ACTIN: CHARACTERISTICS, STRUCTURE, FILAMENTS, FUNCTION - BIOLOGY - 2024

The actin is a cytosolic protein that forms microfilaments. In eukaryotes, actin is one of the most abundant proteins. For example, it represents 10% by weight of the total protein in muscle cells; and between 1 and 5% of the protein in non-muscle cells.

This protein, together with the intermediate filaments and microtubules, forms the cytoskeleton, whose main function is the mobility of the cell, the maintenance of cell shape, cell division, and the movement of organelles in plants, fungi and animals.

Source: Sarcomere.svg: David Richfield (Slashme user) derivative work: Retama

The isoforms of the actin cytoskeleton have different functions, such as: regulation in the development of active tension in smooth muscle, cell cycle, embryo development, tissue development and wound healing.

From an evolutionary point of view, actin is a highly conserved protein. There is about 90% sequence homology in different species. In unicellular organisms, a single gene encodes an actin isoform. Whereas in multicellular organisms, different genes encode multiple isoforms of actin.

Actin, together with myosin, were crucial structures in the evolutionary development of eukaryotic organisms and in their diversification, since they allowed movement in the absence of other structures, such as flagella and cilia.

Structure: actin filaments

Actin is a globular single chain polypeptide protein. In muscle, actin has a molecular mass of approximately 42 KDa.

This protein has two domains. Each has two subdomains, and a gap between the domains. ATP – Mg ⁺² binds to the bottom of the cleft. The amino and carboxyl termini meet at subdomain 1.

Actin G and actin F

There are two main forms of actin: the actin monomer, called G-actin; and a filamentous polymer, made up of G-actin monomers, called F-actin. Actin filaments, observed by electron microscopy, have narrow and wide regions, respectively 7 nm and 9 nm in diameter.

Along the filament, the actin monomers form a tightly packed double helix. A repeating unit along the filament consists of 13 helices and 28 actin monomers, and has a distance of 72 nm.

The actin filament has two ends. One is formed by the gap that joins ATP – Mg ⁺², which is located in the same direction in all the actin monomers of the filament, called the (-) end; and the other end is the opposite, called the (+) end. Therefore, the actin filament is said to have polarity.

These components are often known as microfilaments, since they are the components of the cytoskeleton with the smallest diameter.

Where do we find actin?

Actin is an extremely common protein in eukaryotic organisms. Of all cellular proteins, actin accounts for about 5-10% - depending on the cell type. In the liver, for example, each of the cells that make it up has almost 5,10 ⁸ actin molecules.

characteristics

The two forms of actin, monomer and filament, are continuously in a dynamic balance between polymerization and depolymerization. In general, there are three salient characteristics of this phenomenon:

1) Actin filaments are typical of the structure of muscle tissue and the cytoskeleton of eukaryotic cells.

2) Polymerization and depolymerization is a dynamic process that is regulated. Where the polymerization or aggregation of G – ATP – Mg ⁺² actin monomers occurs at both ends. Whether this process occurs depends on the conditions of the environment and the regulatory proteins.

3) The formation of bundles and reticles, which make up the actin cytoskeleton, gives strength to cell motility. This depends on proteins that are involved in the formation of cross-links.

Features

Muscle contraction

The functional and structural unit of the skeletal muscle is the sarcomere, which has two types of filaments: the thin filaments, formed by actin, and the thick filaments, formed by myosin. Both filaments are arranged alternately, in a precise geometric way. They allow muscle contraction.

The thin filaments are anchored to regions called Z discs. This region consists of a network of fibers, in which the CapZ protein is found, and to which the ends (+) of the actin filaments are anchored. This anchor prevents depolymerization of the (+) end.

On the other hand, tropomodulin is located at the ends (-) of actin filaments, and protects them from depolymerization. In addition to actin, thin filaments possess tropomyosin and troponin, which function to control actomyosin interactions.

How does muscle contraction occur?

During muscle contraction, the thick filaments perform pivoting movements, pulling the thin filaments toward the middle of the sarcomere. This causes coarse and thin fibers to slip.

Thus, the length of the thick and thin filaments remains constant, but the overlap between both filaments increases. The length of the sarcomere decreases due to the anchorage of thin filaments to the Z discs.

How do you stop muscle contraction?

ATP is the energy currency of the cell. Therefore, it is almost always available in living muscle tissues. Taking into account the above, there must be mechanisms that allow the relaxation of the muscle and the arrest of contractions.

Two proteins, called tropomyosin and troponin, play a fundamental role in this phenomenon. These work together to block the binding sites of myosin (thus preventing its binding to actin). As a result, the muscle relaxes.

Conversely, when an animal dies it experiences a phenomenon known as rigor mortis. Responsible for this hardening of the carcass is the blocking of the interaction between myosin and actin, shortly after the death of the animal.

One of the consequences of this phenomenon is the need for ATP for the release of the two protein molecules. Logically, in dead tissues there is no availability of ATP and this release cannot occur.

Other types of movement

The same mechanism that we describe (later we will delve into the mechanism underlying movement) is not restricted to muscle contractions in animals. It is responsible for the amoeboidal movements that we observe in amoebae and in some colonial molds.

Similarly, the cytoplasmic movement that we observe in algae and in land plants is driven by similar mechanisms.

Regulation of actin filament polymerization and depolymerization

Contraction of smooth muscle tissue and cells produces an increase in F-actin and a decrease in G-actin. Actin polymerization occurs in three stages: 1) nucleation, a slow step; 2) elongation, a quick step; and 3) steady state. The rate of polymerization is equal to the rate of depolymerization.

The actin filament grows faster at the (+) end than at the (-) end. The rate of elongation is proportional to the concentration of actin monomers in equilibrium with actin filaments, called the critical concentration (Cc).

The Cc for the (+) end is 0.1 µM, and for the (-) end is 0.8 µM. This means that 8 times less concentration of actin monomers is required to polymerize the (+) end.

Actin polymerization is mainly regulated by thymosin beta4 (TB4). This protein binds G actin and retains it, preventing it from polymerizing. Whereas profilin stimulates actin polymerization. Profilin binds to actin monomers, facilitating polymerization at the (+) end, through dissociation of the actin-TB4 complex.

Other factors such as the increase in ions (Na ⁺, K ⁺ or Mg ⁺²) favor the formation of filaments.

Formation of the actin cytoskeleton

The formation of the actin cytoskeleton requires the making of cross-links between actin filaments. These bonds are formed by proteins, whose outstanding characteristics are: they have actin-binding domains; many have domains homologous to calponin; and each type of protein is expressed in a certain type of cell.

In filopodia and stress fibers, the cross-links between actin filaments are made by fascina and filamin. These proteins, respectively, cause actin filaments to be parallel or have different angles. Thus, actin filaments define the shape of the cell.

The region of the cell with the highest number of actin filaments is located near the plasma membrane. This region is called the cortex. The cortical cytoskeleton is organized in different ways, depending on the type of cell, and is connected to the plasma membrane through binding proteins.

Some of the best described cytoskeletons are muscle cells, platelets, epithelial cells, and erythrocytes. For example, in muscle cells, the dystrophin binding protein binds actin filaments to an integral glycoprotein complex in the membrane. This complex binds to extracellular matrix proteins.

Actin-myosin interaction action model

The researchers led by Rayment proposed a four-step model to explain the interaction of actin and myosin. The first step occurs with the binding of ATP to the heads of myosin. This binding generates a conformational change in the protein, releasing it from the actin in the small filament.

The ATP is then hydrolyzed to ADP, releasing an inorganic phosphate. The myosin molecule attaches itself to a new actin subunit, generating a high-energy state.

The release of inorganic phosphate brings about a change in myosin, returning to the initial conformation and the movement of the small filaments, with respect to the thick filaments, occurring. This movement causes the movement of the two ends of the sarcomere, bringing them closer together.

The last step involves the release of the ADP. At this point the myosin head is free and can bind to a new ATP molecule.

Cell movement driven by actin polymerization

Crawling motility is a type of cell motility. The steps of this type of motility are: projection of the adhesion leader axis towards the substrate; adhesion to the substrate; rear retraction; and dis-adhesion.

The projection of the leader axis requires the participation of proteins, which participate in the polymerization and depolymerization of actin filaments. The leader axis is found in the cell cortex, called the lamellipodium. The axis projection steps are:

- Activation of receptors by extracellular signal.

- Formation of active GTPases and 4,5-bisphosphate phosphoinositol (PIP ₂).

- Activation of the WASp / Scar and Arp2 / 3 proteins, which bind to actin monomers to form branches in actin filaments.

- Rapid growth of actin filaments, at the myosin-decorated end of the branch. The membrane is pushed forward.

- Completion of the elongation produced by coat proteins.

- Hydrolysis of ATP bound to actin in older filaments.

- Depolymerization of actin-ADP of the filaments promoted by ADF / cofilin.

- Exchange of ADP for ATP catalyzed by profilin, generating G-ATP actin ready to begin to elongate branches.

Actin-related diseases

Muscular dystrophy

Muscular dystrophy is a degenerative disease of the skeletal muscle. It is recessively inherited and is linked to the X chromosome. It mainly affects males with a high frequency in the population (one in every 3,500 males). The mothers of these men are heterozygous asymptomatic, and may lack a family history.

There are two forms of muscular dystrophy, Duchenne and Becker, and both are caused by defects in the dystrophin gene. These defects consist of deletions that remove axons.

Dystrophin is a protein (427 KDa) that forms cross-links between actin filaments. It has an actin-binding domain at the N-terminus, and a membrane-binding domain at the C-terminus. Between both domains there is a third tubular domain made up of 24 tandem repeats.

In the muscular cortical reticulum, dystrophin participates in the binding of actin filaments to the plasma membrane through a glycoprotein complex. This complex also binds to extracellular matrix proteins.

In patients lacking functional dystrophin with Duchenne muscular dystrophy, the cortical cytoskeleton does not support the plasma membrane. Consequently, the plasma membrane is damaged by the stress of repeated muscle contractions.

References

1. Devlin, TM 2000. Biochemistry. Editorial Reverté, Barcelona.
2. Gunst, SJ, and Zhang, W. 2008. Actin cytoskeletal dynamic in smooth muscle: a new paradigm for the regulation of smooth muscle contraction. Am J Physiol Cell Physiol, 295: C576-C587.
3. Lodish, H., Berk, A., Zipurski, SL, Matsudaria, P., Baltimore, D., Darnell, J. 2003. Cellular and molecular biology. Editorial Medica Panamericana, Buenos Aires, Bogotá, Caracas, Madrid, Mexico, Sāo Paulo.
4. Nelson, DL, Cox, MM 2008. Lehninger – Principles of biochemistry. WH Freeman, New York.
5. Pfaendtner, J., De La Cruz, EM, Voth, G. 2010. Actin filament remodeling by actin depolymerization factor / cofilin. PNAS, 107: 7299-7304.
6. Pollard, TD, Borisy, GG 2003. Cellular Motility Driven by Assembly and Disassembly of Actin Filaments. Cell, 112: 453-465.