TUBULIN

The protein subunit of microtubules in eukaryotic cells. Tubulin exists in the cell as a dimer of α- and β-tubulin, each with a molecular weight of about 55,000 daltons as determined by electrophoresis. α-Tubulin contains 450 amino acids, with a very acidic Cterminal sequence. β-Tubulin has 445 amino acids with about 42 percent sequence identity with the α-subunit, which suggests a common ancestral origin. The tubulin dimer is associated with two moles of ATP, one at an exchangeable site and the other at a non-exchangeable site. Under appropriate conditions of temperature, pressure, ionic strength and calcium concentration, tubulin dimers self-assemble to form the cytoplasm microtubule complex of interphase cells and the spindle of dividing cells.