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Actin is a protein that contributes to the contractile property of muscle and other cells. It is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton and the thin filaments in muscle fibrils. Actin is found in abundance in all cells of the body, and is involved in muscle contraction, cell crawling, cytokinesis, cytoplasmic organisation, and intracellular transport. It is present in muscle cells as well as non-muscle cells, and is a major constituent of the contractile apparatus.
| Characteristics | Values |
|---|---|
| Actin discovery | W.D. Halliburton (1887) |
| Actin identification and purification | Brunó Ferenc Straub |
| Actin structure | Beadlike molecules |
| Actin form | G-actin (monomeric globular actin) and F-actin (polymeric fibrous actin) |
| Actin mass | 42 kDa |
| Actin diameter | 4 to 7 nm |
| Actin function | Muscle contraction, cell motility, cell division, and cytokinesis |
| Actin location | Muscle fibrils, cytoskeleton |
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What You'll Learn
Actin is found in abundance in all cells
Actin is a protein that is found in abundance in all cells. It was first observed in 1887 by W.D. Halliburton, who extracted it from muscle tissue. However, the discovery of actin is credited to Brunó Ferenc Straub, a biochemist working in Albert Szent-Györgyi's laboratory in Hungary. Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton and thin filaments in muscle fibrils. It exists in two forms: G-actin (monomeric globular actin) and F-actin (polymeric fibrous actin).
In muscle cells, actin plays a crucial role in muscle contraction. Two long strands of bead-like actin molecules twist together to form thin filaments. These actin filaments interact with thicker filaments made of myosin, the most abundant protein found in muscles. The sliding of actin past myosin generates muscle tension, resulting in muscle contraction. This process is known as the sliding filament theory.
Actin is also found in other cell types, where it performs various essential functions. It is a major component of the cytoskeleton, providing a dynamic structure that continuously assembles and disassembles. Actin filaments mediate internal cell motility and are involved in cell division, cytokinesis, vesicle and organelle movement, and cell signaling. The ability of cells to dynamically form microfilaments allows them to remodel themselves in response to environmental changes or internal signals.
In vertebrates, three main groups of actin isoforms have been identified: alpha, beta, and gamma. Alpha actins are found in muscle tissues and are crucial for the contractile apparatus. Beta and gamma actins coexist in most cell types, contributing to the cytoskeleton and internal cell motility. The diverse range of structures formed by actin enables it to perform a wide range of functions, regulated by the binding of tropomyosin along the filaments.
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Actin is a family of globular multi-functional proteins
Actin is a family of globular multifunctional proteins that form microfilaments in the cytoskeleton and thin filaments in muscle fibrils. Actin was first observed in 1887 by W.D. Halliburton, who extracted a protein from muscle that "coagulated" preparations of myosin. However, the discovery of actin is credited to Brunó Ferenc Straub, a biochemist working in Albert Szent-Györgyi's laboratory at the University of Szeged, Hungary.
Actin exists in two forms: G-actin (monomeric globular actin) and F-actin (polymeric fibrous actin). G-actin is a free monomer, while F-actin is part of a linear polymer microfilament. Both are essential for critical cellular functions such as cell division and contraction. Actin is present in almost all eukaryotic cells and is involved in many important cellular processes, including muscle contraction, cell motility, cell division, vesicle and organelle movement, and cell signaling.
In muscle cells, actin and myosin filaments interact to produce muscle contractions. The sliding filament theory explains that the sliding of actin past myosin generates muscle tension, resulting in the shortening of the muscle. This theory is supported by high-resolution microscopy observations of sarcomeres, the basic units that create a striped pattern in muscle cells. The "I band," rich in thinner actin filaments, changes length along with the sarcomere during contraction, while the "A band," composed of thicker myosin filaments, remains relatively constant in length.
Actin's role in muscle contraction was further elucidated by Straub, who found that actin contains bound ATP. During polymerization, the nucleotide is hydrolyzed to ADP and inorganic phosphate, which remain bound to the microfilament. This transformation from ATP-bound actin to ADP-bound actin is crucial for muscular contraction, specifically in smooth muscle.
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Actin forms microfilaments in the cytoskeleton
Actin is a family of globular, multifunctional proteins that form microfilaments in the cytoskeleton and thin filaments in muscle fibrils. Actin is found in essentially all eukaryotic cells and is the major cytoskeletal protein of most cells. It is present at a concentration of over 100 μM and has a mass of roughly 42 kDa, with a diameter of 4 to 7 nm.
Actin filaments are assembled in two general types of structures: bundles and networks. Bundles can be composed of polar filament arrays, where all barbed ends point to the same end of the bundle, or non-polar arrays, where the barbed ends point towards both ends. Networks of actin filaments are highly concentrated at the periphery of the cell, forming a three-dimensional structure beneath the plasma membrane. This network of actin filaments and associated actin-binding proteins (called the cell cortex) determines cell shape and is involved in a variety of cell surface activities, including movement.
Actin can be present as either a free monomer called G-actin (globular) or as part of a linear polymer microfilament called F-actin (filamentous). Both are essential for important cellular functions such as the mobility and contraction of cells during cell division. Actin participates in many other important cellular processes, including cell motility, cell division and cytokinesis, vesicle and organelle movement, cell signalling, and the establishment of cell polarity and morphogenesis in yeasts.
The formation of actin filaments is reversible, and their function often involves undergoing rapid polymerization and depolymerization. The assembly and disassembly of actin filaments, their crosslinking into bundles and networks, and their association with other cell structures (such as the plasma membrane) are regulated by a variety of actin-binding proteins, which are critical components of the actin cytoskeleton.
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Actin is involved in muscle contraction
Actin is a family of globular multifunctional proteins that form microfilaments in the cytoskeleton and thin filaments in muscle fibrils. Actin is involved in muscle contraction, cell motility, cell division, vesicle and organelle movement, cell signalling, and cytokinesis. Actin is present in two forms: G-actin (monomeric globular actin) and F-actin (polymeric fibrous actin). Both forms are essential for important cellular functions such as the mobility and contraction of cells during cell division.
In muscle, two long strands of bead-like actin molecules are twisted together to form a thin filament. Bundles of these filaments alternate and interdigitate with bundles of thick filaments formed of myosin, the most abundant protein found in muscle. When a signal for muscle contraction is sent along a nerve to a muscle cell, actin and myosin are activated. Myosin works as a motor, hydrolysing adenosine triphosphate (ATP) to release energy in such a way that a myosin filament moves along an actin filament, causing the two filaments to slide past each other. This sliding interaction between actin and myosin generates muscle tension and is at the core of our current understanding of sarcomere shortening.
The sarcomere is the basic unit of muscle cells, and they contain many parallel actin and myosin filaments. The shortening of the sarcomere causes the overall length of a muscle to change. The sliding filament theory, proposed by Huxley and Niedergerke (1954) and Huxley and Hanson (1954), states that the sliding of actin past myosin generates muscle tension. The theory was developed through observations of changes in the sarcomeres as muscle tissue shortened. The investigators noted that the "I band," rich in thinner filaments made of actin, changed its length along with the sarcomere.
Actin polymerization is a key cellular process that controls smooth muscle contraction. Actin reorganization in airway smooth muscle cells involves Gq and Gi-2 activation of Rho. Actin-associated proteins are critical in smooth muscle contraction, cell proliferation, and airway hyperresponsiveness.
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Actin is involved in cell crawling and motility
Actin is a protein that contributes to the contractile property of muscle and other cells. It is found in two forms: G-actin (monomeric globular actin) and F-actin (polymeric fibrous actin). Actin is involved in muscle contraction, cell motility, cell division, and cytokinesis.
Actin is also involved in cell crawling and motility. Cell crawling involves a coordinated cycle of movements that can be viewed in three stages:
- Protrusions such as pseudopodia, lamellipodia, or microspikes must be extended from the leading edge of the cell.
- These extensions must attach to the substratum across which the cell is migrating.
- The trailing edge of the cell must dissociate from the substratum and retract into the cell body.
The actin cytoskeleton undergoes dynamic assembly and disassembly during cell crawling, regulating protrusion formation, focal adhesion assembly, and retraction. Actin filaments are involved in the spreading of cells, and myosin can move along actin filaments, carrying out functions such as organelle movement and sensory functions.
Actin-based cell motility is observed in various cell types, including fibroblasts, keratocytes, neutrophils, and amoebae. The actin cytoskeleton is essential for gliding or crawling locomotion, and its dynamic properties enable cells to move over or through substrates.
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Frequently asked questions
No, actin is found in abundance in all cells of the body. It is present in muscle cells as well as non-muscle cells.
Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton. It is one of the most conserved proteins in eukaryote evolution.
Actin is involved in muscle contraction, cell crawling, cytokinesis, cytoplasmic organisation, and intracellular transport. It also plays a role in cell division, cell motility, and cell signalling.
There are three main groups of actin isoforms: alpha, beta, and gamma. Alpha-actin is found exclusively in muscle fibres, while beta and gamma actin coexist in most cell types as components of the cytoskeleton.
