Tubulin. Molecular model of a tubulin dimer. The beta-tubulin subunit (gold) is at top and the alpha-tubulin subunit (black) is at bottom. Polymers of alpha- and beta-tubulin make up microtubules, part of the cell's cytoskeleton. Microtubules help to maintain cell shape, allow some cellular mobility and are involved in intracellular transport and nuclear division.