Microtubules are dynamic in structure in eukaryotic cells and are formed by the self-assembly of the alpha-beta (αβ) tubulin heterodimers. These heterodimers are arranged linearly into protofilaments with 13 protofilaments typically forming the microtubule wall. Microtubules are uniquely equipped to transmit signals originating from receptor occupancy since they fill the cytoplasm and usually span the distance from the plasma membrane to the nucleus. They are involved in numerous functions that include regulation of cell morphology and movement, vesicle transport, and chromosome segregation during mitosis.
Fig. 1 Structure of microtubule (From wiki: Microtubule).
The α- and β-tubulin subunits are approximately 55 kilodaltons (kDa) in molecular mass and are highly homologous. The system is stable while a cap of tubulin-GTP at the ends is maintained, but when this cap is lost, the microtubule comes apart. Microtubules grow and shorten by addition and removal of tubulin subunits from their ends. The faster end is known as the plus end and has the β-tubulin exposed, whereas the slower end is known as the minus end and has the α-tubulin exposed. The plus ends of microtubules are crucial in that they exhibit “dynamic instability”, alternating between phases of growing (rescue) and shortening (catastrophe) while the minus ends are frequently associated at a single site in the cell called the microtubule organizing center (MTOC). Growing microtubules usually have long curved, sheet-like extensions up to several microns in length, whereas shrinkage involves protofilaments curling into tight rings.
In non-motile cells, microtubules are believed to be organized by a perinuclear centrosome, from which microtubules emanate radially towards the cell periphery, with their plus ends free and minus ends remaining centrosome-bound and quiescent. Gamma (γ) tubulin complexes have recently been discovered to be associated with the centrosome, at the peri-centriolar material, and thought to be responsible for microtubule nucleation and organization of the mitotic centrosome. It has been observed that endothelial cells at the edge of a wounded monolayer reposition their MTOC, which has the minus ends of microtubules associated with it, to a position between the cell nucleus and the leading edge of the cell. The importance of microtubules in wound repair and cell motility was demonstrated by studies which showed that centrosome redistribution is essential for endothelial cell migration and efficient wound repair.
Microtubule growth and shortening are thought to activate Rac1 and RhoA signaling, respectively, which then control actin dynamics. RhoA activity may in turn regulate the dynamic state of microtubules by inducing the formation of a subset of stabilized microtubules. It has been demonstrated that specific inhibitors of RhoA block microtubule-depolymerization-induced formation of stress fibers, focal adhesions, and clustering of integrins by fibronectin. Furthermore, microtubule breakdown stimulates stress fiber formation by actin filaments, accompanied by the assembly of focal adhesions and tyrosine phosphorylation of the focal adhesion kinase (FAK).