Microtubules: what they are, their composition, and what they are used for.

An overview of the characteristics, functions and constituent parts of microtubules.

Cells are composed of a multitude of structures that, just like in a watch, make it perform its functions with absolute precision.

One of the structures that we can find within this complex organic machinery are microtubules. We are going to deepen in the characteristics of these elements and which are the functions that they fulfill in our organism.

What are microtubules? Characteristics of these structures

Microtubules are microscopic tubes that are found in each of our cells, starting in the MTOC.starting at 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 an interior diameter of only 12 nanometers, and their length can reach a few microns, a distance that may seem small but which, 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 made up of 13 protofilaments, which in turn are made up of 13 protofilaments.The 13 protofilaments are arranged one against the other to form the cylindrical structure, leaving the hollow center part. In addition, 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 bacterial cells there are some differences with respect to the rest of eukaryotic cells. In this case the tubulins would be bacterial specific, and would form 5 protofilaments instead of the usual 13 that we saw before. In any case, these microtubules function in a similar way to the others.

Dynamic instability

One of the qualities that characterizes microtubules is the so-called dynamic instability.. This is a constant process in this structure whereby 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 may continue to shorten until they are completely broken down to start the cycle all over again by polymerizing again.. This process of polymerization, i.e. growth, occurs most frequently at the + end, i.e. at the b-tubulin end.

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

Note that the speed of the process is critical to understanding what can happen next. If the dimers bind to the microtubules faster than the hydrolysis itself occurs, this means that there will always be a so-called cap of GTPs at the plus end of the dimers. On the contrary, in the event that the hydrolysis is faster than the polymerization itself (because the latter has slowed down its process), what we will obtain at the other end will be a GTP-GDP dimer.

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

The tubulin-GDP dimers that have become loose are soon converted into tubulin-GTP dimers, so that they are once again available to bind to the microtubules again. In this way, the dynamic instability that we were talking about at the beginning occurs, causing the microtubules to grow and shrink without stopping, in a perfectly balanced cycle.

Functions

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

1. Cilia and flagella

Microtubules form a large part of other important elements of the cell, such as cilia and flagella, which are basically microtubules.which are basically microtubules but with a plasma membrane surrounding them. These cilia and flagella are the structure that the cell uses to move and also as a sensitive element to capture various information from the environment essential for certain cellular processes.

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

Both cilia and flagella are complex elements that can harbor 250 types of protein. In each cilium and flagellum we find the axoneme, a central set of microtubules covered by the plasma membrane mentioned above. These axonemes are composed 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 between all of them hollow.

Returning to the axoneme, it is necessary to indicate that the pairs of microtubules that compose it are adhered to each other thanks to the effect of the nexin protein and by spokes of proteins. In turn, in these external pairs we also find dynein, another protein, whose utility 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 a movement at a structural level.

2. Transport

Another key function of microtubules is the transport of organelles within the cell cytoplasm.These can be vesicles or other types of organelles. This mechanism is possible because the microtubules 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. Bearing in mind that axons can measure not only centimeters, but meters in certain species, it gives us an idea of the growth capacity of the microtubules themselves to support this transport function, so essential in cellular rhythms.

With respect to this function, the microtubules would be a mere pathway for the organelles, but no interaction between the two elements would be generated.. Instead, movement would be achieved through motor proteins, such as dynein, which we have already seen, and also kinesin. The difference between the two types of protein is the direction they take in 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 form another of the fundamental structures of the cell, in this case the achromatic, mitotic or meiotic spindle. It is formed by several microtubules that connect the centrioles and centromeres of the chromosomes during the process of cell division, either by mitosis or meiotic mitosis.either by mitosis or meiosis.

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 particular 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 ensure 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 such fundamental organelles as the endoplasmic reticulum or the Golgi apparatus.

5. Organization of filaments

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

Bibliographical 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.