Microtubules are polymers of tubulin, a dimer of two 55-kDa subunits, designated alpha and beta [PMID:3896122, PMID:2194680]. 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 [PMID:8102497].

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 [PMID:3680207].

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 [PMID:8274140].