Tubulin is the protein which makes up microtubules. Microtubules are assembled from dimers of α- and β-tubulin. Each of these subunits has three domains. γ-tubulin is important in the nucleation and polar orientation of microtubule. Tubulin binds GTP and assembles onto the (+) ends of microtubules in the GTP-bound state. Once assembled into microtubules, it hydrolyzes GTP into GDP. The GDP-bound form of tubulin will disassemble from the tip of a microtubule, though it will not spontaneously fall out of the middle. This GTP cycle is essential for the dynamic instability of the microtubule.
Delta and epsilon tubulin have been found to localize at centrioles and may play a role in forming the mitotic spindle during mitosis.