In the service of man and nanotechnology
Anastasia Kruv, "Biomolecule"
Every cell of our body is a real metropolis. Moreover, the buildings in it are very dense – the city hall (core), energy stations (mitochondria), a chemical plant (Golgi apparatus) and many other organelles. There is also a developed road network in this city (microtubules and microfilaments), along which a special type of transport moves: protein motors – complex molecules several nanometers in size. Their services are for every taste. They transport loads (for example, substances that must be removed from cells), participate in the transmission of nerve signals, position the nucleus and separate cells during division, perform muscle contraction, etc. These amazing molecules, the importance of their normal functioning for human health and their potential application in nanotechnology will be discussed in this article.
A protein is a sequence of amino acids connected in a chain, which, in turn, is twisted into a certain volumetric structure. There are a huge number of different proteins – as well as the functions they perform in our body. One of the varieties of proteins is intracellular motors that move inside cells along special "roads" (microtubules and actin microfilaments) and can transport loads (organelles and molecules that are too large to diffuse, such as glucose) to the place of the cell where they are needed . An example of cargo can be mitochondria (energy stations of cells), granules of melanosome pigment that give the skin a brown color as a result of tanning, vesicles – "bubbles" that contain a variety of substances, including enzymes, hormones, neurotransmitters .
There are three large families of protein motors – myosin, kinesin and dynein, each of which consists of several representatives that differ from each other [3, 4]. The families are divided by:
- the type of surface on which they can move (microtubules or actin microfilaments);
- type of cargo being transported;
- movement speeds (myosin – 0.2–60 micrometers per second, dynein - 14, kinesin – 2-3);
- the distances to be overcome (from tenths of a micrometer to several millimeters – real truckers by cellular standards!) .
Despite these differences, there are common features in the structure of the motors. The oblong structure (40-100 nanometers)  and the presence of a tail consisting of chains of amino acids twisted together are characteristic. A certain load can be attached to one end of the tail, and heads can be attached to the other. Despite the name, the function of this part of the motor is more like the legs we are used to, not the heads (although maybe motors have such a quirk – walking on their heads). With the help of the heads, the motors are sequentially disconnected from the road, take a step and reconnect, after which the cycle repeats.
A small step for the motor is of great importance for humanity
To understand the mechanism of movement of protein motors, it is necessary to introduce two terms. The first is conformational changes. The phrase sounds intimidating, but its essence is simple – it is a change in the shape of a macromolecule under the influence of environmental factors. In the case of protein motors, this change leads to the fact that the "leg" is carried forward and takes a step. But, as you know, in order to move, you need energy. For cars – in the form of fuel, for us – in the form of food, and for protein motors in the form of an ATP molecule – the main source of energy of cells. ATP consists of Adenine (one of the four "letters" encoding DNA) and Three Phosphoric acid residues. This molecule contains a large amount of energy, which is released when the above residues are separated. This process (with the separation of one residue and the conversion of ATP into ADP – Adenine and two Phosphoric acid residues) refills the "tank" of protein motors. Armed with this knowledge, let's consider the process of motor movement using the example of kinesin.
Kinesin has two "legs" that work very smoothly (Fig. 1, video). At the beginning of each cycle, the posterior "leg" is firmly connected to the microtubule ("road") and the ATP molecule, the anterior is connected to ADP and weakly connected to the road. Then, a chemical reaction occurs on the hind leg about to take a step, as a result of which ATP turns into ADP, and the connection with the "road" weakens. Meanwhile, the anterior "leg" loses ADP, but ATP joins it (as a sign of consolation) and the "leg" binds tightly to the microtubule. A conformational change occurs, as a result of which the shape of the motor changes so that the rear "leg" moves forward, connects to the microtubule and becomes the leading one . The cycle closes, after which everything repeats again.
Figure 1. Mechanism of kinesin movement
Video. How the protein kinesin steps
And instead of a heart – a protein motor
One of the fascinating examples of the work of protein motors is muscle contraction. Muscles are a collection of elongated fibers (Fig. 2a), consisting of repeatedly repeating links (Fig. 2b). Each of them is assembled from parallel "roads" – actin filaments (Fig. 2b, d). For myosins, this is a real multi-lane highway along which they "rush" in opposite directions. But the fact is that the "tails" of the motors are woven together, and the latter "pull" with the same force. As a result, when myosins "walk", they remain in place. And everything would be like in Krylov's famous fable, but in this case the "roads" are moving towards each other in horizontal directions (Fig. 2d). And this is nothing but muscle contraction.
Figure 2. The mechanism of muscle contraction. Figure from , adapted.
In addition to muscle contraction, protein motors perform many other functions on which the normal functioning of the body depends. But what if something goes wrong in this well-coordinated system? Well, it happens, and it can lead to various diseases. Let's look at some examples.
And everything is wrong, and everything is wrong
At the beginning of the article, an analogy is given between a cell and a megapolis. And what do the residents of the latter most often suffer from? That's right, from traffic jams. In cells, this also happens when too many protein motors gather in one place, the speed of their movement slows down, as a result of which goods are not delivered on time.
Another problem of big cities is the theft of vehicles. When intruders (some viruses, for example, herpes) penetrate our body, they use advanced mechanisms to take over the motors and get to the target . Other viruses are capable of destroying microtubules and actin filaments, as a result of which the motor is at the wrong time and in the wrong place.
Malfunction may occur as a result of mutation of motors or auxiliary proteins, exposure to chemicals (for example, blocking movement). There are also cases when the motor is working properly, but the cells "exceed official authority", which is vaccinated to diseases. For example, as in the case of cancer, when cells begin to divide abnormally (remember, at the beginning of the article it was said that protein motors are involved in cell division) . Errors in the operation of protein motors can also lead to kidney diseases, chronic respiratory tract infections, amyotrophic lateral sclerosis, Alzheimer's disease, etc.
Protein motors in nanotechnology
If viruses use motors for their own purposes, can't we do the same? This idea fascinates many researchers, because the ability of protein motors to recognize and transport certain loads potentially has many applications. For example, the idea was put forward to use them for sorting and filtering substances, as well as the delivery of building materials for the assembly of various structures. Thus, the transfer of gold nanowires (in this case, representing actin microfilaments partially coated with gold) was demonstrated, which theoretically would find application for assembling miniature electrical circuits in a bottom-up approach . This approach, an alternative to the commonly used top–down approach, is designed to overcome the limitations of the latter in terms of miniaturization. These strategies differ in that, drawing an analogy, we can say that "bottom-up" is like assembling a figure from Lego bricks, and "top-down" is closer to carving figures from blocks of marble.
Another suggestion is to use motors in biosensors (so named because of the presence of biological recognition elements, for example, antibodies – fighters of the immune system that react only with the substance for which the test is being done ). It is assumed that protein motors could deliver the analyzed substances directly to the recognizing element. This would allow, firstly, to detect extremely small amounts of substances (up to one molecule, which is much more difficult than finding a needle in a haystack!). This would reduce the volume of tests taken and, thereby, reduce the negative impact on patients. Secondly, it would help the recognizing element and the analyzed substance to meet as quickly as possible, which would have a positive effect on the speed of obtaining analysis results. In addition, this method could become an alternative to nanofluidics, the science of controlling fluid flows in miniature channels. The fact is that the use of nanafluidics is associated with the creation of quite large pressures, which consumes a significant part of the battery energy. At the same time, protein motors are powered by ATP, which reduces electricity costs. As a result, compact, cheap devices are available to a wide mass of people. Thus, the use of motors can potentially expand the possibilities of diagnostics, making it faster, cheaper and more accessible, not only in laboratories, but also at home .
An alternative application of protein motors is targeted delivery [11, 12]. Its goal is to reduce the doses of drugs taken and reduce the side effects of their use. Imagine that you decided to paint the ceiling in the room, but did not cover the furniture and floor with anything. As a result, you will most likely make the ceiling, and it will be as good as new, but all the furniture will be in traces of paint and will suffer for nothing. This also happens in the body when the treatment of one organ has a negative effect on others. But another method is also possible when healthy organs do not interact with the drug, since it is in a protective shell that resolves and releases the medicinal substance only after the drug has been delivered (for example, by motor) to the destination. This revolutionary approach has a high chance of becoming the future of medicine.
In order to realize all these possibilities, it is necessary to learn first-class control over protein motors: to start, stop and direct their movement, attach and release loads, etc. In order to create such a "joystick", scientists are trying to use heat (under the influence of which polymers specially added on the path of the motors shrink and stop blocking movement), chemicals, light, electromagnetic field [9, 13]. In parallel, the ideas of creating artificial motors that could be more resistant to the environment (denaturation – destruction of proteins is a very serious problem) and immediately adapted to respond to control signals are being considered.
The use of protein motors in nanotechnology is currently far from being realized. It is not yet known how practical this approach will be and whether it will find its niche (for most of the applications discussed, there are much more developed and widely used methods, such as, for example, the creation of electrical circuits using lithography in the semiconductor industry). Now scientists are putting forward proposals, proving their theoretical possibility, and making isolated demonstrations. But even if these ideas never come to fruition, their study in itself brings a huge amount of information useful for chemistry, biology, materials science, medicine and electronics.
Nature has been creating and honing various structures and methods of controlling them for millions of years, so it is not easy for humanity to achieve the same perfection. So far, these are dreams, but much of what seems impossible today has a chance to become the most ordinary thing tomorrow.
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