Tubulin, C-terminal <p>Microtubules are polymers of tubulin, a dimer of two 55kDa subunits, designated alpha and beta [<cite idref="PUB00000039"/>, <cite idref="PUB00000978"/>]. 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 [<cite idref="PUB00006112"/>].</p> <p>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 [<cite idref="PUB00002443"/>].</p><p>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 [<cite idref="PUB00000721"/>].</p><p>This entry represents the extreme C-terminal structural domain of both alpha and beta tubulin. It forms a helix hairpin [<cite idref="PUB00020380"/>].</p>