH) through the National Center for Research Resources’ P41 program. This work was supported by the NIH (D.A.A. and T.N.D.) and the Howard Hughes Medical Institute (D.A.A.). J.M.K. was a NIH Ruth L. Kirschstein National Research Service Award (NRSA) postdoctoral fellow.

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Authors and Affiliations



J.M.K. purified and prepared samples for electron microscopy, collected cryo-electron microscopy data, determined the structure and performed microtubule nucleation experiments. J.K.P. explored γ-TuSC assembly conditions and prepared and imaged capped microtubules. A.Z. designed and cloned expression constructs, and generated and tested baculovirus strains. D.A.A and J.M.K. designed experiments and analysed data. J.M.K., D.A.A. and T.N.D. wrote the paper. All the authors discussed the results and commented on the manuscript.

Corresponding author

Correspondence to David A. Agard.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Information

This file contains Supplementary Results and Discussion, References and Supplementary Figures 1-8 with legends. (PDF 9242 kb)

Supplementary Movie 1

γTuSC/Spc110p filament structure: The helical reconstruction is shown rotating. The front clipping plane is then brought in to show features on the filament interior, and to demonstrate the lack of connections between layers of the helix. (MOV 14683 kb)

Supplementary Movie 2

A single ring and a single γTuSC subunit from the filament structure: A single turn of the γTuSC/Spc110p1-220 structure coloured by subunit is shown rotating. The view is rotated to look down the helical axis to demonstrate the 13-fold γ-tubulin symmetry. A single γTuSC subunit is then shown, colored gold for γ-tubulin, dark blue for Spc98p, light blue for Spc97p, and light green for Spc110p1-220. (MOV 18085 kb)

Supplementary Movie 3

Reorganization of the γ-tubulin ring from the filament geometry to microtubule geometry: Initially, a single ring of thirteen γ-tubulins from the γTuSC-Spc110p1-220 filament is shown. The γ-tubulins are then moved by linear interpolation to their corresponding positions in a microtubule lattice. This movie shows the movement with a view down the filament axis. (MOV 2326 kb)

Supplementary Movie 4

Reorganization of the γ-tubulin ring from the filament geometry to microtubule geometry: Initially, a single ring of thirteen γ-tubulins from the γTuSC-Spc110p1-220 filament is shown. The γ-tubulins are then moved by linear interpolation to their corresponding positions in a microtubule lattice. This movie shows the movement with a perpendicular view. (MOV 1724 kb)

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Kollman, J., Polka, J., Zelter, A. et al. Microtubule nucleating γ-TuSC assembles structures with 13-fold microtubule-like symmetry. Nature 466, 879–882 (2010).

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