Biochemistry: The Cytoskeleton

1.1 The Cytoskeleton

The concept and the term ‘cytosquelette’, (in French) were first introduced by a French embryologist Paul Wintrebert in 1931 (Frixione 2000). Cytoskeleton is a complex network array of cytoplasmic fibers that determine and control visco-elastic properties and mechanical strength of cells. It also organizes and gives structure to the cell interior, controls many dynamic processes, such as intracellular trafficking, cell division, adhesion, and locomotion. It is ubiquitously present in all eukaryotic cells and its analogues have been discovered in prokaryotes. Biochemically, the cytoskeleton is defined as a remnant of the cell after treatment with non-ionic detergents, which looks like an empty cage of the cell. Cytoskeleton

These are formed by the polymerization of tubulins. Each tubulin molecule is a hetero dimer of two closely related and tightly associated subunits called α-tubulin and β-tubulin. Tubulins are highly conserved in all eukaryotes throughout the evolution. Each microtubule is typically composed of thirteen linear protofilaments of alternating α- and β-tubulins arranged in parallel to form a cylindrical structure. The microtubules are polar structure i.e. the beta-tubulin is exposed at the minus end and alpha- tubulin towards the plus end and the polymerization is three times faster at the plus end than that of the minus end in vitro. The minus end of the microtubule is embedded inside the centrosome and the plus end directed outwards. Centrosome and basal body are the microtubule organizing centre (MTOC), where nucleation of microtubule occurs. In plant and fungi, microtubule organizing centre is called as Spindle Pole Body (SPB). All these microtubule organizing centers contain universally a tubulin homologue called γ-tubulin that interacts with α- and β-tubulin and helps in the nucleation of microtubule. In a cell, individual microtubule elongates and shrinks back or may completely disappear and is replaced by a new one. This process involves the polymerization and depolymerization with the hydrolysis of GTPs. The microtubule containing ADP-tubulin at the end is highly unstable and depolymerizes rapidly that is called “catastrophe” and the phenomenon is known as “dynamic