Tubulin fibres

Microtubules may function as a form of skeletal support for microfilaments. Agents that increase intracellular cGMP favour the assembly of microtubules, whereas those that increase intracellular Ca2+ and cAMP result in the dissolution of tubulin fibres. Furthermore, the oxidation state of the neutrophil may affect the integrity of the tubulin fibres. Oxidised glutathione (which is increased during oxidative metabolism) regulates tubulin disassembly, and oxidation may increase tubulin tyrosylation, which also promotes disassembly. 

In tumour cells, the 3-D reconstruction gives a clear image of the spatial arrangement of tubulin fibres in relation to cell shape and position of other cellular organelles, particularly the nucleus (Strohmaier et al. 2000). The tubulin forms an intricate network of fibres of variable thickness. The highest tubulin concentrations appear in the cell periphery and particularly in pseudopodia/invado-... 

Vinca alkaloids inhibit formation of tubulin fibres... 

In resting neutrophils it is estimated that there are about 11-23 microtubules per cell, with a diameter of approximately 25 nm and a wall width of 5 nm. They are long, tubular structures made by the helical formation of tubulin molecules, which are either a- or /3-subunits, each with a relative molecular mass of 55 kDa (Fig. 4.9). Each subunit is present in equimolar amounts in a tubulin molecule, and these subunits exist as dimers of one a- and one /3-subunit. Because microtubules are polar, growth of the fibre is biased towards one end, termed the plus end. A number of microtubule-associated proteins (MAPs) affect the dynamic shape of the microtubule, and in the resting neutrophil about 35-40% of the tubulin pool is assembled, whilst the remainder can be assembled very rapidly after cell stimulation. 

The centrioles migrate to opposite poles of the cell and the mitotic spindle is formed, apparently joining the cell membrane through the centrioles to the centromere of each chromosome. Spindle fibres consist of one type of protein, tubulin, of molecular weight 60,000. It is the organisation of these molecules to form the mitotic spindle which is blocked by the drugs colchicine, colcemide, nocodazole, vincristine and vinblastine (Fig. 10.3) with the consequence that mitosis is arrested in metaphase. 

The spindle fibres are attached to the chromosomes at the centromere and align them on the equatorial plate. The spindle arrangement contains microtubules composed of the protein tubulin. Colchicine is a specific spindle poison, which binds to tubulin, and inhibits its polymerization. Consequently, colchicine blocks mitosis, causing polyploidy, the unequal partition of chromosomes and metaphase arrest. 

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