Protein associated with or component of the microtubule. The microtubule is formed by the arrangement of 13 parallel protofilaments arising from end-to-end aggregation of the tubulin alpha/beta-dimers. Microtubules are associated with various other proteins (MAPs, dynein, kinesin) and are involved in structures responsible for cellular movement such as flagella or cilia. Microtubules of the ciliary axoneme are more permanent than cytoplasmic and spindle microtubules. Microtubule formation is inhibited by agents such as colchicine, vinblastine or vincristine.