InterPro domain: IPR004057

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 incorporation into 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 control access 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, has also 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 ]. More recently, epsilon-tubulin has been identified in humans [ 6 ] and Trypanosomes [ 7 ]. In Tetrahymena [ 8 ] is required for the assembly and maintenance of the triplet microtubule blades of basal bodies. In humans, it has been localised to centrosomes [ 9 ]. It is required for centriole duplication and organisation of the pericentriolar material [ 9 ].

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. Delta-tubulin and epsilon-tubulin: two new human centrosomal tubulins reveal new aspects of centrosome structure and function. Nat. Cell Biol. 2, 30-5
7. New tubulins in protozoal parasites. Curr. Biol. 10, R258-9
8. ε-tubulin is essential in Tetrahymena thermophila for the assembly and stability of basal bodies. J. Cell. Sci. 126, 3441-51
9. Epsilon-tubulin is required for centriole duplication and microtubule organization. Nat. Cell Biol. 5, 71-6