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== Structure ==
[[File:Tubulin dimer 1JFF.png|thumb|Cartoon representation of the structure of α(yellow)/β(red)-tubulin heterodimer, GTP and GDP.<ref>{{cite journal | vauthors = Löwe J, Li H, Downing KH, Nogales E | title = Refined structure of alpha beta-tubulin at 3.5 A resolution | journal = Journal of Molecular Biology | volume = 313 | issue = 5 | pages = 1045–57 | date = November 2001 | pmid = 11700061 | doi = 10.1006/jmbi.2001.5077 | url = https://zenodo.org/record/1229896 }}</ref>]]
In eukaryotes, microtubules are long, hollow cylinders made up of polymerised α- and β-[[tubulin]] [[protein dimer|dimers]].<ref name="weisenberg">{{cite journal | vauthors = Weisenberg RC | title = Microtubule formation in vitro in solutions containing low calcium concentrations | journal = Science | volume = 177 | issue = 4054 | pages = 1104–5 | date = September 1972 | pmid = 4626639 | doi = 10.1126/science.177.4054.1104 | bibcode = 1972Sci...177.1104W | s2cid = 34875893 }}</ref> The inner space of the hollow microtubule cylinders is referred to as the lumen. The α and β-tubulin subunits are
These α/β-tubulin [[protein dimer|dimers]] [[polymerize]] end-to-end into linear protofilaments that associate laterally to form a single microtubule, which can then be extended by the addition of more α/β-tubulin dimers. Typically, microtubules are formed by the parallel association of thirteen protofilaments, although microtubules composed of fewer or more [[protofilament]]s have been observed in various species <ref>{{cite journal | vauthors = Chaaban S, Brouhard GJ | title = A microtubule bestiary: structural diversity in tubulin polymers | journal = Molecular Biology of the Cell | volume = 28 | issue = 22 | pages = 2924–31 | date = 2017 | pmid = 29084910 | doi = 10.1091/mbc.E16-05-0271 | pmc = 5662251}}</ref> as well as ''in vitro''.<ref>{{cite journal | vauthors = Chrétien D, Metoz F, Verde F, Karsenti E, Wade RH | title = Lattice defects in microtubules: protofilament numbers vary within individual microtubules | journal = Journal of Cell Biology | volume = 117 | issue = 5 | pages = 1031–40 | date = June 1992 | pmid = 1577866 | doi = 10.1083/jcb.117.5.1031 | pmc = 2289483}}</ref>
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