Tubulin is the major constituent of microtubules (PubMed:28013290). It binds two moles of GTP, one at an exchangeable site on the beta chain and one at a non-exchangeable site on the alpha chain. Dimer of alpha and beta chains. A typical microtubule is a hollow water-filled tube with an outer diameter of 25 nm and an inner diameter of 15 nM. Alpha-beta heterodimers associate head-to-tail to form protofilaments running lengthwise along the microtubule wall with the beta-tubulin subunit facing the microtubule plus end conferring a structural polarity. Microtubules usually have 13 protofilaments but different protofilament numbers can be found in some organisms and specialized cells. Colocalizes with kinesin KIP3 at the plus ends of growing microtubules and along the microtubule lattice. Rhizoxin, an antibiotic that exhibits potent anti-mitotic activity against most eukaryotic cells except for yeasts, binds to beta tubulin. Belongs to the tubulin family.