Introduction

When it comes to muscle, understanding its structural organization is necessary to understand physiology. If there’s altered physiology, then it will be termed Pathology. The sarcomere is highly organized, containing long cylindrical filaments called myofibrils that run parallel to the axis of the fibre. The dark bands on myofibrils are called A bands (anisotropic) and the light bands are called I bands (isotropic).

The A and I banding pattern in sarcomeres is mainly due to the regular arrangement of thick and thin myofilaments composed of myosin and F-actin. The myosin heads bind to the actin, forming transient cross-bridges between thick and thin filaments. The thin, helical actin filaments are each 1 micrometre long and 8 nanometers wide and run between the thick filaments. Each G-actin monomer contains a binding site for myosin. The monomers of G-actin join together to form an F-actin polymer.

Actin has over 360 different amino acid sequences. The amino acids are different in actin just because of their different functions.

Regulatory proteins associated with actin

Two major regulatory proteins add up to make thin filaments. These are troponin and tropomyosin.  The tropomyosin is a 40-nanometer long coil of two polypeptide chains located in the groove between the two twisted strands of F-actin. While troponin is a complex of three subunits: TnT which attaches to tropomyosin, TnC which binds to Calcium ions and TnI which regulates actin-myosin interactions. The troponin molecule attaches to specific sites regularly spaced along each tropomyosin molecule.

Actin and Cell Movement

Actin filaments in association with myosin are responsible for any type of cell movement.

Myosin acts as the molecular motor that converts chemical energy in the form of ATP to mechanical energy, generating force and movement.

The muscle contraction takes place which allows understanding the sliding filament theory.

Interactions of actin and myosin are responsible not only for muscle contraction but also for several movements in non-muscle cells like in cell division.

Actin filaments are also responsible for the crawling movement of the cell across a surface for which the driving force is again actin-myosin interactions.

Mechanical forces which are produced by forming cross-bridges cause actin filaments to slide inward among the myosin filament.

The interaction of a single myosin filament with two actin filaments and calcium ions causes contraction.

The active site of actin is inhibited by troponin and tropomyosin.

These are activated by calcium ions allowing contraction to take place.

Another theory supplements sliding theory which is Walk Along Theory.

This account for the tilting of myosin heads automatically towards the arm that is dragging the actin through is termed as the power stroke.

Just as the tilted head breaks away from the active site it returns backs to its normal position which is at a 90-degree angle to the strand.

This allows the next actin filaments to interact along the length of myosin filaments.

Read more about Muscle Contractions

Actin and Actin Filaments

An actin protein is the monomeric subunit of two types of filaments in cells; microfilaments are among the three major components of the cytoskeleton. The thin filaments are the contractile machinery of the cell. It can be present as a free monomer which is G-actin (globular) which joins up via peptide linkages to form a linear F-actin. It weighs about 42k Daltons approximately. When seen under X-ray crystallography actin is found to consist of two lobules that are separated by a cleft making up a globule. The cleft is known as ATPase fold which allows ATP and calcium to bind here and change into ADP and phosphate after hydrolysis.

F-actin is what is called the actin filament. Two parallel F-actin strands must rotate about 166 degrees to lie over each other in the correct proportion. This creates a double-helical structure of the microfilaments.  Each molecule of actin bound to ATP or ADP that is associated with a magnesium ion. The most common forms of actin which are found are ATP-G-actin and ADP-F-actin. This helical structure of F-actin can be either levorotatory or dextrorotatory which explains the shifting of the strand either to left or right in the presence of plane-polarized light. The F-actin polymer is even termed to have structural polarity with two opposite ends directed towards the cleft. 

Assembly and Disassembly of Actin Filaments

Kenneth Homes in 1990 identified the three-dimensional structure of actin filament. To study the assembly of actin filaments in vitro one needs to regulate closely the ionic concentration of actin solutions. Solutions with less ionic strength and concentration cause the actin filaments to undergo depolymerization. At physiological levels, where homeostasis is maintained, actin assembles itself and undergoes polymerization instantaneously.

 Actin filaments grow when there are several monomeric additions to the side chain of the filaments. The polymerization of actin is a reversible process that solely depends on monomeric concentration.  A phenomenon of treadmilling is evident to this point. The actin filaments have two ends, one is the minus end and the other is the plus end. The minus end grows at a lesser pace when compared to the plus end. This accounts for growth within the filaments. The reason for rapid growth at the plus end is its association with ATP which converts to ADP and P yielding energy.

Actin’s isoforms

In the vertebrates, there are three major isoforms of actin, although there are six in total. These are alpha, beta, and gamma actin.

• Alpha actin is encoded by the ACTA1 gene and is involved in certain major processes which include muscle contraction, the adaptation of muscle fibers, answer to mechanical triggers, and growth of the cell and the development of skeletal muscle fibers.
• Beta-actin is encoded by the ACTB gene and is among one of the actions which are present in the cytoskeleton of non-muscular cells. What it does is allow certain materials to move inside the cell from one place to another. It also allows taking control of the Fc signaling pathway involved in the process of phagocytosis. It also helps in platelet aggregation and cytoskeleton organization. IN CNS, we are having a nucleus named substantia nigra somewhere close to the hypothalamus. Beta-actin also plays a vital role in its development.
• Gamma actin is encoded by the ACTG1 gene. Gamma actin is found at several places like in cytoskeleton, striated muscle cells, and have a major role in transduction and transmission in a muscle cell.  It coordinates the epinephrine dependent signaling pathway and also plays a role in platelet aggregation.

Actin genes

Different isoforms are associated with actin in different tissues having different genes as well.

Skeletal muscle actin genes are

• ACTA1 – alpha-actin
• ACTB – beta-actin

Smooth muscle actin genes are

• ACTA2 – alpha-actin
• ACTG1 – gamma actin

Cardiac muscle actin genes are

• ACTG2 – gamma actin 2
• ACTC1 – cardiac muscle actin 1

Actin’s polymerization

Actin polymerization is not just a normal end to end addition of monomers to form a polymer. Its polymerization differs a bit in this case. This non-linear polymerization is followed by the process of nucleation. This process is mediated by phosphatidyl-inositol-3-phosphate which prevents desiccation allowing no water to pour in. This causes the form of filaments to be polarized. This creates hydrophobic clefts within.

Post-translational modifications of Actin

In two ways actin carries out its post-translational modification. These are acetylation of the N-terminal and methylation of histidine 68. A protein named ubiquitin plays a major role in protein homeostasis by keeping a close look at protein’s turnover. The ubiquitin is also attached covalently in every sixth molecule of actin.

Actin’s identification and recycling

Actin and DNAse I are having a greater affinity for each other in-vivo. This allowed the scientist to view actin as a crystal for the first time although its significance is unclear.

Gelsolin masks actin-binding sites and serves to be an important actin-capping protein. They insert into the targeted cleft. It serves to be an actin’s scavenger system. These acts by causing depolymerization and removal of actin from the circulation when once actin is released into the bloodstream due to the injury.

Coflin is found in almost all eukaryotic organisms and is involved in the recycling of actin subunits.

Macrolides (antibiotic class) is even found to interact with actin monomers. This disrupts polymerization and has a strong anti-tumor effect.

Applications of actin

Actin is used in technological laboratories to track myosin under the principle of molecular motors. It is mainly used as an evident tool for the diagnosis procedures of pathologies. Its main applications are in the fields of

• Nanotechnology
• Western blots
• Food technology

Functions of Actin

A wider range of functions is offered by actin. Actin is an essential protein found within eukaryotic cells and serves the basis of the cellular cytoskeleton. Several actin filaments, when bundled together, can give strong mechanical support to the cell and allows several materials to be transferred within the cells. Hence, it has a major role in signal transduction. It plays a key role in cell migration by assembly and disassembly of actin at certain intervals. Actin is known to be found in the cellular cytoplasm and the nucleus as well. These filaments are numerous in the muscle cells.

Actin is present in different proportions in different types of cells. It is about 2% of total proteins in hepatocytes, 10% in fibroblasts, and 70% in platelets. As stated above the different isoforms in different types of cells accounts for different functions. The cortex of an animal cell just under the cell membrane is found to have greater concentrations of microfilaments. This, in turn, relays several signals to produce the effect of Signal Transduction.

Other functions of actin are as follows:

• The action rings are found in the periodic ring-like structure in the axons. These rings form spectrum tetramers linking actin rings to form a basis of the existing cytoskeleton.
• Actin also helps in several processes called endocytosis, cytokinesis, and cell polarity in a few fungal species, In plants, it is known to find ten types of actin. These actin filaments within the plants’ cells allow them to carry out internal cellular movements.
• It plays a key role in cell division and elongation as well.

Overall, actin is found to influence during the process of cytokinesis, apoptosis, cellular adhesion, ciliary movements, and production of intrinsic cellular chirality.

Pathology associated with actin

Salmonella, Shigella, and Listeria are found to be active pathogens disrupting the cytoskeleton of an individual during an infection. Toxoplasma gondii forms their cytoskeleton disrupting the host’s cellular machinery in the longer run. It works by releasing a G-actin binding protein named as toxofillin. It shows a strong affinity for actin dimers targeting ADP cleft most importantly. Clostridium botulinum and Clostridium perfringens are known to modify actin by adding ribose to ADP resulting in cellular disintegration and cell death.

Pseudomonas Aeruginosa forms a biofilm that has a stronger actin component within. This allows it to stay protected from the host’s defensive mechanisms and hence able to cause diseases in both plants and animals. The major infectious diseases these organisms tend to cause are hospitality acquired infections like ventilator-associated pneumonia and sepsis syndromes.

The other major pathological condition associated with actin is mutation. These induce different myopathies in individuals. The mutations which take place commonly are point mutations that affect normal cellular integration and migration. It too seems to have a profound effect on the stereocilia of ciliated cells. Other normal cellular changes are at the expense of point mutation in any of the genes of the actin monomer.

Summary

Actin serves to be a complementary protein in several cells where it is integrated into the variety of protrusions and extensions.

The cell surface extensions are made up of several actin filaments depending on its function. These cellular extensions are responsible for;

• absorption as in microvilli in the small intestine
• motility and detection in auditory cells via the help of stereocilia
• phagocytosis by pseudopodia
• for assembly and disassembly of actin filaments by microspikes or filopodia.

These filaments even play a major role in maintaining the cell’s integrity and shape by providing mechanical support just beneath the plasma membrane.

Frequently Asked Questions

What is actin?

Actin is a cellular protein majorly found in the cytoskeletal components such as microfilaments. It is mainly found in the eukaryotic cells. Thin filaments of skeletal muscles are also made up of actin protein.

What is the role of actin?

Actin is responsible for maintaining the shape and architecture of the cell. It is also involved in cell movements. Being a component of thin filaments, it also plays an important role in muscle contraction.

How is actin produced?

Actin is produced by the polymerization of actin monomers. These monomers are made in the process of translation. Actin monomers are added together to form long actin filaments by the process of polymerization.

Why do cells need actin?

Actin filaments are responsible for providing mechanical support to the cell. They are also involved in the movement of the cell, division of the cell, as well as cell contraction. 

References

1. PDB: 1J6Z; Otterbein LR, Graceffa P, Dominguez R (Jul 2001). “The crystal structure of uncomplexed actin in the ADP state”. Science. 293 (5530): 708–711. doi:10.1126/science.1059700. PMID 11474115. S2CID 12030018.
2. Doherty GJ, McMahon HT (2008). “Mediation, modulation, and consequences of membrane-cytoskeleton interactions”. Annual Review of Biophysics. 37 (1): 65–95. doi:10.1146/annurev.biophys.37.032807.125912. PMID 18573073. S2CID 17352662.
3. Vindin H, Gunning P (Aug 2013). “Cytoskeletal tropomyosins: choreographers of actin filament functional diversity”. Journal of Muscle Research and Cell Motility. 34 (3–4): 261–274. doi:10.1007/s10974-013-9355-8. PMC 3843815. PMID 23904035.
4. Gunning PW, Ghoshdastider U, Whitaker S, Popp D, Robinson RC (Jun 2015). “The evolution of compositionally and functionally distinct actin filaments”. Journal of Cell Science. 128 (11): 2009–2019. doi:10.1242/jcs.165563. PMID 25788699.
5. Ghoshdastider U, Jiang S, Popp D, Robinson RC (Jul 2015). “In search of the primordial actin filament”. Proceedings of the National Academy of Sciences of the United States of America. 112 (30): 9150–9151. doi:10.1073/pnas.1511568112. PMC 4522752.