Microtubules are polymers of tubulin, a dimer of two 55kDa subunits, designated alpha and beta. Within the microtubule lattice, alpha-betaheterodimers associate in a head-to-tail fashion, giving rise to microtubule polarity. Fluorescent labelling studies have suggested that tubulin isoriented in microtubules with beta-tubulin toward the plus end. 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. 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. This entry represents alpha tubulin.