Microtubules: what are they, composition, and what are they for?

Cells are made up of a multitude of structures that, like in a clock, make them carry out their functions with absolute precision.

One of those that we can find within this complex organic machinery are microtubules. We are going to delve into the characteristics of these elements and what are the functions they fulfill in our body.

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What are microtubules? Characteristics of these structures

Microtubules are microscopic tubes found in each of our cells, beginning in the MTOC or microtubule organizing center and extending throughout the cytoplasm of the cell. Each of these small tubes has a thickness of 25 nanometers, with the diameter of its interior being only 12 nanometers. Regarding the length, they can reach a few microns, a distance that may seem small but that at the cellular level and in proportion to their width makes them long.

At the structural level, microtubules are composed of protein polymers, and are composed of 13 protofilaments

, which in turn are made up of tubulin monomers a and b located alternately, that is, creating a chain of dimers a-b. The 13 protofilaments are arranged one against the other until they form the cylindrical structure, leaving the part of the hollow center. Furthermore, all 13 have the same structure, all having a - end, which begins with tubulin a, the other being the + end, of tubulin b.

In the microtubules of bacteria cells there are some differences with respect to the rest of eukaryotic cells. In this case, the tubulins would be specific to bacteria, and would make up 5 protofilaments instead of the usual 13 that we saw before. In any case, these microtubules work in a similar way to the others.

Dynamic instability

One of the qualities that characterizes microtubules is the so-called dynamic instability. It is a constant process in this structure by which they are continuously polymerizing or depolymerizing. This means that all the time they are incorporating tubulin dimers to increase the length or on the contrary they are eliminating them to be shortened.

In fact, they can continue to shorten until they are completely undone to start the cycle again, going back to polymerize. This polymerization process, that is, growth, occurs more frequently at the + end, that is, at the tubulin b end.

But how does this process occur at the cellular level? Tubulin dimers are found in the cell in the free state. They are all attached to two molecules of guanosine triphosphate, or GTP (a nucleotide triphosphate). When the time comes for these dimers to adhere to one of the microtubules, a known phenomenon takes place. as hydrolysis, whereby one of the GTP molecules is transformed into guanosine diphosphate, or GDP (a nucleotide diphosphate).

Keep in mind that the speed of the process is essential to understand what can happen next. If dimers bind to microtubules faster than hydrolysis itself occurs, this is this means that there will always be the so-called cap or cap of GTPs at the most extreme of the dimers. On the contrary, in the event that the hydrolysis is faster than the polymerization itself (because this has made its process slower), what we will obtain in the extreme extreme will be a GTP-GDP dimer.

As one of the triphosphate nucleotides has changed to a diphosphate nucleotide, an instability is generated in the adhesion between the protofilaments themselves, which causes a chain effect ending with a depolymerization of the entire set. Once the GTP-GDP dimers that were causing this imbalance have disappeared, the microtubules regain normality and resume the polymerization process.

The loosened tubulin-GDP dimers quickly become tubulin-GTP dimers, so they are again available to bind to microtubules again. In this way, the dynamic instability of which we spoke at the beginning occurs, causing the microtubules to grow and decrease without stopping, in a perfectly balanced cycle.

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Features

Microtubules have a fundamental role for various tasks within the cell, of a very varied nature. We will study some of them in depth below.

1. Cilia and flagella

Microtubules make up a large part of other important elements of the cell such as cilia and flagella, which are basically microtubules but with a plasma membrane surrounding them. These cilia and flagella are the structure that the cell uses to be able to move and also as sensitive element to capture diverse information of the fundamental environment for certain processes cell phones.

Cilia differ from flagella in that they are shorter but also much more abundant. In their movement, the cilia drive the fluid that surrounds the cell in a direction parallel to it, while the flagella do the same perpendicular to the cell membrane.

Both cilia and flagella are complex elements that can house 250 types of protein. In each cilium and each flagellum we find the axoneme, a central set of microtubules covered by the plasma membrane that we indicated previously. These axonemes are made up of a pair of microtubules located in the center and surrounded by 9 other pairs on the outside.

The axoneme extends from the basal body, another cellular structure, in this case formed by 9 sets, in this case triple, of microtubules, arranged circularly to leave hollow the central cavity between all they.

Returning to the axoneme, it should be noted that the pairs of microtubules that compose it are adhered to each other thanks to the effect of the nexin protein and by protein radii. In turn, in these outer pairs we also find dynein, another protein, whose usefulness in this case is to generate the movement of the cylinders and flagella, since it is of the motor type. Internally, this happens thanks to a sliding between each pair of microtubules, which ends up generating a movement at the structural level.

2. Transport

Another key function of microtubules is to transport organelles within the cell cytoplasm., being able to be vesicles or of another type. This mechanism is possible because the microtubules would act as a kind of lanes by which the organelles move from one point to another in the cell.

In the specific case of neurons, this phenomenon would also occur for the so-called axoplasmic transport. Taking into account that axons can measure not only centimeters, but meters in certain species, it allows us to get an idea of the growth capacity of the microtubules themselves to be able to support this transport function, so essential in the rhythms cell phones.

Regarding this function, microtubules they would be a mere path for the organelles, but an interaction between the two elements would not be generated. On the contrary, the movement would be achieved through motor proteins, such as dynein, which we have already seen, and also kinesin. The difference between both types of protein is the direction they take in the microtubules, since the dyneins are used for movement going towards the minus end, while kinesin is used to go towards the extreme more.

3. Achromatic spindle

Microtubules also make up another of the fundamental structures of the cell, in this case the achromatic, mitotic or meiotic spindle. It is made up various microtubules that connect the centrioles and centromeres of chromosomes while the process of cell division occurs, either by mitosis or by meiosis.

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4. Cell shape

We already know that there are many types of cells, each with its own characteristics and arrangement. Microtubules would help to provide the cell with the determined shape of each of these types, for example in the case seen above of an elongated cell, such as a neuron with its long axon and dendrites.

At the same time They are also key so that certain elements of the cell are in the place where they must be to fulfill their functions properly. This is the case, for example, of organelles as fundamental as the endoplasmic reticulum or the Golgi apparatus.

5. Filament organization

Another of the essential functions of microtubules is to be responsible for the distribution of the filaments throughout the cytoskeleton (the network of proteins that are found inside the cell and that nourishes all the structures inside), forming a network of increasingly smaller pathways that goes from the microtubules (the largest) towards the intermediate filaments and ending with the narrowest of all, the so-called microfilaments, which can be myosin or actin.

Bibliographic references:

• Desai, A., Mitchison, T.J. (1997). Microtubule polymerization dynamics. Annual review of cell and Developmental Biology.
• Mitchison, T., Kirschner, M. (1984). Dynamic instability of microtubule growth. Nature.
• Nogales, E., Whittaker, M., Milligan, R.A., Downing, K.H. (1999). High-resolution model of the microtubule. Cell. ScienceDirect.