A separate group of cytoskeletal proteins are microtubule proteins. These include tubulin, microtubule-associated proteins (MAP 1, MAP 2, MAP 4, tau, etc.) and translocator proteins (dynein, kinesin, dynamin). Microtubules are protein tubular structures with a diameter of about 25 nm and a length of up to several tens of micrometers; their wall thickness is about 6 nm. They are an essential component of the cytoplasm of eukaryotic cells. Microtubules form the spindle of division (achromatic figure) in mitosis and meiosis, the axoneme (central structure) of motile cilia and flagella, the wall of centrioles and basal bodies. Microtubules play an important, if not key, role in cell morphogenesis and in some types of cell motility.
The walls of microtubules are built from the protein tubulin, which accounts for 90% by weight. Tubulin is a globular protein that exists as a dimer of α- and β-subunits with a molecular weight of ~55 kDa. The microtubule has the shape of a hollow cylinder, the wall of which consists of linear chains of tubulin dimers, the so-called protofilaments. In protofilaments, the α-subunit of the previous dimer is connected to the β-subunit of the next. Dimers in adjacent protofilaments are displaced relative to each other, forming helical rows. The cross section shows 13 tubulin dimers, which corresponds to 13 protofilaments in
microtubule wall (Fig. 9). Each subunit contains about 450 amino acids and the amino acid sequences of the subunits are approximately 40% homologous to each other. Tubulin is a GTP-binding protein, and the β-subunit contains a labile bound GTP or GDP molecule that can exchange with GTP in solution, and the α-subunit contains a tightly bound GTP molecule.
Rice. 9. Microtubule structure.
Tubulin is capable of spontaneous polymerization in vitro. Such polymerization is possible at physiological temperatures and favorable ionic conditions (absence of Ca2+ ions) and requires two factors: a high concentration of tubulin and the presence of GTP. Polymerization is accompanied by hydrolysis of GTP, and tubulin in the microtubule remains bound to GDP, while inorganic phosphate goes into solution.
Tubulin polymerization consists of two phases: nucleation and elongation. During nucleation, seeds are formed, and during
elongation - their elongation with the formation of microtubules. It should be noted that during the polymerization of tubulin, subunits are added only at the ends of microtubules.
Opposite ends of microtubules differ in growth rates. The fast growing end is called the plus end, and the slow growing end is called the minus end of the microtubule (see Fig. 9). In the cell, the (–) ends of microtubules are usually associated with the centrosome, while the (+) ends are directed toward the periphery and often reach the very edge of the cell.
Microtubules are susceptible dynamic instability.
At a constant amount of polymer, spontaneous growth or shortening of individual microtubules occurs up to their complete disappearance. Due to the delay of GTP hydrolysis in relation to the incorporation of tubulin, a GTP cap is formed at the end of the microtubule, which is in the process of growth, consisting of 9-18 GTP-tubulin molecules. The GTP cap stabilizes the end of the microtubule and promotes its further growth. If the rate of incorporation of new heterodimers is less than the rate of GTP hydrolysis, or in the case of mechanical rupture of the microtubule, an end without a GTP cap is formed. This end has a reduced affinity for new tubulin molecules; he starts to figure it out.
The polymerization and depolymerization of microtubules is induced by changes in temperature, ionic conditions, or the use of special chemical agents. Among the substances that cause irreversible disassembly, indole alkaloids (colchicine, vinblastine, vincristine, etc.) are widely used.
MICROTUBE ASSOCIATED PROTEINS
Microtubule-associated proteins are divided into two groups: structural MAPs (microtubule-associated proteins) and microtubule-associated proteins.
translocators.
Structural IDAs
A common property of structural MAPs is their permanent association with microtubules. Another common property of this group of proteins is that, unlike translocator proteins, when interacting with tubulin, they all bind to the C-terminal part of the molecule about 4 kDa in size.
There are high molecular weight MAP 1 and MAP 2, tau proteins with a molecular weight of about 60-70 kDa and MAP 4 or MAP U with a molecular weight of about 200 kDa.
Thus, the MAP 1B molecule (a representative of the MAP 1 protein group) is a stoichiometric complex of one heavy and two light chains, it is an elongated rod-shaped molecule 190 nm long, having at one end a globular domain 10 nm in diameter (apparently, a microtubule-binding site). ); its molecular weight is 255.5 kDa.
MAP 2 is a thermostable protein. It retains the ability to interact with microtubules and remain in their composition in several assembly-disassembly cycles after heating to 90°C.
Structural MAPs are able to stimulate initiation and elongation and stabilize finished microtubules; stitch microtubules into bundles. Short α-
helical hydrophobic sequences at the N-terminus of MAP and tau, closing the MAP molecules sitting on neighboring microtubules, like a zipper. The biological role of such cross-linking may be to stabilize the structures formed by microtubules in the cell.
To date, experimental studies have established that, in addition to regulating the dynamics of microtubules, structural MAPs have two more main functions: cellular morphogenesis and participation in the interaction of microtubules with other intracellular structures.
Translocator proteins
A distinctive feature of the proteins of this group is the ability to convert the energy of ATP into a mechanical force that can move particles along microtubules or microtubules along the substrate. Accordingly, translocators are mechanochemical ATPases, and their ATPase activity is stimulated by microtubules. Unlike structural MAPs, translocators are associated with microtubules only at the time of ATP-dependent movement.
Translocator proteins are divided into two groups: kinesin-like proteins (mediate movement from the (–) end to the (+) end of microtubules) and dynein-like proteins (movement from the (+) end to the (–) end of microtubules) (Fig. 10 ).
Kinesin is a tetramer of two light (62 kDa) and two heavy (120 kDa) polypeptide chains. Kinesin molecule
has the shape of a rod with a diameter of 2–4 nm and a length of 80–100 nm with two globular heads at one end and a fan-shaped extension at the other (Fig. 11).
Rice. 10. Proteins-translocators.
There is a hinge section in the middle of the rod. The N-terminal fragment of the heavy chain with a size of about 50 kDa, which has mechanochemical activity, is called the kinesin motor domain.
Rice. 11. The structure of the kinesin molecule.
General characteristics of microtubules. The essential components of the cytoskeleton include microtubules (Fig. 265), filamentous non-branching structures, 25 nm thick, consisting of tubulin proteins and their associated proteins. During polymerization, tubulins form hollow tubes (microtubules), which can be several microns long, and the longest microtubules are found in the sperm tail axoneme.
Microtubules are located in the cytoplasm of interphase cells singly, in small loose bundles, or in the form of densely packed formations as part of centrioles, basal bodies in cilia and flagella. During cell division, most of the microtubules of the cell are part of the division spindle.
By structure, microtubules are long hollow cylinders with an outer diameter of 25 nm (Fig. 266). The wall of microtubules consists of polymerized tubulin protein molecules. During polymerization, tubulin molecules form 13 longitudinal protofilaments, which are twisted into a hollow tube (Fig. 267). The size of the tubulin monomer is about 5 nm, equal to the thickness of the microtubule wall, in the cross section of which 13 globular molecules are visible.
The tubulin molecule is a heterodimer consisting of two different subunits, a-tubulin and b-tubulin, which upon association form the tubulin protein itself, initially polarized. Both subunits of the tubulin monomer are bound to GTP; however, GTP on the a-subunit does not undergo hydrolysis, in contrast to GTP on the b-subunit, where GTP is hydrolyzed to GDP during polymerization. During polymerization, tubulin molecules are combined in such a way that the a-subunit of the next protein associates with the b-subunit of one protein, and so on. Consequently, individual protofibrils arise as polar filaments, and accordingly the entire microtubule is also a polar structure, having a rapidly growing (+) end and a slowly growing (-) end (Fig. 268).
With a sufficient concentration of protein, polymerization occurs spontaneously. But during spontaneous polymerization of tubulins, hydrolysis of one molecule of GTP associated with b-tubulin occurs. During microtubule growth, tubulin binding occurs at a faster rate at the growing (+)-end. However, if the concentration of tubulin is insufficient, the microtubules can be disassembled from both ends. The disassembly of microtubules is facilitated by lowering the temperature and the presence of Ca ++ ions.
Microtubules are very dynamic structures that can emerge and disassemble fairly quickly. In the composition of isolated microtubules, additional proteins associated with them, the so-called microtubules, are found. MAP proteins (MAP - microtubule accessory proteins). These proteins, by stabilizing microtubules, accelerate the process of tubulin polymerization (Fig. 269).
The role of cytoplasmic microtubules is reduced to two functions: skeletal and motor. The skeletal, scaffold, role is that the location of microtubules in the cytoplasm stabilizes the shape of the cell; when dissolving microtubules, cells that had a complex shape tend to acquire the shape of a ball. The motor role of microtubules is not only that they create an ordered, vector, system of movement. Cytoplasmic microtubules, in association with specific associated motor proteins, form ATPase complexes capable of driving cellular components.
In almost all eukaryotic cells in the hyaloplasm one can see long unbranched microtubules. In large quantities, they are found in the cytoplasmic processes of nerve cells, in the processes of melanocytes, amoebas and other cells that change their shape (Fig. 270). They can be isolated by themselves, or it is possible to isolate their forming proteins: these are the same tubulins with all their properties.
microtubule organization centers. The growth of microtubules of the cytoplasm occurs polarly: the (+) end of the microtubule grows. The lifetime of microtubules is very short, so new microtubules are constantly being formed. The process of beginning the polymerization of tubulins, nucleation, occurs in clearly defined areas of the cell, in the so-called. microtubule organizing centers (MOTC). In the CMTC zones, the laying of short microtubules occurs, their (-) ends facing the CMTC. It is believed that the (--)-ends in the COMT zones are blocked by special proteins that prevent or limit the depolymerization of tubulins. Therefore, with a sufficient amount of free tubulin, an increase in the length of microtubules extending from the COMT will occur. As COMT in animal cells, mainly cell centers containing centrioles are involved, as will be discussed below. In addition, the nuclear zone can serve as the CMT, and during mitosis, the poles of the fission spindle.
One of the purposes of cytoplasmic microtubules is to create an elastic, but at the same time stable intracellular skeleton, necessary to maintain the shape of the cell. In disc-shaped amphibian erythrocytes, a tourniquet of circularly laid microtubules lies along the cell periphery; bundles of microtubules are characteristic of various outgrowths of the cytoplasm (axopodia of protozoa, axons of nerve cells, etc.).
The role of microtubules is to form a scaffold to support the cell body, to stabilize and strengthen cell outgrowths. In addition, microtubules are involved in cell growth processes. Thus, in plants, in the process of cell elongation, when a significant increase in cell volume occurs due to an increase in the central vacuole, large numbers of microtubules appear in the peripheral layers of the cytoplasm. In this case, microtubules, as well as the cell wall growing at this time, seem to reinforce, mechanically strengthen the cytoplasm.
By creating an intracellular skeleton, microtubules are factors in the oriented movement of intracellular components, setting spaces for directed flows of various substances and for the movement of large structures. Thus, in the case of fish melanophores (cells containing melanin pigment) during the growth of cell processes, pigment granules move along microtubule bundles.
In the axons of living nerve cells, one can observe the movement of various small vacuoles and granules that move both from the cell body to the nerve ending (anterograde transport) and in the opposite direction (retrograde transport).
Proteins responsible for the movement of vacuoles have been isolated. One of them is kinesin, a protein with a molecular weight of about 300,000.
There is a whole family of kinesins. Thus, cytosolic kinesins are involved in the transport of vesicles, lysosomes, and other membrane organelles through microtubules. Many of the kinesins bind specifically to their cargoes. So some are involved in the transfer of only mitochondria, others only synaptic vesicles. Kinesins bind to membranes through membrane protein complexes - kinectins. Spindle kinesins are involved in the formation of this structure and in chromosome segregation.
Another protein, cytoplasmic dynein, is responsible for retrograde transport in the axon (Fig. 275). It consists of two heavy chains - heads that interact with microtubules, several intermediate and light chains that bind to membrane vacuoles. Cytoplasmic dynein is a motor protein that carries cargo to the minus end of microtubules. Dyneins are also divided into two classes: cytosolic - involved in the transfer of vacuoles and chromosomes, and axonemic - responsible for the movement of cilia and flagella.
Cytoplasmic dyneins and kinesins have been found in almost all types of animal and plant cells.
Thus, in the cytoplasm, the movement is carried out according to the principle of sliding filaments, only along the microtubules it is not filaments that move, but short molecules - movers associated with moving cellular components. The similarity with the actomyosin complex of this system of intracellular transport lies in the fact that a double complex (microtubule + mover) is formed, which has a high ATPase activity.
As can be seen, microtubules form radially diverging polarized fibrils in the cell, the (+)-ends of which are directed from the center of the cell to the periphery. The presence of (+) and (-)-directed motor proteins (kinesins and dyneins) creates the possibility for the transfer of its components in the cell both from the periphery to the center (endocytic vacuoles, recycling of ER vacuoles and the Golgi apparatus, etc.), and from the center to periphery (ER vacuoles, lysosomes, secretory vacuoles, etc.) (Fig. 276). This polarity of transport is created due to the organization of a system of microtubules that arise in the centers of their organization, in the cell center.
Microtubules are one of the essential components of the plant cell cytoplasm. Morphologically, microtubules are long hollow cylinders with an outer diameter of 25 nm. The wall of microtubules consists of polymerized tubulin protein molecules. During polymerization, tubulin molecules form 13 longitudinal protofilaments, which are twisted into a hollow tube. The exchange of the tubulin monomer is about 5 nm, which is equal to the thickness of the microtubule wall, in the cross section of which 13 globular molecules are visible.
A microtubule is a polar structure with a rapidly growing plus end and a slowly growing minus end.
Microtubules are very dynamic structures that can emerge and disassemble fairly quickly. When using electronic signal amplification systems in a light microscope, one can see that microtubules grow, shorten, and disappear in a living cell; are constantly in dynamic instability. It turned out that the average half-life of cytoplasmic microtubules is only 5 minutes. Thus, in 15 minutes, about 80% of the entire population of microtubules is renewed. As part of the fission spindle, microtubules have a lifetime of about 15–20 s. However, 10–20% of microtubules remain relatively stable for quite a long time (up to several hours).
Microtubules are structures in which 13 protofilaments, consisting of α- and β-tubulin heterodimers, are stacked around the circumference of a hollow cylinder. The outer diameter of the cylinder is about 25 nm, the inner diameter is about 15.
One end of the microtubule, called the plus end, constantly attaches free tubulin to itself. From the opposite end - the minus end - tubulin units are split off.
There are three phases in microtubule formation:
delayed phase, or nucleation. This is the stage of microtubule nucleation, when tubulin molecules begin to combine into larger formations. This connection is slower than the attachment of tubulin to an already assembled microtubule, which is why the phase is called delayed;
polymerization phase, or elongation. If the concentration of free tubulin is high, its polymerization occurs faster than the depolymerization at the minus end, thereby elongating the microtubule. As it grows, the concentration of tubulin drops to a critical one and the growth rate slows down until entering the next phase;
steady state phase. Depolymerization balances polymerization and microtubule growth stops.
Laboratory studies show that the assembly of microtubules from tubulins occurs only in the presence of guanosine triphosphate and magnesium ions.
Fig.1. Microtubule self-assembly steps
Recently, the assembly and disassembly of microtubules has been observed in living cells. After introducing antibodies to tubulin labeled with fluorochromes into the cell and using electronic signal amplification systems in a light microscope, it can be seen that microtubules grow, shorten, and disappear in a living cell; are constantly in dynamic instability. It turned out that the average half-life of cytoplasmic microtubules is only 5 min. Thus, in 15 minutes, about 80% of the entire microtubule population is renewed. At the same time, individual microtubules can slowly (4-7 µm/min) elongate at the growing end, and then shorten rather quickly (14-17 µm/min). In living cells, microtubules as part of the fission spindle have a lifetime of about 15–20 s. It is believed that the dynamic instability of cytoplasmic microtubules is associated with a delay in GTP hydrolysis, which leads to the formation of a zone containing non-hydrolyzed nucleotides (“GTP cap”) at the plus end of the microtubule. In this zone, tubulin molecules bind with high affinity to each other.
each other, and, consequently, the growth rate of the microtubule increases. On the contrary, with the loss of this site, microtubules begin to shorten.
However, 10–20% of microtubules remain relatively stable for quite a long time (up to several hours). Such stabilization is observed to a large extent in differentiated cells. Stabilization of microtubules is associated with either modification of tubulins or their binding to microtubule accessory (MAP) proteins and other cellular components.
Acetylation of lysine in the composition of tubulins significantly increases the stability of microtubules. Another example of tubulin modification may be the removal of terminal tyrosine, which is also characteristic of stable microtubules. These modifications are reversible.
Fig.2. Location of microtubules in the cytoplasm of fibroblast (a), melanocyte (b) and neuron (c)
Microtubules themselves are not capable of contraction, however, they are essential components of many moving cellular structures, such as cilia and flagella, like the cell spindle during mitosis, as microtubules of the cytoplasm, which are essential for a number of intracellular transports, such as exocytosis, mitochondrial movement, etc. .
In general, the role of cytoplasmic microtubules can be reduced to two functions: skeletal and motor. The skeletal, scaffold, role is that the location of microtubules in the cytoplasm stabilizes the shape of the cell; when dissolving microtubules, cells that had a complex shape tend to acquire the shape of a ball. The motor role of microtubules is not only that they create an ordered, vector, system of movement. Cytoplasmic microtubules in association with specific associated motor proteins form ATPase complexes capable of driving cellular components.
In almost all eukaryotic cells in the hyaloplasm one can see long unbranched microtubules. In large quantities, they are found in the cytoplasmic processes of nerve cells, in the processes of melanocytes, amoebas and other cells that change their shape (Fig. 270). They can be isolated by themselves, or it is possible to isolate their forming proteins: these are the same tubulins with all their properties.
Microtubules themselves are not capable of contraction, however, they are essential components of many moving cellular structures, such as the cell spindle during mitosis as microtubules of the cytoplasm, which are essential for a number of intracellular transports, such as exocytosis, mitochondrial movement, etc.
In general, the role of cytoplasmic microtubules can be reduced to two functions: skeletal and motor. The skeletal, scaffold, role lies in the fact that the location of microtubules in the cytoplasm stabilizes the shape of the cell. The motor role of microtubules is not only that they create an ordered, vector system of movement. Cytoplasmic microtubules and associations with specific associated motor proteins form ATPase complexes capable of driving cellular components. In addition, microtubules are involved in cell growth processes. In plants, in the process of cell elongation, when a significant increase in cell volume occurs due to an increase in the central vacuole, large numbers of microtubules appear in the peripheral layers of the cytoplasm. In this case, microtubules, as well as the cell wall growing at this time, seem to reinforce, mechanically strengthen the cytoplasm.
Chemical composition of microtubules
Microtubules are composed of tubulin proteins and their associated proteins. The tubulin molecule is a heterodimer consisting of two different subunits, which upon association form the tubulin protein itself, initially polarized. During polymerization, tubulin molecules are combined. Consequently, individual protofibrils arise as polar filaments, and accordingly the entire microtubule is also a polar structure, having a rapidly growing plus-end and a slowly growing minus-end. With a sufficient concentration of protein, polymerization occurs spontaneously. During spontaneous polymerization of tubulins, one molecule of GTP is hydrolyzed. During the lengthening of the microtubule, tubulin binding proceeds at a faster rate at the growing plus-end. However, if the concentration of tubulin is insufficient, the microtubules can be disassembled from both ends. Disassembly of microtubules is facilitated by lowering the temperature and the presence of Ca 2 ions.
There are a number of substances that affect the polymerization of tubulin. Thus, the alkaloid colchicine binds to individual molecules of tubulin and prevents their polymerization. This leads to a drop in the concentration of free tubulin capable of polymerization, which causes a rapid disassembly of cytoplasmic microtubules and spindle microtubules. Colcemid and nocodozol have the same effect, when washed off, complete restoration of microtubules occurs. Taxol has a stabilizing effect on microtubules, which promotes tubulin polymerization even at low concentrations. Microtubules also contain additional proteins associated with them, the so-called MAP proteins. These proteins, by stabilizing microtubules, accelerate the process of tubulin polymerization.
Functions of microtubules
Microtubules in the cell are used as "rails" to transport particles. Membrane vesicles and mitochondria can move along their surface. Transportation through microtubules is carried out by proteins called motor proteins. These are high-molecular compounds, consisting of two heavy (weighing about 300 kDa) and several light chains. Heavy chains are divided into head and tail domains. The two head domains bind to microtubules and act as motors, while the tail domains bind to organelles and other intracellular formations to be transported.
There are two types of motor proteins: cytoplasmic dyneins; kinesins.
Dyneins move cargo only from the plus-end to the minus-end of the microtubule, that is, from the peripheral regions of the cell to the centrosome. Kinesins, on the contrary, move towards the plus-end, that is, towards the cell periphery.
The movement is carried out due to the energy of ATP. The head domains of motor proteins for this purpose contain ATP-binding sites.
In addition to the transport function, microtubules form the central structure of cilia and flagella - the axoneme. A typical axoneme contains 9 pairs of united microtubules along the periphery and two complete microtubules in the center. Microtubules also consist of centrioles and a division spindle, which ensures the divergence of chromosomes to the poles of the cell during mitosis and meiosis. Microtubules are involved in maintaining the shape of the cell and the arrangement of organelles (in particular, the Golgi apparatus) in the cytoplasm of cells.
In almost all eukaryotic cells in the hyaloplasm, one can see long unbranched microtubules. In large quantities, they are found in the cytoplasmic processes of nerve cells, fibroblasts and other cells that change their shape. They can be isolated themselves, or the proteins that form them can be isolated: these are the same tubulins with all their properties.
Main functional value of such microtubules of the cytoplasm is to create an elastic, but at the same time stable intracellular scaffold (cytoskeleton), necessary to maintain the shape of the cell.
Non-membrane organelles include microtubules - a tubular formation of various lengths with an outer diameter of 24 nm, a wall thickness of about 5 nm and a "lumen" width of 15 nm. They occur in the free state in the cytoplasm of cells or as structural elements of flagella (spermatozoa), cilia (ciliated epithelium of the trachea), mitotic spindle and centrioles (dividing cells).
Microtubules are built by assembly (polymerization) of the tubulin protein. microtubules polar: the ends (+) and (-) are distinguished in them. Their growth comes from a special structure of non-dividing cells - microtubule organizing center, with which the organelle is connected by the end (-) and which is represented by two elements identical in structure to the centrioles of the cell center. Microtubules are elongated by attachment of new subunits at the end (+). In the initial phase, the direction of growth is not determined, but of the formed microtubules, those that come into contact with their (+) end with a suitable target remain. In plant cells, in which microtubules are present, structures such as centrioles have not been found.
Microtubules are involved in:
- in maintaining the shape of cells,
- in the organization of their motor activity (flagella, cilia) and intracellular transport (chromosomes in the anaphase of mitosis).
The functions of intracellular molecular motors are performed by the proteins kinesin and dynein, which have the activity of the ATPase enzyme. During flagellar or ciliary movement, dynein molecules, attaching to microtubules and using the energy of ATP, move along their surface towards the basal body, that is, towards the end (-). The displacement of microtubules relative to each other causes wave-like movements of the flagellum or cilia, inducing the cell to move in space. In the case of immobile cells, such as the ciliated epithelium of the trachea, the described mechanism is used to remove mucus from the respiratory tract with particles settling in it (drainage function).
The participation of microtubules in the organization of intracellular transports illustrates the movement of vesicles (vesicles) in the cytoplasm. Kinesin and dynein molecules contain two globular "heads" and "tails" in the form of protein chains. With the help of heads, proteins contact with microtubules, moving along their surface: kinesin from the end (-) to the end (+), and dynein in the opposite direction. At the same time, they pull the bubbles attached to the "tails" behind them. Presumably, the macromolecular organization of the "tails" is variable, which ensures the recognition of various transported structures.
With microtubules as an essential component of the mitotic apparatus, the divergence of centrioles to the poles of a dividing cell and the movement of chromosomes in the anaphase of mitosis are associated. Animal cells, cells of a part of plants, fungi and algae are characterized by a cell center (diplosome) formed by two centrioles. Under an electron microscope, the centriole looks like a "hollow" cylinder with a diameter of 150 nm and a length of 300-500 nm. The wall of the cylinder is formed by 27 microtubules grouped into 9 triplets. The function of centrioles, similar in structure to the elements of the center of organization of microtubules (see here, above), includes the formation of filaments of the mitotic spindle (the division spindle, the achromatin spindle of classical cytology), which are microtubules. Centrioles polarize the process of cell division, providing a regular divergence to its poles of sister chromatids (daughter chromosomes) in the anaphase of mitosis
The structure of kinesin (a) and the transport of vesicles along the microtubule (b)
Around each centriole is a structureless, or finely fibrous, matrix. Often you can find several additional structures associated with centrioles: satellites (satellites), foci of convergence of microtubules, additional microtubules that form a special zone, the centrosphere around the centriole.
| Parameter name | Meaning |
| Article subject: | microtubules |
| Rubric (thematic category) | Ecology |
General characteristics of microtubules. The essential components of the cytoskeleton include microtubules (Fig. 265), filamentous non-branching structures, 25 nm thick, consisting of tubulin proteins and their associated proteins. During polymerization, tubulins form hollow tubes (microtubules), which can be several microns long, and the longest microtubules are found in the sperm tail axoneme.
Microtubules are located in the cytoplasm of interphase cells singly, in small loose bundles, or in the form of densely packed formations in the composition of centrioles, basal bodies in cilia and flagella. During cell division, most of the microtubules of the cell are part of the division spindle.
By structure, microtubules are long hollow cylinders with an outer diameter of 25 nm (Fig. 266). The wall of microtubules consists of polymerized tubulin protein molecules. During polymerization, tubulin molecules form 13 longitudinal protofilaments, which are twisted into a hollow tube (Fig. 267). The size of the tubulin monomer is about 5 nm, equal to the thickness of the microtubule wall, in the cross section of which 13 globular molecules are visible.
The tubulin molecule is a heterodimer consisting of two different subunits, a-tubulin and b-tubulin, which upon association form the tubulin protein itself, initially polarized. Both subunits of the tubulin monomer are bound to GTP; however, GTP on the a-subunit does not undergo hydrolysis, in contrast to GTP on the b-subunit, where GTP is hydrolyzed to GDP during polymerization. During polymerization, tubulin molecules are combined in such a way that the a-subunit of the next protein associates with the b-subunit of one protein, and so on. Consequently, individual protofibrils arise as polar filaments, and accordingly the entire microtubule is also a polar structure, having a rapidly growing (+) end and a slowly growing (-) end (Fig. 268).
With a sufficient concentration of protein, polymerization occurs spontaneously. But during spontaneous polymerization of tubulins, hydrolysis of one molecule of GTP associated with b-tubulin occurs. During microtubule growth, tubulin binding occurs at a faster rate at the growing (+)-end. However, if the concentration of tubulin is insufficient, the microtubules can be disassembled from both ends. The disassembly of microtubules is facilitated by lowering the temperature and the presence of Ca ++ ions.
Microtubules are very dynamic structures that can emerge and disassemble fairly quickly. The isolated microtubules contain additional proteins associated with them, the so-called microtubules. MAP proteins (MAP - microtubule accessory proteins). These proteins, by stabilizing microtubules, accelerate the process of tubulin polymerization (Fig. 269).
The role of cytoplasmic microtubules is reduced to two functions: skeletal and motor. The skeletal, scaffolding, role is essentially that the location of microtubules in the cytoplasm stabilizes the shape of the cell; when dissolving microtubules, cells that had a complex shape tend to acquire the shape of a ball. The motor role of microtubules is not only that they create an ordered, vector, system of movement. Cytoplasmic microtubules, in association with specific associated motor proteins, form ATPase complexes capable of driving cellular components.
In almost all eukaryotic cells, long unbranched microtubules can be seen in the hyaloplasm. In large quantities, they are found in the cytoplasmic processes of nerve cells, in the processes of melanocytes, amoebas and other cells that change their shape (Fig. 270). Οʜᴎ are isolated by themselves, or it is possible to isolate their forming proteins: these are the same tubulins with all their properties.
microtubule organization centers. The growth of microtubules of the cytoplasm occurs polarly: the (+) end of the microtubule grows. The lifetime of microtubules is very short, in connection with this, new microtubules are constantly being formed. The process of beginning the polymerization of tubulins, nucleation, occurs in clearly defined areas of the cell, in the so-called. microtubule organizing centers (MOTC). In the CMTC zones, the laying of short microtubules occurs, their (-) ends facing the CMTC. It is believed that the (--)-ends in the COMT zones are blocked by special proteins that prevent or limit the depolymerization of tubulins. For this reason, with a sufficient amount of free tubulin, an increase in the length of microtubules extending from the COMT will occur. As COMT in animal cells, mainly cell centers containing centrioles are involved, as will be discussed below. In addition, the nuclear zone can serve as a CMT, and during mitosis, the poles of the division spindle.
One of the purposes of cytoplasm microtubules is to create an elastic, but at the same time stable intracellular skeleton, which is extremely important for maintaining the shape of the cell. In disc-shaped amphibian erythrocytes, a tourniquet of circularly laid microtubules lies along the cell periphery; bundles of microtubules are characteristic of various outgrowths of the cytoplasm (axopodia of protozoa, axons of nerve cells, etc.).
The role of microtubules is to form a scaffold to support the cell body, to stabilize and strengthen cell outgrowths. At the same time, microtubules are involved in cell growth processes. Thus, in plants, in the process of cell elongation, when a significant increase in cell volume occurs due to an increase in the central vacuole, large numbers of microtubules appear in the peripheral layers of the cytoplasm. In this case, microtubules, as well as the cell wall growing at this time, seem to reinforce, mechanically strengthen the cytoplasm.
By creating an intracellular skeleton, microtubules are factors in the oriented movement of intracellular components, setting their location of space for directed flows of various substances and for the movement of large structures.
Hosted on ref.rf
Thus, in the case of fish melanophores (cells containing melanin pigment) during the growth of cell processes, pigment granules move along microtubule bundles.
In the axons of living nerve cells, one can observe the movement of various small vacuoles and granules that move both from the cell body to the nerve ending (anterograde transport) and in the opposite direction (retrograde transport).
Proteins responsible for the movement of vacuoles have been isolated. One of them is kinesin, a protein with a molecular weight of about 300,000.
There is a whole family of kinesins. Thus, cytosolic kinesins are involved in the transport of vesicles, lysosomes, and other membrane organelles through microtubules. Many of the kinesins bind specifically to their cargoes. So some are involved in the transfer of only mitochondria, others only synaptic vesicles. Kinesins bind to membranes through membrane protein complexes - kinectins. Spindle kinesins are involved in the formation of this structure and in chromosome segregation.
Another protein, cytoplasmic dynein, is responsible for retrograde transport in the axon (Fig. 275). It consists of two heavy chains - heads that interact with microtubules, several intermediate and light chains that bind to membrane vacuoles. Cytoplasmic dynein is a motor protein that carries loads to the minus end of microtubules. Dyneins are also divided into two classes: cytosolic - involved in the transfer of vacuoles and chromosomes, and axonemic - responsible for the movement of cilia and flagella.
Cytoplasmic dynesins and kinesins have been found in virtually all types of animal and plant cells.
Τᴀᴋᴎᴍ ᴏϬᴩᴀᴈᴏᴍ, and in the cytoplasm, movement is carried out according to the principle of sliding threads, only not threads move along microtubules, but short molecules - movers associated with moving cellular components. The similarity with the actomyosin complex of this system of intracellular transport lies in the fact that a double complex (microtubule + mover) is formed, which has a high ATPase activity.
As can be seen, microtubules form radially diverging polarized fibrils in the cell, the (+)-ends of which are directed from the center of the cell to the periphery. The presence of (+) and (-)-directed motor proteins (kinesins and dynesins) creates the possibility for the transfer of its components in the cell both from the periphery to the center (endocytic vacuoles, recycling of ER vacuoles and the Golgi apparatus, etc.) and from the center to the periphery (ER vacuoles, lysosomes, secretory vacuoles, etc.) (Fig. 276). This polarity of transport is created due to the organization of a system of microtubules that arise in the centers of their organization, in the cell center.
Microtubules - concept and types. Classification and features of the category "Microtubules" 2017, 2018.
