Structure of Microtubules

Microtubules are long, hollow cylinders with an outer diameter of 250A. The wall thickness is about 50A, leaving a hollow core of about 150A diameter. The length of microtubules varies from a fraction of a micron to several microns. In some cases the microtubules are probably several millimeters long.

A transvers section of a cytoplasmic microtubule shows 13 subunits (protofilaments) which lie parallel to the long axis of the microtubuline. The microtubules of crayfish sperm arms and cockroach epidermis show 15 protofilaments in cross sections, while those of the crayfish nerve cord show 12. Neurotubules are made up of 13 protoilaments.

Tubulin

The protofilaments appears to be made up of a linar series of globular protein (tubulin) units, like a string a beads. Each unit is about 50AX40A in size. Under normal physiological conditions tubulin is a dimmer composed of two similar, but not identical, polypeptides. Brain tubulin dimmers have a sedimentation coefficient of 6S and molecular weight of 115,000. The two subunits of a tubulin dimer are called α-tubulin and β-tubulin, with molecular weights of 55,000 and 57,000. Although the amino acid compositions of the two subunits are similar, there are differences in their peptide maps. The α- and β-tubulin units are arranged alternately in the protofilament. The basic arrangement of tubulin appears to be helical, with 13 tubulin molecules per turn of the helix. In addition to the tubulins there are about 20 to 25 secondary proteins which have been termed microtubule-associated proteins (MAPs). They are of importance in the function of microtubules and in the control of assembly.

Tubulins from the different sources such as cilia and brain are similar but not identical. It is also likely that tubulin from the cytoplasm differs from both flagellar tubulins.
Guanine nucleotides are found associated with both flagellar and brain tubulin. Each tubulin dimer has two nucleotide binding sites and is bound to two molecules of GTP. One molecule is bound strongly and the other weakly. Brain tubulin contains at least one mole of strongly bound Mg++ per mole of tubulin dimer.

Effect of colchicine. The alkaloid poison colchicine is produced by many species of the family Liliaceae. It specifically blinds to tubulin of microtubules, one mole to each mole of tubulin. As a result of these binding the microtubules break down. In dividing cells mitosis is blocked at metaphase because of the breakdown of the spindle microtubules, leading to polyploidy. Colchicine is therefore used to produce polyploid varieties of plants. Where microtubules provide the skeletal framework of the cell, breakdown of the microtubules leads to retraction of structures supported by microtubules, e.g. in axopodia of Protozoa.

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