Lecture: musculoskeletal system of the cell. cytoskeleton
Once again dedicating the publication to biological topics, let's talk about one of the most important in it - the cytoskeleton (from the Greek "cytos", which means "cell"). We will also consider the structure and functions of the cytoskeleton.
General concept
Before talking on this topic, the concept of cytoplasm should be given. This is the internal semi-fluid environment of the cell, which is limited by the cytoplasmic membrane. This internal environment does not include the cell nucleus and vacuoles.
And the cytoskeleton is the framework of the cell, which is located in the cells of eukaryotes (living organisms containing a nucleus in their cells). It is a dynamic structure that is capable of change.
Some sources discussing the structure and functions of the cytoskeleton give a slightly different definition, formulated in different words. It is a musculoskeletal system of cells, which is formed by protein filamentous structures. Participates in cell movement.
Structure
The cytoskeleton was formed by proteins. In its structure, several systems are distinguished, the name of which comes from the main structural elements, or from the main proteins that are part of these systems.
Since the cytoskeleton is a structure, it has three main components. They play an important role in the life and movement of cells.
The cytoskeleton consists of microtubules and microfilaments. The latter are otherwise called actin filaments. All of them are unstable by nature: they are constantly assembled and disassembled. Thus, all components have a dynamic equilibrium with the proteins corresponding to them.
Cytoskeletal microtubules, which are a rigid structure, are present in the cytoplasm of eukaryotes, as well as in its outgrowths, which are called flagella and cilia. Their length can vary, with some reaching several micrometers in length. Sometimes microtubules are connected by handles or bridges.
Microfilaments are made of actin, a protein similar to that found in muscle. They also contain other proteins in small quantities. The main difference between actin filaments and microtubules is that some of them cannot be seen under a light microscope. In animal cells they are combined into a plexus under the membrane and are thus associated with its proteins.
Animal microfilaments and plant cells also interact with the protein myosin. Moreover, their system has the ability to contract.
Intermediate filaments are composed of various proteins. The structural component not studied enough. There is a possibility that plants do not have it at all. Also, some scientists believe that intermediate filaments are in addition to microtubules. It has been precisely proven that when the microtubule system is destroyed, the filaments are rearranged, and with the reverse procedure, the influence of the filaments has practically no effect on the microtubules.
Functions
Speaking about the structure and functions of the cytoskeleton, we will list exactly how it affects the cell.
Thanks to microfilaments, proteins move along the cytoplasmic membrane. The actin contained in them takes part in muscle contractions, phagocytosis, cell movements, as well as in the process of fusion of sperm and eggs.
Microtubules are actively involved in maintaining cell shape. Another of their functions is transport. They transport organelles. They can perform mechanical work, which includes the movement of mitochondria and cilia. Microtubules play a particularly important role in the process of cell division.
They are aimed at creating or maintaining a certain cellular asymmetry. Under certain influences, microtubules are destroyed. This may lead to the loss of this asymmetry.
The functions of the cytoskeleton also include cell adaptation to external influence, processes of endo- and exocytosis.
Thus, we examined what functions the cytoskeleton performs in a living organism.
Eukaryotes
There are certain differences between eukaryotes and prokaryotes. Therefore, it is important to consider the cytoskeleton of these animals. Eukaryotes (animals that have a nucleus in their cell) have three types of filaments.
Actin filaments (in other words, microfilaments) are located near the cell membrane. They take part in intercellular interaction and also transmit signals.
Intermediate filaments are the least dynamic part of the cytoskeleton.
Microtubules are hollow cylinders and are a very dynamic structure.
Prokaryotes
Prokaryotes include single-celled organisms- bacteria and archaea that do not have a formed nucleus. Prokaryotes were thought to have no cytoskeleton. But since 2001, active research on their cells began. Homologues (similar, similar) of all elements of the eukaryotic cytoskeleton were found.
Scientists have found that one of the protein groups of the bacterial cell skeleton has no analogues among eukaryotes.
Conclusion
Thus, we examined the structure and functions of the cytoskeleton. It plays an extremely important role in the life of the cell, providing its most important processes.
All cytoskeletal components interact. This is confirmed by the existence of direct contacts between microfilaments, intermediate filaments and microtubules.
According to modern ideas, the most important link that unites various cellular parts and carries out data transfer is the cytoskeleton.
The cytoskeleton is a set of thread-like protein structures - microtubules and microfilaments that make up the musculoskeletal system of the cell. Only eukaryotic cells have a cytoskeleton; prokaryotic (bacterial) cells do not have it, which is an important difference between these two types of cells. The cytoskeleton gives the cell a certain shape even in the absence of a rigid cell wall. It organizes the movement of organelles in the cytoplasm (the so-called flow of protoplasm), which underlies amoeboid movement. The cytoskeleton is easily rebuilt, providing, if necessary, a change in cell shape. The ability of cells to change shape determines the movement of cell layers in the early stages embryonic development . During cell division ( mitosis
) the cytoskeleton “disassembles” (dissociates), and its self-assembly occurs again in the daughter cells.
The cytoskeleton performs three main functions.
1. Serves as a mechanical framework for the cell, which gives the cell its typical shape and provides communication between the membrane and organelles. The framework is a dynamic structure that is constantly updated as external conditions and the state of the cell change.
2. Acts as a “motor” for cellular movement.
24. The role of the immunocytochemistry method in the study of the cytoskeleton. Features of the organization of the cytoskeleton in muscle cells.
Immunocytochemical analysis is a method that allows for immunological analysis of cytological material while maintaining cell morphology. ICC is one of many types of immunochemical method: enzyme immunoassay, immunofluorescence, radioimmune, etc. The basis of the ICC method is the immunological reaction of antigen and antibody.
The cytoplasm of eukaryotic cells is permeated by a three-dimensional network of protein threads (filaments) called the cytoskeleton. Depending on their diameter, filaments are divided into three groups: microfilaments (6-8 nm), intermediate fibers (about 10 nm) and microtubules (about 25 nm).
All these fibers are polymers consisting of subunits of special globular proteins.
Microfilaments (actin filaments) are composed of actin, a protein most abundant in eukaryotic cells. Actin can exist as a monomer (G-actin, “globular actin”) or a polymer (F-actin, “fibrillar actin”). G-actin is an asymmetric globular protein (42 kDa), consisting of two domains. As ionic strength increases, G-actin reversibly aggregates to form a linear, coiled-coil polymer, F-actin. The G-actin molecule carries a tightly bound ATP molecule, which, when converted to F-actin, is slowly hydrolyzed to ADP, i.e. F-actin exhibits the properties of ATPase. B. Squirrels
intermediate fibers
The structural elements of intermediate fibers are proteins belonging to five related families and exhibiting a high degree of cellular specificity. Typical representatives of these proteins are cytokeratins, desmin, vimentin, glia fibrillary acidic protein [GFAP] and neurofilament.
All of these proteins have a basic core structure in the central part, which is called a supercoiled α-helix. Such dimers associate antiparallel to form a tetramer.
Microtubules are built from the globular protein tubulin, which is a dimer of α- and β-subunits. Tubulin monomers bind GTP, which is slowly hydrolyzed by GDP and GTP. Two types of proteins are associated with microtubules: structural translocator proteins.
Cytoskeletal structures include microtubules, thin microfilaments, and intermediate filaments (microfibrils).
They are made of proteins and do not have membranes. These organelles perform not only scaffolding and form-building, but also many other functions.
Microtubules. They are found in the cytoplasm of almost all cells multicellular organisms, except prokaryotes. Microtubules are examined by electron microscopy. Microtubules are arranged separately as an independent structure or form complex structures of centrioles, cilia, flagella, and spindles.
The organelle is a straight, non-branching, hollow structure. In the cytoplasm of most cells, microtubules are constantly undergoing assembly and disassembly. As a result of this dynamic equilibrium, the entire system of distribution of cytoplasmic organelles, their position in the cell, the shape of the cell, and the movement of substances in it are maintained. If you induce depolymerization of microtubules in a cell by introducing colchicine or significantly lowering the temperature, the shape of the cell will greatly change and the distribution of transport flows in it will be disrupted. Consequently, microtubules of the cytoplasm form an elastic, but quite stable intracellular skeleton - the cytoskeleton.
With light microscopy, microtubule clusters can be detected using specific antibodies to tubulin. They form a cluster near the cell center, participating in the formation of the centrosphere.
Microtubules are hollow cylinders with a total diameter of 24 nm, the internal lumen is 15 nm wide, and the wall thickness is 5 nm. Microtubules consist of globular proteins - tubulins (13 in a cross section). Tubulin globules have a diameter of about 5 nm, a molecular weight of 60 · 10 3 and a sedimentation coefficient of 3...4 S. Tubulins are divided into alpha and beta tubulins. Tubulins form a dimer - a protein consisting of two tubulin globules. The dimers are connected in a chain that forms a helix. Tubulins can be in two forms: globular (dispersed in the matrix) and fibrillar (in the form of microtubules). A significant amount of guanine diphosphate (GDP) is always found in the composition of tubulins.
Microtubules are formed in microtubule organizing centers, or microtubule organizing centers: centrioles, basal bodies of cilia and flagella, kinetochore zones of mitotic chromosomes.
Microtubule formation occurs by self-assembly. This requires: tubulin globules, GTP (guanine triphosphate), proteins that stimulate polymerization, a high content of Mg 2+ ions and the absence of Ca 2+ ions. If these conditions are met, then the formation of new microtubules occurs even in a test tube (in vitro).
At the beginning of polymerization of the organelle, nucleation occurs, a “seed” is formed from a very short chain of tubulins in three rows, then new tubulins begin to attach to both ends, and the size of the microtubule increases.
Microtubules have positive and negative poles. On the negative pole side, which lies closer to the microtubule organizer, tubulins polymerize more slowly and easily disintegrate into globular particles. On the side of the positive pole, directed towards the periphery of the cell, polymerization proceeds faster.
Microtubules quickly disintegrate into globular particles suspended in the hyaloplasm. The breakdown of the organelle can be provoked by increasing the content of calcium ions inside the cell.
Microtubules form centrioles, have a supporting scaffold function, control transport flows in the cytoplasm, participating in cyclosis, provide the framework basis for cilia and flagella, form the spindle in mitosis and meiosis, etc.
By creating an intracellular skeleton, microtubules can be factors in the oriented movement of the cell as a whole and its intracellular components, and by their arrangement, set vectors for directed flows of various substances and for the movement of large structures.
When the microtubules of fibroblasts in culture were destroyed, the shape of the cells changed from elongated to round or polygonal (polygonal), their movements became chaotic, that is, these organelles control the direction of cell movement.
The destruction of microtubules by colchicine disrupts the transport of substances in the axons of nerve cells, leading to a blockade of secretion, etc. Various small vacuoles, for example, synaptic vesicles containing neurotransmitters in the axon of a nerve cell or mitochondria, can move along cytoplasmic interphase microtubules, as if on rails. These movements are possible due to the connection of microtubules with special proteins - translocators (dyneins and kinesins), which, in turn, contact the transported structures.
Associated with microtubule tubulins is the protein kinesin, which has ATPase activity and ensures the transport of organelles and other structures from the center to the periphery (from the negative to the positive pole of the microtubule). A similar function, but in the opposite direction, is performed by cytoplasmic dynein.
Due to this, microtubules can control transport flows and the distribution of structures in the cell.
If both ends of the microtubules are “closed” (copied), that is, connected, for example, to the cell center and the outer membrane, then the microtubules do not disintegrate and can be methylated (attach methyl groups), acquiring a stable form. Such methylated, stable microtubules can perform specialized functions: serve as the basis for cilia, flagella and the cell center. In the neuron they form a special-purpose organelle - the neurotubule.
Neurotubules perform a variety of functions: support-framework, provide transport of substances (axocurrents), control the release of mediators, regulate regeneration processes in damaged nerve fibers, etc.
The ends of microtubules can be copied by proteins from microtubule organizing centers (MOTCs) or microtubule organizing centers (MTOCs).
Low molecular weight t-proteins and high molecular weight MAPs (microtubule associated proteins) can attach to the sides of microtubules. These proteins form “spikes” on microtubules, connect cytoskeletal elements with each other, stabilize microtubules, can be located at the end of a microtubule, cover it (cap) and thereby prevent their disintegration (depolymerization).
Microtubules are integral part cell center, cilia and flagella. The microtubule system develops along with the centriole, where the initial polymerization of tubulins and the growth of cytoskeletal microtubules occurs.
Intermediate filament. These are threads with a transverse diameter of 8...11 nm. Their accumulations form thicker structures - microfibrils, which in neurons participate in the formation of neurofibrils. They provide a supporting frame function. Intermediate filaments lie in the central regions of cells in the form of a three-dimensional network. At the periphery, filaments are often combined into bundles and attached to the inner surface of desmosomes and hemidesmosomes. Intermediate filaments give cells elasticity and rigidity. By joining with the help of desmosomes to similar areas of neighboring cells, they form an extensive network - a framework that connects cells into a mechanically strong and at the same time flexible and elastic system. This is especially important in epithelial tissues, which are often subject to mechanical stress.
Intermediate filaments are non-branching filaments located in navels (microfibrils). These fibrillar structures are relatively stable compared to microtubules and thin microfilaments. They consist of fibrillar protein monomers. These fibrillar proteins in the form of an α-helix are intertwined with each other and therefore the organelle resembles a rope. Intermediate filaments are especially well developed in cells that experience significant mechanical stress (epithelial, muscle tissue).
Microfibrils are tissue-specific, since they are formed by fibrillar proteins that differ in composition depending on the origin of the cells and tissues. Desmins form intermediate filaments of muscle tissue of mesodermal origin; vimentins - cells of mesenchymal origin (tissue of the internal environment); cytokeratins - epithelial cells; proteins of the neurofibrillary triplet - neurons; glial fibrillary acidic protein - astrocytes.
A feature of intermediate filaments is that the fibrillar proteins that form them are complementarily connected to each other: acidic cytokeratins with cytokeratins that have basic properties. The three monomers of cytokeratins are joined together in the form of an α-helix. Each such thread has a thickness of about 2 nm. These thin threads are connected into thicker formations - hollow tubes with a cross section of 8...11 nm. In some areas, the filaments become unfibered, which facilitates the connection of threads in the organelle. The threads in such a filament are curled into a loosely twisted spiral. Intermediate filaments can form large complexes (microfibrils).
Intermediate filaments in the epithelium are called tonofilaments, and microfibrils are called tonofibrils.
Unlike microtubules, intermediate filaments have no polarity and are stable components of the cytoskeleton. On the inner surface of the nuclear envelope there are structures similar to intermediate filaments. They are formed by lamin proteins and participate in the formation of the nuclear lamina. Chromatin is attached to them.
Using immunomorphological methods, the tissue origin of certain tumors is determined precisely by the proteins of their intermediate filaments, which is very important for diagnosis and the right choice type of chemotherapeutic anticancer drugs.
The chemical composition and molecular weight of intermediate filament proteins are quite diverse. Thus, it was revealed that there are about 15 types of acidic cytokeratins. There are approximately the same number of main cytokeratins. The molecular weight of basic cytokeratins ranges from 50,000 to 70,000, acidic - from 40,000 to 60,000. About 8 of the cytokeratins are part of skin derivatives (hair, claws, horns, nails, etc.). Their distribution depends on the type of epithelium. In multilayered epithelium, cytokeratins are different in different layers of the epithelium, and the predominance of one or another cytokeratin is an indirect sign of the degree of differentiation of keratinocytes (multilayered epithelial cells).
Intermediate filaments of a nerve cell - neurofilaments in vertebrates are formed by the proteins NF-Z, NF-M, NF-H, which differ significantly in molecular weight (from 57 to 150 kDa). These proteins and intermediate filaments maintain the shape of the cell bodies and processes of nerve tissue, and also fix ion channel proteins on the surface.
When the cell is significantly damaged, the intermediate filaments form a tangle and collapse. Damaged organelles and other macromolecular formations are immersed in such a ball. This probably facilitates their subsequent hydrolysis (self-digestion).
During regeneration, the networks of intermediate filaments are restored from the central regions of the cell, from the cell center, which suggests its role as a center for the formation of not only microtubules, but also intermediate filaments.
Thin microfilaments. They are thin threads with a transverse diameter of about 6 nm. Microfilaments are found in almost all cells and are universal elements of the cytoskeleton. They are concentrated on the periphery of the cell, forming the so-called “cortical” peripheral region of the cell, and in the thickness of the cytoplasm they lie in the form of a network, individual fibers or in the form of bundles. In the cortical layer of the cytoplasm, thin microfilaments form condensations under the plasmalemma in the form of dense bundles or layers. In the apical zone of the epithelium, such thickenings are called cuticle.
Thin microfilaments are visible as tightly packed bundles, directed into cell processes, where they serve as the basis for their formation (microvilli and stereocilia).
Along with support, microfilaments are an intracellular contractile apparatus that ensures not only cell mobility during active amoeboid movement, but also during the movement of cytoplasm, the movement of vacuoles, mitochondria, and cell division.
In addition, actin microfilaments also perform a scaffolding function, connecting with a number of stabilizing proteins; they can form temporary or permanent bundles or networks.
In most cells, actins (the major proteins of thin microfilaments) constitute about 5% of the total protein content. There are five forms of actin (isoforms). All isoforms are similar in amino acid sequences, but the structure and composition of the terminal sections of the polypeptide chains are different. This leads to a difference in the rate of actin polymerization, which is necessary for motor activity cells and the rate of formation of protrusions and invaginations of the cell membrane.
Actin molecules in thin microfilaments are twisted in an α-helix, arranged in the form of two chains. This actin is called F-actin. Like microtubule tubulins, actin filaments readily polymerize and break apart again into individual globules. The actin dispersed in the hyaloplasm is called G-actin.
Thin microfilaments have negative and positive poles. The region of the positive pole polymerizes more easily, and the negative pole disintegrates more easily.
The formation of a thin microfilament, like microtubules, begins with the formation of a trimer (nucleation). This is a chain of three actins. Then new actins begin to attach to this trimer (elongation) and the length of the thin filament increases. Proteins that control these processes have been identified. Thus, profillin blocks nucleation. It attaches to the active zone of the monomer and forms a dimer that cannot contact other proteins - actins. Fragmin suppresses nucleation and elongation by also binding the terminal elements of the chain.
With the help of supporting scaffold proteins, microfilaments can connect to the cell membrane - these are α-actinin, talin, vinculin, spectrin, fragmin, ankyrin, adjucin. The diversity of adhesion proteins is due to different ways attachment of microfilaments: parallel to the membrane, in the form of bundles (copy type), etc.
Microfilaments adhere to each other using the proteins fascin, α-actinin, fimbrin, filamin, and villin. These proteins can bind thin microfilaments in the form of dense (fimbrin) or loose (α-actinin) bundles or networks (filamin). Thus, the filamin protein, being also a stabilizer protein for thin microfilaments, forms cross-links at the intersections of organelles. As a result, networks of interlocking threads are formed. If both ends of the microfilaments are adhered to the membrane or some other structure (copied), they do not disintegrate and become stable. Subsequent methylation prevents microfilament breakdown.
Stable thin microfilaments are characteristic of muscle tissue, where they are called thin myofilaments. Together with myosins, they form a specialized organelle of muscle tissue - the myofibril. The protein tropomyosin stabilizes thin myofilament.
Gelsolin, villin and fragmin copy the positive pole of a thin microfilament. Acumentin performs a similar function from the negative pole.
Thin microfilaments provide a scaffolding function, control cyclosis, and participate in the formation of adhesive contacts (adhesion belt or ribbon desmosome). In adhesive bands, thin microfilaments lie parallel to the cytomembrane along the adhesive contact. They strengthen this contact by also contacting elements of the intracellular cytoskeleton.
Along with microtubules, microfilaments control the direction of transport flows and the distribution of macromolecular formations and organelles. In cyclosis important has the polarity of thin microfilaments opposite to microtubules.
Microfilaments are involved in cell movement. One of the leading factors ensuring movement is the interaction of actin with thick microfilaments containing myosins. In the presence of calcium ions in striated muscles, this interaction leads to contraction of the symplast. In smooth myocytes and non-muscle cells, a similar role is played by interaction with minimyosins, as well as the ability of actins to rapidly decompose and polymerize.
As a result of the redistribution of thin microfilaments in the cortical zone, the cell can form invaginations (pseudopodia, lamellipodia). This allows for local movements and movements of the entire cell. A similar process underlies phagocytosis and exocytosis.
If a cell is at rest, in a liquid environment and without contact with other cells, it is distinguished by a rounded shape and a uniform network of thin filaments in the cytoplasm. In the process of studying cell movement in tissue cultures, it was proven that the movement of a cell, for example fibroblast, begins with the formation of filopodia - a filamentous outgrowth of the cytoplasm with a diameter of 0.3...0.5 μm and a length of up to 20 μm. Then flat plate-like outgrowths are formed - lamellipodia or outgrowths resembling frills - “raffles”. The lamellipodia then merge so that a special zone is formed - lamellar cytoplasm, in which there are almost no organelles and ribosomes, but many microfilaments. If a cell is evenly spread out, then it is distinguished by the concentration of organelles around the nucleus, which lies in the center. To the outside of the organelles, thin microfilaments form a ring.
During the formation of lamellipodia, cell movement can be activated. The movement is caused by the predominance in one of the directions of adhesive or so-called chemotactic factors.
Chemotactic factors are substances that stimulate the movement of cells in the direction of their highest concentration. The onset of movement is accompanied by a redistribution of organelles and other structures (polarization) of the cell. Such a cell activated for movement is distinguished by the fact that the pseudopodia and lamellar cytoplasm are preserved on one side of the cell. It is this side of the cell that is the direction of its further movement. Side surfaces the cells remain inactive. The moving surface interacts with extracellular structures using point (focal) contacts. Thin filaments are distributed in the form of bundles along the axis of movement. The lamellipodia region contains numerous thin microfilaments and microtubules. With their help, the elements of the cell membrane are transported from the pole with a low content of chemotaxins to the pole with their high concentration. As a result, the cell is pulled in the direction of movement. Subsequently, the movement cycle is repeated.
During the cycle, thin microfilaments and microtubules are continuously redistributed. The microfilament network is extremely unstable and is constantly being rebuilt. In a cell freely floating in the intercellular substance, thin microfilaments are distributed diffusely. At rest, thin actin microfilaments are concentrated in the form of a ring, and some of them lie in the form of radial bundles. During movement, thin microfilaments are distributed along the main direction of movement. Along the lamellar edge, individual fibers or bundles of them are visible, which lie parallel to the surface of the cell.
Cell movement is necessary for the normal functioning and development of tissues and organs. Thus, migration processes ensure the development of germ layers, extra-embryonic cells, the formation of central and peripheral nervous systems. Without active movements, immune reactions, the functioning of epithelial tissues and fibroblasts, and many other processes are impossible.
Thin microfilaments are the support (basis) for microvilli and stereocilia. In the structure of these specialized formations, thin filaments are arranged in the form of closely lying bundles.
Thick microfilaments. They are formed by proteins called myosins (meromyosins). Thick microfilaments in cross section have a diameter of 10...12 nm. These structures are located in muscle tissue and provide muscle contraction when interacting with actin filaments.
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The cytoskeleton is a network of fibers that provides structural support (framework) to archaea. In eukaryotic cells, these fibers are composed of a complex network of protein filaments and motor proteins that help in movement and stabilization.
Cytoskeleton function
The cytoskeleton is distributed throughout and performs a number of important functions:
- Gives cells shape and provides structural support.
- Keeps you close.
- Helps in education.
- The cytoskeleton is not a static structure, and is capable of disassembling and reassembling its internal parts to provide internal and general cell motility. Types of intracellular movement supported by the cytoskeleton include transport of vesicles in and out of the cell, manipulation during or and organelle migration. The cytoskeleton allows cell migration because cell mobility is essential for tissue creation and repair, cytokinesis (cytoplasmic division) in formation, and in immune cell responses to microbes.
- The cytoskeleton helps in transporting communication signals between cells.
- It forms cellular appendage projections such as (in some cells).
Cytoskeleton structure
The cytoskeleton is composed of at least three different types of fibers: microtubules, microfilaments, and intermediate fibers. These fibers vary in size, with microtubules being the thickest and microfibers being the thinnest.
Protein fiber
- Microtubules are hollow rods that function primarily to support or form the cell and act as “routes” along which organelles can move. Microtubules are commonly found in all eukaryotic cells. They vary in length and are about 25 nm (nanometers) in diameter.
- Microfilaments or actin filaments are thin, hard rods that are active during muscle contraction. They are especially common in muscle cells. Like microtubules, they are commonly found in all eukaryotic cells. Microfilaments consist mainly of the contractile protein actin and have a diameter of up to 8 nm.
- Intermediate filaments can be numerous in many cells and provide support to microfilaments and microtubules, holding them in place. These filaments form keratins, found in epithelial cells and neurofilaments in neurons. They have a diameter of about 10 nm.
Motor proteins
A number of motor proteins are contained in the cytoskeleton. As their name suggests, these proteins actively move cytoskeletal fibers. As a result, molecules and organelles are transported around the cell. Motor proteins are powered by ATP, which is produced by
The cytoskeleton is formed by proteins; there are several main systems, called either by the main structural elements visible in electron microscopic studies (microfilaments, intermediate filaments, microtubules), or by the main proteins included in their composition (actin-myosin system, keratins, tubulin-dynein system).
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Subtitles
Cytoskeleton of eukaryotes
Actin filaments (microfilaments)
About 7 nm in diameter, microfilaments are two chains of actin monomers twisted into a spiral. They are mainly concentrated near the outer membrane of the cell, since they are responsible for the shape of the cell and are capable of forming protrusions on the cell surface (pseudopodia and microvilli). They are also involved in intercellular interaction (formation of adhesive contacts), signal transmission and, together with myosin, in muscle contraction. With the help of cytoplasmic myosins, vesicular transport can be carried out along microfilaments.
Intermediate filaments
Prokaryotic cytoskeleton
For a long time, it was believed that only eukaryotes possess a cytoskeleton. However, with the publication of the 2001 article by Jones et al. (PMID 11290328), describing the role of bacterial homologs of actin in cells Bacillus subtilis, a period of active study of the elements of the bacterial cytoskeleton began. To date, bacterial homologues of all three types of eukaryotic cytoskeletal elements - tubulin, actin and intermediate filaments - have been found. It has also been established that at least one group of bacterial cytoskeletal proteins, MinD/ParA, has no eukaryotic counterparts.
Bacterial homologs of actin
The most studied actin-like cytoskeletal components include MreB, ParM and MamK.
MreB and its homologues
MreB proteins and its homologues are actin-like components of the bacterial cytoskeleton that play an important role in maintaining cell shape, chromosome segregation, and organization of membrane structures. Some types of bacteria such as Escherichia coli, have only one MreB protein, while others may have 2 or more MreB-like proteins. An example of the latter is the bacterium Bacillus subtilis, in which the proteins MreB, Mbl ( M re B-l ike) and MreBH ( MreB h omolog).
In genomes E. coli And B. subtilis the gene responsible for the synthesis of MreB is located in the same operon with the genes for the MreC and MreD proteins. Mutations that suppress the expression of this operon lead to the formation of spherical cells with reduced viability.
Subunits of the MreB protein form filaments that wrap around the rod-shaped bacterial cell. They are located on the inner surface of the cytoplasmic membrane. The filaments formed by MreB are dynamic, constantly undergoing polymerization and depolymerization. Immediately before cell division, MreB is concentrated in the region in which the constriction will form. It is believed that MreB also functions to coordinate the synthesis of murein, a cell wall polymer.
Genes responsible for the synthesis of MreB homologues were found only in rod-shaped bacteria and were not found in cocci.
ParM
The ParM protein is present in cells containing low-copy plasmids. Its function is to propagate plasmids to the cell poles. In this case, the protein subunits form filaments elongated along the major axis of the rod-shaped cell.
The structure of the filament is a double helix. The growth of filaments formed by ParM is possible from both ends, in contrast to actin filaments, which grow only at the ± pole.
MamK
MamK is an actin-like protein Magnetospirillum magneticum, responsible for the correct location of magnetosomes. Magnetosomes are invaginations of the cytoplasmic membrane surrounding iron particles. The MamK filament acts as a guide along which magnetosomes are located, one after another. In the absence of the MamK protein, magnetosomes are distributed randomly over the cell surface.
Tubulin homologues
Currently, 2 tubulin homologues have been found in prokaryotes: FtsZ and BtubA/B. Like eukaryotic tubulin, these proteins have GTPase activity.
FtsZ
The FtsZ protein is extremely important for bacterial cell division; it is found in almost all eubacteria and archaea. Also, homologs of this protein were found in eukaryotic plastids, which is another confirmation of their symbiotic origin.
FtsZ forms a so-called Z-ring, which acts as a scaffold for additional cell division proteins. Together they represent the structure responsible for the formation of the constriction (septum).
BtubA/B
Unlike the widespread FtsZ, these proteins are found only in bacteria of the genus Prosthecobacter. They are closer in structure to tubulin than FtsZ.
Crescentin, a homologue of intermediate filament proteins
The protein was found in cells Caulobacter crescentus. Its function is to give cells C. crescentus vibrio forms. In the absence of expression of the crescentin gene, cells C. crescentus take on the shape of a stick. Interestingly, the cells of the double mutants, crescentin − and MreB − , have a spherical shape.
MinD and ParA
These proteins have no homologs among eukaryotes.
MinD is responsible for the position of the division site in bacteria and plastids. ParA is involved in the partitioning of DNA into daughter cells.
