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Tubulin is the major constituent of microtubules, a cylinder consisting of laterally associated linear protofilaments composed of alpha- and beta-tubulin heterodimers. Tubulin α- and β-subunits have molecular weights of ~ 50 kDa and are 36%–42% identical and 63% homologous. Both tubulin subunits bind guanine nucleotides. The binding to α-tubulin at the N-site is nonexchangeable, while the binding to β-tubulin at the E-site is exchangeable. Nucleotide in microtubules does not exchange with the solution, except for terminal subunits at microtubule ends.