DefinitionThis section has been translated automatically.
Microtubules are composed of fine tubular protein structures, and together with the so-called intermediate filaments and microfilaments form the cytoskeleton of the eukaryotic cell. Thus, they guarantee the stability and functionality of the cell to a large extent. The filaments usually start from a center (at the minus end), the so-called microtubule organizing center (MTOC). Examples are the centrioles at the poles of the cells. Microtubules are often arranged as singlet, duplet or triplet organizing units.
General informationThis section has been translated automatically.
Structure of the microtubules: Microtubules are organized in directed protein units, whose ends are marked with (+) and (-) because of their polymerization direction. The microunits of the microtubules consist of heterodimers (without their own covalent bond) which are each composed of a molecular building block α- (negative) and β-tubulin (positive). From this dimeric basic structure the filamentous basic structure of the microtubules is organized with longitudinally connected subfilaments (so-called protofilaments) of which 13 laterally connected subfilaments usually form the wall of the microtubules. The typical spiral-shaped structure of the microtubules (see figure) and their hollow body structure is due to a large number of protofilaments.
Lifetime of the microtubules: Microtubules have an average lifetime of about 10 minutes. It is longer when the microtubules are integrated into larger structures and thus stabilized. Basically, two different populations of microtubules can be defined: short-lived dynamic microtubules and long-lived, stable microtubules.
Composition of the microtubules: The composition of the microtubules is subject to a constant build-up and breakdown in the cell. Thus, the tubulin units are permanently built up and broken down (poly and depolymerised) at both their plus and minus ends. The build-up and breakdown are equally balanced, so that a physiological balance is created. If the balanced ratio shifts in favour of degradation, the microtubules may dissolve completely. Exhaustion of the supply of tubulin units is also possible.
MAPs: Microtubule-associated proteins (MAPs) interact specifically with the microtubules of the cytoskeleton. They thus influence the microtubule dynamics. As stabilizing factors, the proteins bind to microtubules and slow down the depolymerization of the tubulin subunits. Some MAPs additionally accelerate their polymerization, such as the assembly MAPs Tau and MAP4.
Mitosis inhibitors: The build-up and breakdown of microtubules can be inhibited by mitosis inhibitors (see below cytostatics, see below taxanes, see below vinca alkaloids). Thus, the vinca alkaloids vincristine or vinblastine block the polymerization process by specifically binding to α tubulin, which makes polymerization with β tubulin impossible. The formation of the microtubules stops.
Function of the microtubules: The function of the microtubules is broad and multifunctional. Microtubules form the spindle apparatus before cell division. Via the spindle apparatus, the chromatids are drawn to the poles of the cell (minus ends of the microtubules), an important process in cell division (see mitosis inhibitor below).
Microtubules are also involved in the rapid axonal transport of the nerve cell. Here, mainly vesicles are moved along the microtubules by motor proteins (kinesin, dynein). With this type of transport, speeds of 25 to 40 centimetres per day can be achieved. The transport can take place downstream in the direction of the synapse as well as in the opposite direction from the synapse to the soma.
Microtubules continue to form flagella or cilia on certain cell types for locomotion. A process co-responsible for microtubules is the movement of sperm. The microtubules are also actively involved in the process of phagocytosis.