Microtubules are one of three principal components of the cytoskeleton. A microtubule is composed of tubulin dimers, consisting of two closely related polypeptides called α- and β-tubulin. Tubulin dimers polymerize into protofilaments assembled around a hollow core. Tubulin dimers polymerize at the plus end of a microtubule. The polymerized tubulin dimers form a sheet that zips up into the mature microtubule just behind the region of growth. The tubulin dimers that add to the microtubule are bou