It's composed of one type of protein tubulent that comes in two forms. We have alpha and beta tubulin. So basically the alpha and beta globular tubulent proteins create a helical structure that is hollow at the center and that's why we call it a tubule. Now, the function of microtubules is to increase the compressive strength so we see that microfilaments and intermediate filaments give our cell the ability to resist pulling. But our micro tubules give the cells the ability to resist compression so it gives increases the cells compression rest of strength. It also is involved in forming the mitotic spindle and it's involved in forming cilia fluagella as well as involved in intracellular movements, basically moving things within our cell.