Abstract
The atomic structure of tubulin in a polymerized, straight protofilament is clearly distinct from that in a curved conformation bound to a cellular depolymerizer. The nucleotide contents are identical, and in both cases the conformation of the GTP-containing, intra-dimer interface is indistinguishable from the GDP-containing, inter-dimer contact. Here we present two structures corresponding to the start and end points in the microtubule polymerization and hydrolysis cycles that illustrate the consequences of nucleotide state on longitudinal and lateral assembly. In the absence of depolymerizers, GDP-bound tubulin shows distinctive intra-dimer and inter-dimer interactions and thus distinguishes the GTP and GDP interfaces. A cold-stable tubulin polymer with the non-hydrolysable GTP analogue GMPCPP, containing semi-conserved lateral interactions, supports a model in which the straightening of longitudinal interfaces happens sequentially, starting with a conformational change after GTP binding that straightens the dimer enough for the formation of lateral contacts into a non-tubular intermediate. Closure into a microtubule does not require GTP hydrolysis.
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Acknowledgements
We thank G. Barnes for fluorescent tubulin; S. Westermann for help with the optical microscopy; K. Downing, P. Grob and A. Leschziner for their comments on the manuscript; and W. Wriggers for generating the energy-minimized atomic models after the addition of missing loops. This work was funded by a grant from NIHGMS to E.N. and by the Office of Biological and Environmental Research of the US Department of Energy. E.N. is a Howard Hughes Medical Investigator.
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The density maps have been deposited at the Macromolecular Structure Database under accession numbers EMD-1129 (GDP-tube, inner layer), EMD-1130 (GDP-tube, outer layer), and EMD-1131 (GMPCPP-tube). Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
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Wang, HW., Nogales, E. Nucleotide-dependent bending flexibility of tubulin regulates microtubule assembly. Nature 435, 911–915 (2005). https://doi.org/10.1038/nature03606
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DOI: https://doi.org/10.1038/nature03606
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