Actin is included in the composition. Structure and functions of microfilaments
the mechanical function is performed by protein: hemoglobin, myosin, collagen, melanin, or insulin??? and got the best answer
Answer from Polina Feigina[guru]
1. Polymer is a high-molecular compound, a substance with a large molecular weight (from several thousand to several million), in which atoms connected by chemical bonds form linear or branched chains, as well as spatial three-dimensional structures. Often in its structure one can distinguish a monomer - a repeating structural fragment that includes several atoms. A polymer is formed from monomers through polymerization. Polymers include numerous natural compounds: proteins, nucleic acids, polysaccharides, rubber and other organic substances. In most cases, the concept refers to organic compounds, but there are also many inorganic polymers. A large number of polymers are obtained synthetically based on the simplest compounds of elements of natural origin through polymerization reactions, polycondensation and chemical transformations.
Special mechanical properties:
elasticity - the ability to undergo high reversible deformations under a relatively small load (rubbers);
low fragility of glassy and crystalline polymers (plastics, organic glass);
the ability of macromolecules to orient under the influence of a directed mechanical field (used in the manufacture of fibers and films).
Features of polymer solutions:
high solution viscosity at low polymer concentration;
The dissolution of the polymer occurs through the swelling stage.
Special chemical properties:
the ability to dramatically change its physical and mechanical properties under the influence of small quantities of a reagent (vulcanization of rubber, tanning of leather, etc.).
The special properties of polymers are explained not only by their large molecular weight, but also by the fact that macromolecules have a chain structure and have a unique property for inanimate nature - flexibility.
2. Proteins are complex high-molecular natural compounds built from amino acids. Proteins contain 20 different amino acids, which means there is a huge variety of proteins with different combinations of amino acids. Just as we can form an infinite number of words from 33 letters of the alphabet, we can form an infinite number of proteins from 20 amino acids. There are up to 100,000 proteins in the human body.
Proteins are divided into proteins (simple proteins) and proteids (complex proteins).
The number of amino acid residues included in the molecules is different: insulin - 51, myoglobin - 140. Hence Mr protein from 10,000 to several million.
The functions of proteins in the body are varied. They are largely due to the complexity and diversity of the forms and composition of the proteins themselves. Proteins are an irreplaceable building material. One of the most important functions of protein molecules is plastic. All cell membranes contain a protein, the role of which is varied. The amount of protein in the membranes is more than half the mass.
Many proteins have a contractile function. These are primarily the proteins actin and myosin, which are part of the muscle fibers of higher organisms. Muscle fibers - myofibrils - are long thin filaments consisting of parallel thinner muscle filaments surrounded by intracellular fluid. It contains dissolved adenosine triphosphoric acid (ATP), necessary for contraction, glycogen - a nutrient, inorganic salts and many other substances, in particular calcium.
The role of proteins in the transport of substances in the body is great. Having different functional groups and a complex macromolecule structure, proteins bind and transport many compounds through the bloodstream. This is primarily hemoglobin, which carries oxygen from the lungs to the cells. In muscles, this function is taken over by another transport protein - myoglobin.
Another function of protein is storage. Storage proteins include ferritin - iron, ovalbumin - egg protein, casein - milk protein, zein - corn seed protein.
The regulatory function is performed by hormone proteins.
Hormones are biologically active substances that affect metabolism. Many
There are five main sites where the action of actin-binding proteins can be exerted. They can bind to the actin monomer; with a “pointed”, or slowly growing, end of the filament; with a "feathered" or rapidly growing end; with the side surface of the filament; and finally, with two filaments at once, forming a cross-link between them. In addition to the five types of interaction indicated, actin-binding proteins can be calcium sensitive or insensitive. With such a variety of possibilities, it is hardly surprising that a variety of actin-binding proteins have been discovered and that some are capable of more than one type of interaction.
Proteins that bind to monomers inhibit the formation of primers by weakening the interaction of monomers with each other. These proteins may or may not reduce the rate of elongation, depending on whether the actin-actin-binding protein complex is able to attach to the filaments. Profilin and fragmin are calcium-sensitive proteins that interact with actin monomers. Both require calcium to bind to actin. The complex of profilin with the monomer can build on preexisting filaments, but the complex of fragmin with actin cannot. Therefore, profilin primarily inhibits nucleation, whereas fragmin inhibits both nucleation and elongation. Of the three calcium-insensitive actin-interacting proteins, two—DNase I and vitamin D-binding protein—function outside the cell. The physiological significance of their ability to bind actin is unknown. In the brain, however, there is a protein that, by binding to monomers, depolymerizes actin filaments; its depolymerizing effect is explained by the fact that the binding of monomers leads to a decrease in the concentration of actin available for polymerization.
The “feathered” or rapidly growing end of actin filaments can be blocked by so-called capping proteins, as well as cytochalasin B or D. By blocking the point of rapid filament assembly, capping proteins promote nucleation, but inhibit elongation and end-to-end joining of filaments. The overall effect is the appearance of shortened filaments, this is due to both an increase in the number of seeds competing for free monomers and a lack of docking. At least four proteins are known that act in a similar way in the presence of calcium: gelsolin, villin, fragmin, and also a protein with a mol. weighing 90 kDa from platelets. All of them are capable of reducing the lag phase caused by nucleation during the polymerization of purified monomers and shortening already formed filaments. There are also calcium-insensitive capping proteins. So, squirrels with a pier. weighing 31 and 28 kDa from Acanthamoeba and protein with a mol. weighing 65 kDa from platelets exert their effect regardless of the presence or absence of calcium.
Another point at which protein interaction with filaments is possible is at the “pointed” or slow-growing end. Protein binding therein can initiate nucleation and interfere with filament docking. It also affects the elongation rate, and this effect depends on the actin concentration. When the latter is in the range between the critical concentrations for the slow and fast growing ends, binding of the protein to the slow end will increase the elongation rate by preventing the loss of monomers on it. If, however, the actin concentration exceeds the critical one, binding of the protein to the slow end will lead to a decrease in the overall elongation rate due to blocking one of the points of monomer attachment. The overall result of these three effects (stimulation of nucleation, suppression of docking and suppression of elongation) will be an increase in the number and a decrease in the length of filaments. These effects are similar to those caused by proteins that bind to the "feather" end. That is why, in order to determine which of the two classes a given protein belongs to, i.e., at which end of the filaments it acts, it is necessary to conduct either experiments on the competition of this protein with those that obviously bind to the fast end, or experiments with polymerization on pre-existing seeds. Currently, only one protein is definitely known to bind to the “pointed,” or slow-growing, end of actin filaments, namely acumentin, which is found in large quantities in macrophages. It is possible that this is also true for brevin, a whey protein that causes a rapid decrease in the viscosity of F-actin solutions, shortening the filaments without increasing the concentration of free monomers. Neither Brevin nor Acumentin are sensitive to calcium concentrations.
The fourth type of binding to actin filaments is binding to their lateral surface without subsequent cross-linking of them to each other. The attachment of proteins to the surface can either stabilize or destabilize filaments. Tropomyosin binds in a calcium-insensitive manner and stabilizes F-actin, while severin and villin bind to actin filaments and “cut” them in the presence of calcium.
But perhaps the most effective of the actin-binding proteins are those that can cross-link actin filaments with each other and thereby cause the formation of a gel. By binding to F-actin, these proteins usually also induce nucleation. At least four fibrillar actin cross-linking proteins are capable of inducing gelation in the absence of calcium. These are α-actinin from platelets, villin, fimbrin and actinogelin from macrophages. All of them turn the F-actin solution into a rigid gel that can interfere with the movement of the metal ball; the addition of calcium causes the gel to dissolve. All four of these proteins are monomeric. In the case of villin, the protein molecule can be divided into separate domains: the core, which is calcium sensitive and is able to bind to and cap actin filaments, and the head, which is needed to cross-link the filaments in the absence of calcium. There are also numerous calcium-insensitive cross-linking proteins. Two of them, filamin and actin-binding protein from macrophages, are homodimers; they consist of long, flexible protein subunits. Muscle α-actii is another calcium-insensitive cross-linking protein. Vinculin and high molecular weight protein from BHK cells are also capable of forming crosslinks without the help of additional proteins. At the same time, fascin from sea urchins by itself can ensure the formation of only narrow, needle-like bundles of actin filaments, and in order to cause gelation, it needs the assistance of a protein called mol. weighing 220 kDa.
The spectrin family is one of the most interesting among those cross-linking proteins that are not directly affected by calcium. Spectrin itself is an (ar)g tetramer, originally discovered in the membrane skeleton of erythrocytes. The ap-dimers bind to each other tail-to-tail, while the heads of the molecules remain free and can interact with actin oligomers. The α-subunit of each dimer can also interact with calmodulin, a calcium-binding protein involved in many calcium-regulated processes. It is still unknown what effect calmodulin binding has on spectrin activity. Spectrin-like molecules have now been found in many types of cells, so it would be more correct to talk about the spectrin family. The spectrin subunit from erythrocytes has a mol. mass 240 kDa. An immunologically related protein with the same pier. mass was found in most cell types examined. Mol. the mass of the β3-subunit of spectrin from erythrocytes is 220 kDa. In combination with protein with mol. weighing 240 kDa, reacting with antibodies against a-spectrin, a subunit with a mol. weighing 260 kDa (found in the terminal network) or, for example, 235 kDa (found in nerve cells and other types of cells). These related, immunologically cross-reactive complexes were first described as independent proteins and were named TW260/240 and fodrin. Thus, like many other cytoskeletal proteins, spectrin family proteins are tissue specific. That all of these proteins contain a calmodulin-binding domain has only recently been established, and what follows from this remains to be understood.
Myosin is the only actin-related protein capable of generating mechanical force. The mechanical work it produces due to ATP underlies muscle contraction and is believed to provide the tension developed by fibroblasts and other cells in contact with the extracellular matrix. The interaction of myosin with actin is very complex - so much so that a separate book in this series was dedicated to it1. Myosin produces work by cyclically interacting with actin. Myosin-ADP binds to actin filaments, a change in myosin conformation occurs, accompanied by the release of ADP, and then ATP, if present in solution, replaces the ADP released from myosin and induces detachment of actin filaments from myosin. After ATP hydrolysis, the next cycle can begin. Calcium regulates this process at several points. In some muscle cells, it interacts with troponin to control the binding of tropomyosin to actin. Such cells are said to be regulated at the level of thin filaments. In other muscles, calcium acts on the myosin molecule, either directly or by activating enzymes that phosphorylate its light chains.
In some non-muscle cells, calcium regulates contraction at the level of myosin filament assembly.
The relationship between different classes of actin-binding proteins becomes clearer when viewed from the perspective of Flory's gel theory. This theory states that when the probability of cross-links between polymers is high enough, a cross-linked: three-dimensional network is formed. This predicts the existence of a “gel point,” at which an abrupt transition from solution to gel should occur, somewhat similar in mathematical terms to such phase transitions as melting and evaporation; a further increase in the number of cross-links - beyond the point of gelation - should only lead to a change in the rigidity of the gel. Thus, proteins that form cross-links will convert the viscous solution of F-actin into a gel state, and those proteins that destroy filaments or cause an increase in their number will begin to dissolve the gel by reducing the average length of the polymers, not accompanied by an increase in the number of cross-links: the gel will dissolve , when the crosslink distribution density drops below the level determined by the gelation point. Myosin can interact with the gel and cause it to contract. Gel theory turns out to be useful in comparing the properties of actin-binding proteins of different classes and in developing methods for studying their functions. It should, however, be borne in mind that the theory of gels considers only isotropic structures and does not itself take into account the topological features of specific systems. As will become clear from. Further, the topology of the cytoskeleton is an extremely important characteristic, which the gel theory cannot yet predict.
To meaningfully interpret the results of chemical studies of proteins, detailed knowledge of the conditions inside the cell is necessary, including the exact stoichiometry of all proteins relevant to the processes being studied, and regulatory factors such as pH, pCa,. nucleotide concentration, as well as, apparently, the phospholipid composition of adjacent membranes. In a situation where proteins can effectively induce phenomena with the features of abrupt cooperative transitions at a stoichiometry of 1:500, quantitative predictions obviously become questionable.
ACTIN
one of the main proteins will be reduced. muscle fiber elements. It can exist in the form of a monomer (G-A., mol. wt. approx. 42 thousand) and in polymerization. condition (F-A.).
Molecule G-A. has a globular two-domain form and is associated with one ATP molecule, which is converted into adenosine diphosphate during the polymerization of G-A. In salt-free water solutions G-A. did not polymerize. In the case of adding KS1 or MgCl 2, the process begins at a concentration of resp. 0.1-0.15 or 0.01 M. Possibility of polymerization of G-A. in the body depends on actin-binding proteins, for example. filamin, actinin.
FA is a linear polymer that forms a flat helix (its threads are polar) with a pitch of 38 nm and a subunit diameter of 5.5 nm. One turn of the helix contains 13-14 G-A molecules. Polymerization of the monomer leads to a sharp increase in the viscosity of the solution. F. forms a complex with others. protein - myosin - and has a strong activating effect on its adenosine triphosphatase. An important property of FA is the ability to coordinate metabolic processes, which manifests itself during its interaction. with a number of enzymes (phosphorylase kinase, aldolase, glyceraldehyde-3-phosphate dehydrogenase, etc.).
A. is present in all eukaryotic cells (10-15% by weight of all proteins). In non-muscle cells, it forms the “cytoskeleton” (microfilaments of the cell cytoplasm).
Lit.: Fundamentals of biochemistry, trans. from English, vol. 3, M., 1981, p. 1406-10. B. F. Poglazov.
Chemical encyclopedia. - M.: Soviet Encyclopedia. Ed. I. L. Knunyants. 1988 .
Synonyms:See what "ACTIN" is in other dictionaries:
Actin is a protein, the polymerized form of which forms microfilaments, one of the main components of the cytoskeleton of eukaryotic cells. Together with the protein myosin, it forms the main contractile elements of actomyosin muscles... ... Wikipedia
Actin(s)- * actin(s) *actin(s) is a protein of muscle fibers with an MW of 42 kDa, existing in two forms, fibrillar (actin) and globular (actin). A. has sections complementary to sections of myosin molecules (see), and is part of the main actomyosin... ... Genetics. encyclopedic Dictionary
Muscle fiber protein. Mol. m. 42,000. Two forms: globular (GA) and fibrillar (FA), edges are formed during the polymerization of GA in the presence of ATP and Mg + + ions. On each molecule of A. there are sections complementary to certain sections on ... Biological encyclopedic dictionary
A protein whose fibrillar form forms with myosin the main contractile element of muscles, actomyosin... Big Encyclopedic Dictionary
ACTIN, a muscle fiber protein involved in contractile processes in the cell. Contained primarily in muscle tissue cells; reacting with myosin to form ACTOMYOSIN... Scientific and technical encyclopedic dictionary
Noun, number of synonyms: 1 protein (99) ASIS Dictionary of Synonyms. V.N. Trishin. 2013… Synonym dictionary
actinidia- the name of the female family... Spelling dictionary of Ukrainian language
actin- A periodically contracting protein found inside a eukaryotic cell. Biotechnology topics EN actin... Technical Translator's Guide
Muscle fiber protein. Molecular weight is about 70,000. It exists in two forms: globular (G actin) and fibrillar (F actin), which is a product of polymerization of G actin. In resting muscle, A. is in the form of F actin, forming... ... Great Soviet Encyclopedia
A protein whose fibrillar form forms with myosin the main contractile element of muscles, actomyosin. * * * ACTIN ACTIN, a protein whose fibrillar form forms with myosin the main contractile element of muscles, actomyosin... encyclopedic Dictionary
Actin actin. Muscle fiber protein (molecular weight 42 kDa), exists in two forms, fibrillar and globular, has sections complementary to sections of myosin molecules
Cilia and flagella
Cilia and flagella - organelles of special importance, involved in the processes of movement, are outgrowths of the cytoplasm, the basis of which is a card of microtubules called the axial thread, or axoneme (from the Greek axis - axis and nema - thread). The length of cilia is 2-10 microns, and their number on the surface of one ciliated cell can reach several hundred. The only type of human cell that has a flagellum - sperm - contains only one long flagellum of 50-70 microns. The axoneme is formed by 9 peripheral pairs of microtubules by one centrally located pair; such a structure is described by the formula (9 x 2) + 2 (Fig. 3-16). Within each peripheral pair, due to partial fusion of microtubules, one of them (A) is complete, the second (B) is incomplete (2-3 dimers shared with microtubule A).
The central pair of microtubules is surrounded by a central shell, from which radial doublets diverge to the peripheral doublets. The peripheral doublets are connected to each other by nexin bridges, and “handles” of the dynein protein extend from microtubule A to microtubule B of the neighboring doublet (see Fig. 3- 16), which has ATPase activity.
The beating of the cilium and flagellum is caused by the sliding of adjacent doublets in the axoneme, which is mediated by the movement of dynein handles. Mutations that cause changes in the proteins that make up the cilia and flagella lead to various dysfunctions of the corresponding cells. For Kartagener's syndrome (fixed cilia syndrome), usually caused by the absence of dynein handles; patients suffer from chronic diseases of the respiratory system (associated with impaired function of cleansing the surface of the respiratory epithelium) and infertility (due to sperm immobility).
The basal body, similar in structure to the centriole, lies at the base of each cilium or flagellum. At the level of the apical end of the body, microtubule C of the triplet ends, and microtubules A and B continue into the corresponding microtubules of the axoneme of the cilium or flagellum. During the development of cilia or flagellum, the basal body plays the role of a matrix on which the assembly of axoneme components occurs.
Microfilaments- thin protein filaments with a diameter of 5-7 nm, lying in the cytoplasm singly, in the form of septa or in bundles. In skeletal muscle, thin microfilaments form ordered bundles, interacting with thicker myosin filaments.
The corticole (terminal) network is a zone of condensation of microfilaments under the plasmalemma, characteristic of the majority of cells. In this network, microfilaments are intertwined and “cross-linked” with each other using special proteins, the most common of which is filamin. The cortical network prevents sharp and sudden deformation of the cell under mechanical influences and ensures smooth changes in its shape through rearrangement, which is facilitated by actin-dissolving (converting) enzymes.
Attachment of microfilaments to the plasmalemma is carried out due to their connection with its integral (“anchor”) proteins (integrins) - directly or through a number of intermediate proteins talin, vinculin and α-actinin (see Fig. 10-9). In addition, actin microfilaments are attached to transmembrane proteins in special areas of the plasmalemma, called adhesion junctions or focal contacts, which connect cells to each other or cells to components of the intercellular substance.
Actin, the main protein of microfilaments, occurs in a monomeric form (G-, or globular actin), which is capable of polymerizing into long chains (F-, or fibrillar actin) in the presence of cAMP and Ca2+. Typically, an actin molecule looks like two helically twisted filaments (see Figures 10-9 and 13-5).
In microfilaments, actin interacts with a number of actin-binding proteins (up to several dozen types) that perform various functions. Some of them regulate the degree of actin polymerization, others (for example, filamin in the cortical network or fimbrin and villin in the microvillus) contribute to the connection of individual microfilaments into systems. In non-muscle cells, actin accounts for approximately 5-10% of the protein content, only about half of which is organized into filaments. Microfilaments are more resistant to physical and chemical influences than microtubules.
Functions of microfilaments:
(1) ensuring contractility of muscle cells (when interacting with myosin);
(2) providing functions associated with the cortical layer of the cytoplasm and plasmalemma (exo- and endocytosis, formation of pseudopodia and cell migration);
(3) movement of organelles, transport vesicles and other structures within the cytoplasm due to interaction with certain proteins (minimyosin) associated with the surface of these structures;
(4) ensuring a certain rigidity of the cell due to the presence of a cortical network, which prevents the action of deformations, but itself, when rearranged, contributes to changes in cellular shape;
(5) formation of a contractile constriction during cytotomy, which completes cell division;
(6) formation of the basis (“framework”) of some organelles (microvilli, stereocilia);
(7) participation in organizing the structure of intercellular connections (encircling desmosomes).
Microvilli are finger-shaped outgrowths of the cell cytoplasm with a diameter of 0.1 μm and a length of 1 μm, the basis of which is formed by actin microfilaments. Microvilli provide a manifold increase in the surface area of the cell on which the breakdown and absorption of substances occurs. On the apical surface of some cells actively participating in these processes (in the epithelium of the small intestine and renal tubules) there are up to several thousand microvilli, which together form a brush border.
Rice. 3-17. Scheme of the ultrastructural organization of microvilli. AMP – actin microfilaments, AB – amorphous substance (apical part of the microvillus), F, V – fimbrin and villin (proteins that form cross-links in the AMP bundle), mm – minimyosin molecules (attaching the AMP bundle to the microvillus plasmalemma), TC – terminal network AMP, C – spectrin bridges (attach the TC to the plasmalemma), MF – myosin filaments, PF – intermediate filaments, GC – glycocalyx.
The framework of each microvilli is formed by a bundle containing about 40 microfilaments lying along its long axis (Fig. 3-17). In the apical part of the microvilli, this bundle is fixed in an amorphous substance. Its rigidity is due to cross-links from the proteins fimbrin and villin; from the inside, the bundle is attached to the plasmalemma of the microvillus by special protein bridges (minimyosin molecules. At the base of the microvillus, the microfilaments of the bundle are woven into the terminal network, among the elements of which there are myosin filaments. The interaction of actin and myosin filaments of the terminal network is likely , determines the tone and configuration of the microvillus.
Stereocilia- modified long (in some cells - branching) microvilli - are detected much less frequently than microvilli and, like the latter, contain a bundle of microfilaments.
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Microfilaments, microtubules and intermediate filaments as the main components of the cytoskeleton.
Actin microfilaments - structure, functions
Actin microfilaments They are polymer filamentous formations with a diameter of 6-7 nm, consisting of the actin protein. These structures are highly dynamic: at the end of the microfilament facing the plasma membrane (plus end), polymerization of actin from its monomers in the cytoplasm occurs, while at the opposite end (minus end) depolymerization occurs.
Microfilaments, thus, have structural polarity: the thread grows from the plus end, shortening - from the minus end.
Organization and functioning actin cytoskeleton are provided by a number of actin-binding proteins that regulate the processes of polymerization-depolymerization of microfilaments, bind them to each other and impart contractile properties.
Among these proteins, myosins are of particular importance.
Interaction one of their family - myosin II with actin underlies muscle contraction, and in non-muscle cells gives actin microfilaments contractile properties - the ability to undergo mechanical tension. This ability plays an extremely important role in all adhesive interactions.
Formation of new actin microfilaments in the cell occurs by branching from previous threads.
In order for a new microfilament to form, a kind of “seed” is necessary. The key role in its formation is played by the Af 2/3 protein complex, which includes two proteins very similar to actin monomers.
Being activated, the Af 2/3 complex attaches to the side of the preexisting actin microfilament and changes its configuration, acquiring the ability to attach another actin monomer.
This is how a “seed” appears, initiating the rapid growth of a new microfilament, extending in the form of a branch from the side of the old thread at an angle of about 70°, thereby forming a branched network of new microfilaments in the cell.
The growth of individual filaments soon ends, the filament is disassembled into individual ADP-containing actin monomers, which, after replacing ADP in them with ATP, again enter into the polymerization reaction.
Actin cytoskeleton plays a key role in the attachment of cells to the extracellular matrix and to each other, in the formation of pseudopodia, with the help of which cells can spread out and move directionally.
— Return to section " oncology"
- Methylation of suppressor genes as a cause of hemoblastoses - blood tumors
- Telomerase - synthesis, functions
- Telomere - molecular structure
- What is the telomere position effect?
- Alternative ways to lengthen telomeres in humans - immortalization
- The importance of telomerase in the diagnosis of tumors
- Cancer treatment methods affecting telomeres and telomerase
- Cell telomerization does not lead to malignant transformation
- Cell adhesion - consequences of disruption of adhesive interactions
- Actin microfilaments - structure, functions
Microfilaments(thin filaments) - a component of the cytoskeleton of eukaryotic cells. They are thinner than microtubules and in structure are thin protein filaments with a diameter of about 6 nm.
The main protein they contain is actin. Myosin can also be found in cells. In a bundle, actin and myosin provide movement, although actin alone can do this in a cell (for example, in microvilli).
Each microfilament consists of two twisted chains, each of which consists of actin molecules and other proteins in smaller quantities.
In some cells, microfilaments form bundles under the cytoplasmic membrane, separate the mobile and stationary parts of the cytoplasm, and participate in endo- and exocytosis.
Also functions are to ensure the movement of the entire cell, its components, etc.
Intermediate filaments(not found in all eukaryotic cells; they are not found in a number of groups of animals and all plants) differ from microfilaments in their greater thickness, which is about 10 nm.
Microfilaments, their composition and functions
They can be built and destroyed from either end, while thin filaments are polar, their assembly occurs at the “plus” end, and disassembly occurs at the “minus” end (just like microtubules).
There are different types of intermediate filaments (differing in protein composition), one of which is found in the cell nucleus.
The protein strands that form the intermediate filament are antiparallel.
This explains the lack of polarity. At the ends of the filament there are globular proteins.
They form a kind of plexus near the nucleus and diverge to the periphery of the cell. Provide the cell with the ability to withstand mechanical stress.
The main protein is actin.
Actin microfilaments.
Microfilaments in general.
Found in all eukaryotic cells.
Location
Microfilaments form bundles in the cytoplasm of motile animal cells and form the cortical layer (under the plasma membrane).
The main protein is actin.
- Heterogeneous protein
- Found in different isoforms and encoded by different genes
Mammals have 6 actins: one in skeletal muscle, one in cardiac muscle, two types in smooth muscle, two non-muscle (cytoplasmic) actin = a universal component of all mammalian cells.
All isoforms are similar in amino acid sequences, only the terminal sections are variant. (They determine the rate of polymerization and do NOT affect contraction)
Actin properties:
- M=42 thousand;
- in monomeric form it looks like a globule containing an ATP molecule (G-actin);
- actin polymerization => thin fibril (F-actin, represents a flat spiral ribbon);
- actin MFs are polar in their properties;
- at a sufficient concentration, G-actin begins to spontaneously polymerize;
- very dynamic structures that are easy to disassemble and assemble.
During polymerization (+), the end of the microfilament quickly binds to G-actin => grows faster
(–) end.
Low concentration of G-actin => F-actin begins to disassemble.
Critical concentration of G-actin => dynamic equilibrium (microfilament has a constant length)
Monomers with ATP are attached to the growing end; during polymerization, ATP hydrolysis occurs, the monomers become associated with ADP.
Actin+ATP molecules interact more strongly with each other than ADP-bound monomers.
The stability of the fibrillar system is maintained:
- protein tropomyosin (gives rigidity);
- filamin and alpha-actinin.
Microfilaments
They form cross-links between f-actin filaments => a complex three-dimensional network (gives a gel-like state to the cytoplasm);
- Proteins that attach to the ends of fibrils, preventing disassembly;
- Fimbrin (binds filaments into bundles);
- Myosin complex = acto-myosin complex capable of contraction when ATP is broken down.
Functions of microfilaments in non-muscle cells:
Be part of the contractile apparatus;
