What is the definition of microtubule
Microtubules are tubular intracellular polymers made up of globular tubulin subunits. The protein structures have a diameter of about 20-30 nm and, together with the microfilaments and the intermediate filaments, form the cytoskeleton, which gives the cell its shape and strength. Microtubules serve e.g. motor proteins as "rails" through the cell on which the vesicle transport takes place. They also play a special role in cell movement.
- Step 1 - αβ tubulins form a protofilament: Microtubules are made up of α and β tubulin dimers (heterodimers). Via head-to-tail connections, the heterodimers form the individual subunits of a microtubule, which are called protofilaments.
- Step 2 - Several protofilaments form a microtubule: The hollow body of the microtubule is built up in a spiral shape by usually slightly vertically offset lateral assembly of the protofilaments. Due to the head-to-tail arrangement, a microtubule has a certain polarity. The protofilaments within a microtubule have the same polarity, so there are only α-tubulin units at one end while the other end ends in a ring of β-tubulin subunits.
Both α- and β-tubulin bind 1 molecule of GTP. β-tubulin has an additional GTP-ase activity, while GTP is bound irreversibly and non-hydrolyzable to α-tubulin. No exchange of bound GDP or GTP is possible in the filament structure. The heterodimers preferentially attach to the plus end (β-tubulin), so that a microtubule "grows" in the plus direction, while the minus end (α-tubulin) forms the stable starting point. The length of a microtubule can range from fractions of a micrometer to several hundred micrometers.
Microtubules are often arranged as a singlet, doublet or triplet. The filaments usually start from a center (at the minus end), the so-called microtubule organizing center (MTOC). Examples are the centrioles or basal bodies (in cilia, flagella). The stability of the microtubules is ensured by 3 forces:
- Interactions between α- and β-tubulin
- Interactions alongside, within a protofilament and
- Interactions across, between the subunits of the protofilaments.
Two different populations occur in microtubules: long-lived, stable microtubules and dynamic, short-lived microtubules.
3.2 Stable microtubules
In addition to their important function in the cytoskeleton and the associated vesicle transport, stable microtubules also form the framework of centrioles, cilia and flagella. The long-lived structures of microtubules also occur primarily in cells that no longer replicate, such as in the axons of neurons or as axonem in the flagella of sperm cells. These structures ensure stability, flexibility and, with the help of motor proteins, mobility.
3.3 Dynamic microtubules
Dynamic microtubules occur where a quick remodeling of microtubule structures is necessary. During mitosis, the cytosolic network of the interphase cells disappears, and the tubulin it contains forms the cell's spindle apparatus. The very dynamic microtubules strive, for example, starting from the MTOC into the interior of the cell in order to "grab" the microtubules of the other MTOC. If there is no contact, the depolymerization takes place and a new start-up is started. The microtubules ensure that the chromosomes are distributed in the daughter cells. In the picture, the spindle apparatus can be seen colored orange.
4 Dynamic instability
The build-up and breakdown of microtubules is concentration-dependent and takes place in a dynamic change. Both construction and dismantling are preferably carried out at the plus end. Above the critical concentration (Cc) tubulin dimers are polymerized, below the depolymerization takes place.
The factor that determines the stability is the speed with which the heterodimers attach to the plus end. If the step takes place too slowly, the bound GTP is hydrolyzed to GDP + P and this reduces the stability. This leads to "fraying" at the plus end and causes the loss of the lateral stabilization of the protofilaments. If there is a high concentration of GTP-bound tubulin, a so-called "GTP cap" protecting the filament is formed.
Over time, a microtubule grows until there are no longer enough heterodimers available and then begins to depolymerize ("catastrophe"). The hydrolysis of the bound heterodimers in turn increases the concentration of dissolved tubulin and the microtubule begins to grow again ("rescue"). Microtubule-associated proteins (MAPs) can bind to the plus end as stabilizing factors and reduce the frequency of disasters. The hormone catastrophin, on the other hand, destabilizes the GTP cap and thus promotes depolarization. In the presence of catastrophin, the microtubules are therefore shorter and very dynamic.
5 microtubule motor proteins
Motor proteins that bind to microtubules can be differentiated according to their direction of movement. Here, minus-end motor proteins (dyneins) orientate themselves in the direction of the MTOC, while plus-end motor proteins (kinesins) strive with their cargo to the periphery and thus, for example, transport proteins from the endoplasmic reticulum (ER) to the cell membrane.
6 microtubule poisons
Polymerization and depolymerization of microtubules are suppressed by various substances that can be used to treat various diseases. This fact is also used in human genetics when creating a karyogram to lock the chromosomes in the metaphase. The herbal active ingredients colchicine, taxol and vinblastine bind exclusively to αβ tubulin or microtubules.
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