These filaments consist of tubulin dimers (consisting of one a-tubulin and one B-tubulin subunit) When GTP binds to the B subunit of the dimer it changes the conformation of the dimer and allows for polymerization to form a microtubule filament. The B-subunit will eventually hydrolyze the GTP to GDP, at which point the conformation of the dimer will change again such that the dimers will depolymerize and the microtubule will dissolve into the cytoplasm.
