Microtubules

Microtubules are strand-like cylinders composed of a protein building block, tubulin. Tubulin consist of two subunits, alpha and beta, which form a dimer that is freely available within the cytoplasm. Thirteen tubulin molecules polymerize together in a helical manner to form one loop of the microtubule structure. Polymerization can continue in a polarized - to one end of the strand - manner to give tubulin strands of relatively great lengths.

The state of polymerization is dynamic. Guanine triphosphate, GTP, has a stabilizing effect on the structure and permits polymerization. When GTP is hydrolyzed, the microtubule is rapidly disassembled. Modification of the amino acid residues of tubulin can also stabilize the microtubule.

Microtubules have a central role in:

• formation of the mitotic spindle and movement of chromosomes; microtubular failure can result in failure of chromosome separation
• formation of centrioles
• cellular locomotion
• cilia movement
• exo- and endocytosis
• axonal movement in neurons: microtubules act as the pathway for microtubule-associated proteins (MAP) to move transmitter vesicles from cell body to dendrites

Microtubule formation is disrupted by drugs which bind tubulin monomer e.g. vinblastine and colchicine.