a,b,g tubulin are eukaryotic cytoskeletal proteins (Mr 50,000). The a,b tubulin dimer assembles forming microtubules, and g tubulin is involved in microtubule organisation. Microtubules are long hollow, dynamic polymers and behave like conveyer belts inside the cell. They help to move organelles, vesicles, granules and chromosomes via specially attached motor proteins which bind to their surface and travel along them. Microtubules are the main constituent of the mitotic spindle and are the target of antimitotic drugs such as colchilie and Taxol. They also serve a cytoskeletal role.

Tubulin is regulated by the binding of the gamma phosphate of GTP and a coordinated Mg²⁺ ion. GTP bound to the exchangeable site of the subunit is hydrolysed upon assembly and tubulin is transformed into the inactive GDP-liganated form.

A profilament is a linear row of tubulin. The picture below shows the complex structure of tubulin :-

The figure below shows the 3-dimensional view of a microtubule being formed. The tubulin molecules are the bead like structures and combine to form long hollow, dynamic polymers, microtubules.

Taxol stabilises microtubules by binding to a polymer of tubulin. This results in microtubules not being able to undergo depolymerization. Other anticancer agents such as Colchicine, colcemid, and nocadazol inhibit polymerisation and hence prevents the formation of microtubules.