What are Actin Filaments?
Actin is given as a globular protein, which polymerizes to form the long filaments. Because every actin subunit faces in a similar direction, the filament definition biology is polar, with different ends, termed “barbed” and “pointed.” An abundant protein in approximately all the eukaryotic cells, actin has been deeply studied in muscle cells. Actin filaments are arranged into regular arrays in muscle cells, which are complemented by a group of thicker filaments made from a second protein called myosin. The force that causes muscle contraction is generated by these two proteins.
Actin Filaments Function
Let us discuss the actin filaments function in detail. When the signal to contract is sent along the nerve to muscle, the myosin and actin are activated. Myosin acts as a hydrolyzing adenosine triphosphate (ATP) and motor to release energy in such a way that a myosin filament slides past an actin filament. The thick myosin and thin actin filaments are organized in a structure known as sarcomere that shortens as the filaments slide over one another. Skeletal muscles are composed of bundles of several long muscle cells; when the sarcomeres contract, every of these giant muscle cells shortens, and the entire effect is the contraction of the complete muscle. Although the stimulation pathways vary, smooth muscle (found in several blood vessels and internal organs) and heart muscle contract by the same sliding filament mechanism.
Actin Filament Structure
Let us understand the actin filament structure in detail.
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The structure of skeletal, or striated, muscle. Striated muscle tissue, such as human biceps muscle tissue, has long fine fibres, each of which is essentially a bundle of finer myofibrils. Within every myofibril are filaments of the proteins actin and myosin; these filaments slide past one another as the muscle expands and contracts. On every myofibril, regularly taking place, dark bands, known as Z lines, may be seen where myosin and actin filaments overlap. A sarcomere is an area between the two Z lines; sarcomeres can be thought of as the main functional and structural unit of muscle tissue.
Also, actin is present in the non-muscle cells, where it produces a meshwork of filaments responsible for several cellular movement types. The meshwork consists of actin filaments, which are attached to the cell membrane and every other. The filaments’ length and the architecture of their attachments define the cell’s consistency and shape. Actin filament length, number, location, and attachments are all regulated by a large number of accessory proteins that bind to it.
Various tissues and cells contain multiple accessory proteins, which account for the different movements and shapes of multiple cells. For example, in a few cells, actin filaments are bundled by the accessory proteins, and that bundle is attached to the cell membrane to produce microvilli, stable protrusions, which resemble tiny bristles. Microvilli on the epithelial cell’s surface, such as those lining the intestine, increase the surface area of the cell and hence facilitate the absorption of ingested water and food molecules. Other microvilli are involved in sound perception in the ear, where sound waves induce their movement, which sends electrical signals to the brain.
Actin Filaments in Non-Muscle Cells
Several actin filaments found in non-muscle cells are only present for a short time, polymerizing and depolymerizing in regulated ways to generate movement. For example, several cells continually send out and retract small “filopodia,” long needlelike projections of the cell membrane, which are thought to enable the cells to probe their environment and decide the particular direction to go.
Similar to microvilli, filopodia are formed when the actin filaments push out the membrane, but due to these actin filaments being less stable, filopodia contain only a brief existence. The contractile ring is the other actin structure only transiently associated with the cell membrane that is composed of actin filaments running around the cell’s circumference during the cell division. This ring, as its name implies, pinches the cell in half by pulling in the cell membrane through a myosin-dependent mechanism.
Microtubules
Microtubules are the long filaments formed from 13-15 protofilament strands of a globular subunit known as tubulin, with the strands arranged in the hollow cylinder form. Similar to actin filaments, microtubules are polar, with the “plus” and “minus” ends. Most of the microtubule plus ends are growing and constantly shrinking by respectively adding and losing the subunits at their ends.
Stable microtubules can be found in flagella and cilia. Cilia are hair-like structures located on the surface of certain epithelial cell types that beat in unison to transport fluid and particles across the cell surface. Cilia are closely related to the flagella’s structure. Flagella, such as those found on the sperm cells, form a helical wavelike motion, which enables a cell to propel itself through fluids rapidly. A microtubule collection is bound in a regular array in flagella and cilia by a number of accessory proteins that serve as links and spokes in the assembly.
FAQs on Actin Filaments
1. What exactly are actin filaments?
Actin filaments, also known as microfilaments, are the thinnest protein filaments that make up a cell's cytoskeleton. They are primarily composed of a protein called actin and are crucial for providing shape, support, and movement to the cell.
2. What is the basic structure of an actin filament?
An actin filament is built from individual, globular protein subunits called G-actin (globular actin). These subunits link together to form a long, two-stranded helical chain called F-actin (filamentous actin). This structure has a distinct polarity, with a fast-growing 'plus' end and a slow-growing 'minus' end.
3. What are the main functions of actin filaments in a cell?
Actin filaments are involved in many essential cellular processes. Key functions include:
- Muscle Contraction: They interact with myosin filaments to generate force.
- Cell Motility: They enable cells to crawl and move.
- Maintaining Cell Shape: They form a network beneath the cell membrane, providing structural support.
- Cytokinesis: They form the contractile ring that divides the cell in two during cell division.
4. How are actin filaments different from microtubules?
While both are parts of the cytoskeleton, they differ significantly. Actin filaments are thinner (about 7 nm in diameter) and made of actin protein, primarily involved in cell shape and muscle contraction. Microtubules are thicker (about 25 nm), hollow tubes made of tubulin protein, and are mainly responsible for intracellular transport and forming the spindle during cell division.
5. How do actin filaments help muscles contract?
In muscle cells, actin filaments work with another protein called myosin. According to the sliding filament theory, myosin heads bind to actin filaments and pull them, causing the filaments to slide past each other. This sliding movement shortens the muscle unit (sarcomere), resulting in muscle contraction.
6. Why are actin filaments so important during cell division?
During the final stage of cell division, called cytokinesis, actin filaments assemble into a structure known as the contractile ring just inside the cell membrane. This ring tightens like a drawstring, pinching the cytoplasm and ultimately splitting the one cell into two separate daughter cells.
7. Are actin filaments only found in muscle cells?
No, this is a common misconception. While they are highly abundant and well-known for their role in muscle contraction, actin filaments are present in all eukaryotic cells, including plant and animal cells. They perform essential cytoskeletal functions in every cell, not just muscle.
