Tubulin is a major cytoskeleton protein that has five distinct forms designated alpha, beta, gamma, delta and epsilon tubulin. The alpha and beta tubulins form a heterodimer that polymerize into the cylindrical microtubule fibers. Both alpha and beta tubulins bind to GTP but only beta hydrolyses GTP to GDP, leading to tubulin polymerization and microtubule formation. The alpha tubulin form can be modified by addition of a C-terminal tyrosine residue that modulates the polymerization rate. The gamma tubulin is localized in the centrosomes, which compose the core of the microtubule organizing center for microtubule fiber formation.