Microtubules: What They Are, Composition, And What They Are For

Cells are made up of a multitude of structures that, like a clock, make it perform its functions with absolute precision.

One of those that we can find within this complex organic machinery are the microtubules Let’s delve into the characteristics of these elements and what functions they perform in our body.

What are microtubules? Characteristics of these structures

The microtubules are microscopic tubes found in each of our cells, starting 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. As for length, they can reach a few microns, a distance that may seem small but at the cellular and in proportion to their width makes them long.

At a structural level, microtubules They are composed of protein polymers, and are made up of 13 protofilaments, which in turn are formed by tubulin monomers a and b located alternately, that is, creating a chain of ab dimers. The 13 protofilaments are arranged against each other until they form the cylindrical structure, leaving the center part hollow. Furthermore, all 13 have the same structure, all having a – end, which begins with tubulin a, and the other being the + end, of tubulin b.

In the microtubules of bacterial cells there are some differences compared 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 function similarly 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 they are constantly incorporating tubulin dimers to increase length or, on the contrary, they are eliminating them to look shorter.

In fact, They can continue to shorten until they are completely destroyed to begin the cycle again, polymerizing again This polymerization process, that is, growth, occurs more frequently at the + end, that is, at the b-tubulin end.

But how does this process occur at the cellular level? In the cell there are tubulin dimers that are in a 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 phenomenon known as hydrolysis takes place, by which one of the GTP molecules is transformed into guanosine diphosphate, or GDP (a nucleotide diphosphate).

It must be taken into account that the speed of the process is essential to understand what can happen next. If the dimers bind to the microtubules faster than the hydrolysis itself occurs, this means that the so-called GTP cap or cap will always exist at the plus end of the dimers. On the contrary, in the event that the hydrolysis is faster than the polymerization itself (because it has made its process slower), what we will obtain in the extreme will be a GTP-GDP dimer.

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

The loose tubulin-GDP dimers soon become tubulin-GTP dimers, making them available to bind to microtubules again. In this way, that dynamic instability that we talked about at the beginning occurs, causing the microtubules to grow and decrease without stopping, in a perfectly balanced cycle.

Features

Microtubules play a fundamental role in several tasks within the cell, of a very varied nature. Below we will study some of them in depth.

1. Cilia and flagella

The microtubules They 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 a sensitive element to capture various information from the environment that is essential for certain cellular processes.

Cilia differ from flagella in that they are shorter but also much more abundant In their movement, the cilia propel the liquid 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 that is located in the center and is 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 the central cavity hollow between all of them.

Returning to the axoneme, it must be indicated that The pairs of microtubules that compose it are attached to each other thanks to the effect of the protein nexin and by protein spokes In turn, in these outer couples we also find dynein, another protein, whose usefulness in this case is to generate the movement of the cilia 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 movement at a structural level.

2. Transportation

Another key function of microtubules is to transport organelles within the cell cytoplasm, which may be vesicles or another type. This mechanism is possible because the microtubules would act as a kind of rails along 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 cellular rhythms. .

Regarding this function, microtubules They would be a mere path for the organelles, but an interaction would not be generated between both elements On the contrary, 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 on the microtubules, since dyneins are used for movement towards the minus end, while kinesin is used to move towards the plus end.

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 several microtubules that connect the centrioles and centromeres of chromosomes while the process of cell division occurs either by mitosis or meiosis.

4. Cellular shape

We already know that there are many types of cells, each with its own characteristics and disposition. Microtubules would help provide the cell with the specific 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 to ensuring that certain elements of the cell are in the place where they should be to perform their functions properly This is the case, for example, of such fundamental organelles as the endoplasmic reticulum or the Golgi apparatus.

5. Organization of filaments

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