What are microtubules?
Microtubules are tubular polymers whose protein structures are about 24nm in diameter. Together with other filaments they form the cytoskeleton, which gives the cells strength and form. In addition, they also play an important role in cell movement and are also important elements of cilia, flagella, centrioles and nuclear spindles. Microtubules are also very important for cancer therapy. Some drugs that affect the division of tumor cells are already being used in the form of chemotherapeutics or cytotoxic drugs.
Anatomy & Construction
Microtubules consist of alpha and beta tubulin dimers (heterodimers). The heterodimers are the subunits of the microtubules, which are also called protofilaments. The protofilaments build by co-storage of the hollow body in the form of a spiral, which are at one end only alpha-tubulin units and at the other end only beta-tubulin subunits. Alpha and beta-tubulin possess the property of binding 1 molecule of GTP. The alpha-tubulin GTP is irreversibly bound.
The heterodimers are preferably at the plus end, so a microtubule grows in that direction, while the minus end forms the stable side. A microtubule is between one micron and several hundred microns long. The arrangement of the microtubules takes place either as a singlet, doublet or triplet. The filaments usually start from the microtubule organizing center, which includes, for example, the centrioles or the basal bodies. In addition, a distinction is made between two different populations: dynamic, short-lived and stable, long-lived microtubules. The stable microtubules are the skeleton of flagella, cilia and centrioles.
Furthermore, the long-lived microtubules also occur in the axons of the neurons and in the scourges of sperm cells. There they provide flexibility, stability and agility. Dynamic microtubules are also found where a quick remodeling is needed. In addition, they ensure the distribution of chromosomes in the daughter cells. Microtubules are alternately built up or broken down, whereby assembly and disassembly take place mainly at the plus end. A microtubule grows until there are not enough heterodimers left.
Then the depolymerization begins, causing the concentration of tubulin to rise again and begin to grow again. Different substances prevent the depolymerization or the polymerization, these are used for the treatment of diseases.
Function & Tasks
Microtubules have multifunctional tasks. They influence the arrangement of the chromosomes and the vesicle movement, which works like a rail system. The vesicle activity is the prerequisite for the transport of motor proteins. The transport is due to the proteins kinesin and dynein, which are located on the vesicle surface. Dynes-occupied vesicles are transported from the plus to the minus end, and kinesin occupies vesicles in the opposite direction.
If individual microtubules store together, complex structures form. These include the centrioles and the basal bodies. Centrioles are composed of nine microtubule triplets consisting of two incomplete and one complete microtubule. Basal bodies have the same structure as centrioles. They are located below the cell surface and have the task of anchoring flagella and kinetic cilia. Kinetic cilia are composed of a central microtubule pair and nine microtubule doublets. Cilia are mainly found on epithelial cells and transport small particles on the surface of the cell. Cilia consist of a plasma membrane and are found on the surface of eukaryotic cells.
Its center consists of stable microtubules, which are arranged in the form of a bundle. Cilia provide movement of fluid across the cell surface. For example, they are used by some protozoa to collect food particles. Many cilia are found on the epithelial cells, where they carry mucus layers with dead cells or dust particles up to the throat, so that they can subsequently be eliminated.
In addition, cilia create a flow on the fallopian tube so that the eggs can be transported through the fallopian tube. Flagella (Flagella) have the same structure as Cinémécs, but they are much longer and serve for cell movement. These include, for example, the movement of sperm and the transport of protozoa.
In primary ciliary dysplasia, the kinetic cilia are defective and the number of dynein molecules is reduced. Primary ciliary dysplasia is a hereditary disease that is very rare and in which the transport mechanism that carries inhaled bacteria and particles does not function properly. As a result, the movement of the cinema cilia is missing or it is very uncoordinated.
For this reason, the dirt particles with the bronchial mucus or the secretion of the paranasal sinuses can not be properly removed, resulting in bronchiectasis (irreversible bronchodilation), chronic bronchitis or chronic sinusitis. If the Flagellenschlag of the sperm is disturbed in men, then infertility occurs. In the context of Alzheimer's disease, altered microtubules are found in the brain of the patient. In this disease, the enzyme MARK2 has an influence on the protein tau. In normal cells, tau is bound to microtubules, stabilizing them. However, when MARK2 acts on tau, instability of the cytoskeleton and disruption in the cell transport system occur, which is one of the hallmarks of Alzheimer's disease.