Microtubules are tubular polymers of the protein tubulin, which constitute the skeleton of eukaryotic cells. They are found in all dividing and most nondividing cells, where they play important roles in cell division, intracellular transport, and organization of the cytoskeleton. Microtubules are also involved in a number of cellular processes that require movement, such as cell motility, embryogenesis, and nerve growth.
In addition to their structural role, microtubules also serve as tracks for motor proteins that move along them. These motor proteins generate the force necessary for many essential cellular functions, such as chromosome separation during cell division, trafficking of vesicles and organelles within cells, and movement of cells themselves. Several different families of motor proteins interact with microtubules, including kinesins (which move towards the plus end of microtubules), dyneins (which move towards the minus end), and myosins (which move away from both ends).
The structure of a microtubule is an assembly of tubulin dimers arranged in a helical lattice around a central axis. Each tubulin monomer consists of two structurally similar subunits: α-tubulin and β-tubulin. When these subunits assemble into dimers, they form an extended sheet that can wrap around other sheets to form hollow tubes. The resulting structure is incredibly strong and resilient; however, it is also dynamic, meaning that it can rapidly disassemble and reassemble under certain conditions. For example, during cell divisionmicrotubules must be quickly assembled to help separate chromosomes into daughter cells; afterward they must just as quickly disassemble so that the cell can return to its normal shape.
The flexibility of microtubule dynamics is controlled by several factors, including post-translational modifications on tubulin dimers (such as phosphorylation)and the binding of regulatory molecules (such as MAPs). Post-translational modifications can stabilize or destabilize microtubule assemblies by altering interactions between individual tubulin dimers; this allows for rapid changes in microtubule stability when needed (for example during cell division). Regulatory molecules bind to specific sites on either α- or β-subunits; some promote assembly while others promote disassembly. The overall effect depends on which regulatory molecule is bound where—thus providing another layerof control over microtubule dynamics .