microtubules are long, hollow cylinders made up of polymerized ?- and ?-tubulin dimers. The inner space of the hollow microtubule cylinders is referred to as the lumen. The ? and ?-tubulin subunits are ~50% identical at the amino acid level, and both have a molecular weight of approximately 50 kDa.

These ?/?-tubulin dimers polymerize end-to-end into linear protofilaments that associate