1. Howard Hughes Medical Institute and Department of Biochemistry and Biophysics, University of California, San Francisco, 600 16th Street, San Francisco, California 94143, USA
2. Department of Biological Sciences, Stanford University, Stanford, California 94305-5020, USA
3. †Present address: Department of Molecular and Cell Biology, University of California, Berkeley, 16 Barker Hall, Berkeley, California 94720-3202, USA
4. *These authors contributed equally to this work

Correspondence to: David A. Agard¹ Correspondence and requests for materials should be addressed to D.A.A. (Email: agard@msg.ucsf.edu).

Abstract

Microtubules are hollow polymers of -tubulin that show GTP-dependent assembly dynamics and comprise a critical part of the eukaryotic cytoskeleton. Initiation of new microtubules in vivo requires -tubulin, organized as an oligomer within the 2.2-MDa -tubulin ring complex (-TuRC) of higher eukaryotes^{1, 2, 3}. Structural insight is lacking regarding -tubulin, its oligomerization and how it promotes microtubule assembly. Here we report the 2.7-Å crystal structure of human -tubulin bound to GTP-S (a non-hydrolysable GTP analogue). We observe a 'curved' conformation for -tubulin–GTPS, similar to that seen for GDP-bound, unpolymerized -tubulin⁴. Tubulins are thought to represent a distinct class of GTP-binding proteins, and conformational switching in -tubulin might differ from the nucleotide-dependent switching of signalling GTPases. A crystal packing interaction replicates the lateral contacts between - and -tubulins in the microtubule⁵, and this association probably forms the basis for -tubulin oligomerization within the -TuRC. Laterally associated -tubulins in the -TuRC might promote microtubule nucleation by providing a template that enhances the intrinsically weak lateral interaction between -tubulin heterodimers. Because they are dimeric, -tubulins cannot form microtubule-like lateral associations in the curved conformation⁵. The lateral array of -tubulins we observe in the crystal reveals a unique functional property of a monomeric tubulin.

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