Microtubules are polymers of tubulin, a dimer of two 55kDa subunits, designated alpha and beta [1, 2]. Within the microtubule lattice, alpha-beta heterodimers associate in a head-to-tail fashion, giving rise to microtubule polarity. Fluorescent labelling studies have suggested that tubulin is oriented in microtubules with beta-tubulin toward the plus end [3].

For maximal rate and extent of polymerisation into microtubules, tubulin requires GTP. Two molecules of GTP are bound at different sites, termed N and E. At the E (Exchangeable) site, GTP is hydrolysed during incorporationinto the microtubule. Close to the E site is an invariant region rich in glycine residues, which is found in both chains and is thought to controlaccess of the nucleotide to its binding site [4].

Most species, excepting simple eukaryotes, express a variety of closely-related alpha- and beta-isotypes. A third family member, gamma tubulin, hasalso been identified in a number of species. Gamma tubulin is found at microtubule-organising centres, such as the spindle poles or the centrosome, suggesting that it is involved in minus-end nucleation of microtubule assembly [5].

This entry represents the extreme C-terminal structural domain of both alpha and beta tubulin. It forms a helix hairpin [6].

1. Molecular biology and genetics of tubulin. Annu. Rev. Biochem. 54, 331-65
2. Diversity among tubulin subunits: toward what functional end? Cell Motil. Cytoskeleton 16, 159-63
3. Localization of an exchangeable GTP binding site at the plus end of microtubules. Science 261, 1044-7
4. Tubulin sequence region beta 155-174 is involved in binding exchangeable guanosine triphosphate. J. Biol. Chem. 262, 15472-5
5. Gamma-tubulin: the hub of cellular microtubule assemblies. Bioessays 15, 637-43
6. The 4 A X-ray structure of a tubulin:stathmin-like domain complex. Cell 102, 809-16

Microtubules are polymers of tubulin, a dimer of two 55kDa subunits, designated alpha and beta [<
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