Microtubule is the third principal component of the cytoskeletal system. These are rigid hollow rods approximately 25 nm in diameter.Like actin filaments, microtubules are dynamic structures that undergo continual assembly and disassembly within the cell. They function both movements, including some forms of cell locomotion, the intercellular transport of organelles, and the separation of chromosomes during mitosis.\\
Structure and organization of microtubule :
Microtubules are composed of single type of globular protein called tubulin. The building blocks of microtubules are tubulin dimers consisting of two closely related 55 kd polypeptides: alpha and beta tubulin.In addition a third type of tubulin gamma tubulin is concentrated in controsomes where it plays a critical role in initiating microtubule assembly. The evolutionary ancestor of all plant and animal tubulins appears to be a protein similar to the prokaryotic protein FtsZ.
Tubulin dimers polymerise to form microtubules, which generally consist of 13 linear protofilaments assembled around a hollow core. The protofilaments which composed of head to tail arrays of tubulin dimers are arranged in parallel. Microtubules are polar structures with two distinct ends: a fast growing plus ends and a slow growing minus end. This polarity is an important consideration in determining the direction of movement along microtubules, just as the polarity of actin filaments defines the direction of myosin movement.
Tubulin dimers can depolymerise as well as polymerise, and microtubules can undergo rapid cycles of assembly and disassembly. Both alpha- and beta- tubulin bind GTP, which functions analogously to the ATP bound to actin to regulate polymerisation. In particular the GTP bound beta- tubulin is hydrolysed to the GDP during or shortly after polymerization. The GTP hydrolysis as weakens the binding affinity of tubulin for adjacent molecules, thereby favouring depolymerization, resulting in the dynamic behaviour of microtubules.
Like actin filaments microtubules undergo treadmilling, a dynamic behaviour in which tubulin molecules bound to GDP are continuously lost from the minus end replaced by the addition of tubulin molecules bound to GTP to the plus end of the same microtubule.
Functions of microtubules:-
1) Microtubules participate in a wide variety of cell activities.
2) Most involve motion that is provided by motors that use ATP.
3) They determine the positions of membrane enclosed organelles and direct intracellular transport.
4) The migration of chromosomes during mitosis and meiosis takes place on microtubules that makes up the spindle fibre.
