text: Microtubulus composed of monomers (it's a kind of building blocks, if you're not familiar with the term) called tubulin. In short, there is an GTP binding grrove in each tubulin monomer, allowing them to form dimer of alpha and beta tubulin. In the GTP binding state, their conformation favors formation of microtubulus polymer. There is a special protein that acts to "charge" GDP binding monomer with new GTP molecule.