Microtubules are thin, branched, hollow cylinders of about 20 – 25 nm in diameter and several microns in length.
They were first observed in nerve cells by De Roberties and Franchi (1953). About a decade later these were studied in plant cells by Ledbetter and porter (1963). Most commonly each microtubule is composed of a ring of about 11-13 longitudinal strands of protein protofilaments.
The right encloses a central core of about 12 – 15 nanometer. The protofilaments are constituted by the polymerization of a and p tubulin protein subunits. The tubulins continuously polymerize and depolymerize to present a dynamic structure.
The half- life of tubulin ranges from 10 minutes in no dividing cells to 20 seconds in dividing cells. In many cells microtubules organize from nucleation centers near the centre of the cell and radiate towards the periphery. Both the ends of microtubule possess distinct polarity and are designated as’+’ end (away from nucleation centre) and’-‘ end (toward the nucleation centre).
ADVERTISEMENTS:
The microtubules grow at each end by polymerization of tubulin dimmers (of a and b tubulin) powered by the hydrolysis of GTP and shrink at each end by depolymerization. However, both processes always occur more rapidly at ‘Plus (+) ends’.
Functions
1. Maintain the form of the cells.
2. Constitute the axoneme of cilia and flagella responsible for their movements.
ADVERTISEMENTS:
3. Constitute the mitotic spindle responsible for movement of chromosome during nuclear division. (Alkaloids like colchicines and vinblastine inhibits microtubules assembly by binding with tubulin).
4. Synthesis of the cell plate during cytokinesis of plant cell division.
5. Special motor proteins move cellular organelles around the cell on microtubules using ATP. Kinesin protein move organelles toward plus end (periphery) and dyneins move toward minus end. Charcot Marie -tooth disease is a rare disorder due to mutation of kinesin gene. In these patients axon-transport is defective accounting for muscle weakness.