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Structural insight into filament formation by mammalian septins

Abstract

Septins are GTP-binding proteins that assemble into homo- and hetero-oligomers and filaments. Although they have key roles in various cellular processes, little is known concerning the structure of septin subunits or the organization and polarity of septin complexes. Here we present the structures of the human SEPT2 G domain and the heterotrimeric human SEPT2–SEPT6–SEPT7 complex. The structures reveal a universal bipolar polymer building block, composed of an extended G domain, which forms oligomers and filaments by conserved interactions between adjacent nucleotide-binding sites and/or the amino- and carboxy-terminal extensions. Unexpectedly, X-ray crystallography and electron microscopy showed that the predicted coiled coils are not involved in or required for complex and/or filament formation. The asymmetrical heterotrimers associate head-to-head to form a hexameric unit that is nonpolarized along the filament axis but is rotationally asymmetrical. The architecture of septin filaments differs fundamentally from that of other cytoskeletal structures.

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Figure 1: Structure and dimerization of SEPT2.
Figure 2: Structural analysis of the human septin complex.
Figure 3: Electron microscopic studies of septin complexes.
Figure 4: The septin filament.

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Acknowledgements

Data collection was done at the Swiss Light Source, beam line X10SA, Paul Scherrer Institute, Villigen, Switzerland, and we thank the beam line staff for assistance. We would like to thank I. Vetter, I. Schlichting, T. Meinhart, W. Blankenfeldt, N. Schrader, E. Hofmann, K. Kühnel, A. Scrima, R. Gasper and R. Rose for data collection and crystallographic advice. M.S. and F.H. thank the International Max Planck Research School for financial support. This work was supported by the 3D Repertoire project, within the EU Sixth Framework Program, and the Fondation Louis-Jeantet.

Author Contributions M.S. purified and crystallized SEPT2-315 and the human septin complex and solved the structures. M.F. made the human septin complex constructs, developed the purification procedure and purified the yeast septin complex used in the electron microscopy. F.H. and H.S. did the electron microscopy analysis. D.K. made SEPT2-315 mutants. M.W. assisted M.S. throughout data collection and structure determination. I.G.M. provided the clones for human septins, and valuable hints. A.W. supervised the project and wrote the paper. All authors discussed the results and commented on the manuscript.

The atomic coordinates of SEPT2-315 and the human septin complex are deposited in the Protein Data Bank with accession numbers 2QA5 and 2QAG, respectively.

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Correspondence to Alfred Wittinghofer.

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Supplementary information

Supplementary Information

This file contains Supplementary Tables 1-4 describing the data collection, phasing, and refinement statistics and EM particle statistics, Supplementary Figures 1-3 with Legends showing sequence alignment of Sept2, 6 and 7 with secondary structural elements; details of Sept2 interface and raw electron density pictures of HSC. (PDF 569 kb)

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Sirajuddin, M., Farkasovsky, M., Hauer, F. et al. Structural insight into filament formation by mammalian septins. Nature 449, 311–315 (2007). https://doi.org/10.1038/nature06052

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