So microtubules are the largest of the three and are rigid hollow tubes made of a protein known as tubulin. And there are two versions of tubulin. We have alpha as well as beta tubulin and together the alpha and the beta tubulin proteins which are globular proteins, they basically create a helical structure, structure that winds as shown. So we have this winding in a helical like fashion and we create a hollow center. So this structure here is the microtubule. So notice that the actin and the intermediate filament.