Structure of the αβ tubulin dimer by electron crystallography

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  • An Erratum to this article was published on 14 May 1998

Abstract

The αβ tubulin heterodimer is the structural subunit of microtubules, which are cytoskeletal elements that are essential for intracellular transport and cell division in all eukaryotes. Each tubulin monomer binds a guanine nucleotide, which is non-exchangeable when it is bound in the α subunit, or N site, and exchangeable when bound in the β subunit, or E site. The α- and β-tubulins share 40% amino-acid sequence identity, both exist in several isotype forms, and both undergo a variety of post-translational modifications1. Limited sequence homology has been found with the proteins FtsZ2 and Misato3, which are involved in cell division in bacteria and Drosophila, respectively. Here we present an atomic model of the αβ tubulin dimer fitted to a 3.7-Å density map obtained by electron crystallography of zinc-induced tubulin sheets. The structures of α- and β-tubulin are basically identical: each monomer is formed by a core of two β-sheets surrounded by α-helices. The monomer structure is very compact, but can be divided into three functional domains: the amino-terminal domain containing the nucleotide-binding region, an intermediate domain containing the Taxol-binding site, and the carboxy-terminal domain, which probably constitutes the binding surface for motor proteins.

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Figure 1: Experimental phase and intensity data and fitted curves for two representative reciprocal lattice lines.
Figure 2: Sections of the experimental density map with the fitted model for different regions in the α- and β-tubulin molecules.
Figure 3: Sequences of pig brain α- and β-tubulin28 used in the model (in the absence of tubulin sequences from cow we have used its closest known relative).
Figure 4: Ribbon diagram of the tubulin dimer showing α-tubulin with bound GTP (top), and β-tubulin containing GDP and taxotere (bottom).

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Acknowledgements

We thank R. F. Ludueña for isotypically purified αβII and αβIII tubulin, M. Le for help with electron diffraction processing, and R. M. Glaeser and Y. L. Han for comments on the manuscript. Taxol was provided by the Drug Synthesis and Chemistry Branch, Division of Cancer Treatment of the National Cancer Institute. This work was supported by the NIH.

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Correspondence to Eva Nogales.

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