 The inner space of the hollow microtubule cylinders is referred to as the lumen.The α and β-tubulin subunits are approximately 50% identical at the amino acid level, and each have a molecular weight of approximately 50 kDa.These α/β-tubulin 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.