Muscles are made up of specialized proteins that contract to produce movement. Of these proteins, actin and myosin play a vital role in muscle contraction. They work together to facilitate both voluntary and involuntary movements in humans and other animals. Actin and myosin are essential for the functioning of muscle cells and are also involved in various cellular processes. In this note, we will discuss the difference between actin and myosin and focus on their roles, structure, and function in muscle contraction.
Actin
Definition and Structure
Actin is a globular protein known to form thin filaments in muscle cells. In its monomeric form it is called G-actin (globular actin). Upon polymerization, G-actin forms filamentous actin (F-actin), which is a major component of the cytoskeleton in eukaryotic cells. Actin filaments are essential for maintaining cell shape, enabling cell motility, and facilitating various cellular processes such as division and intracellular transport.
Actin filaments have a diameter of about 7 nanometers and vary in length, typically ranging from 2 to 2.6 micrometers in muscle cells. Filaments consist of two intertwined threads of actin molecules arranged in a helical structure.
Regulatory Proteins
Actin filaments are associated with several regulatory proteins such as tropomyosin and troponin. Tropomyosin binds along the grooves of actin filaments and blocks the myosin binding sites. Troponin, on the other hand, binds to tropomyosin and controls the interaction of actin with myosin. Binding of calcium ions to troponin leads to a conformational change that moves tropomyosin away from its binding site, allowing myosin to bind to actin and initiate contraction.
position
In muscle cells, actin filaments are present in both the A-band (anisotropic) and I-band (isotropic) of the sarcomere. The I-band contains only actin filaments, while the A-band contains both actin and myosin filaments. Actin filaments are anchored to Z-discs at the ends of the sarcomeres.
Appearance and Surface
Actin filaments have a smooth surface. When viewed under a microscope, they appear less striped than myosin filaments. The smooth surface of actin is due to the consistent arrangement of G actin subunits along the filament.
function
Actin filaments are primarily responsible for maintaining cell shape, facilitating movement, and interacting with myosin to produce muscle contraction. During contraction, actin filaments slide into the H-zone (the central region of the A-band) of the sarcomere.
Abundant
Actin filaments are more abundant than myosin filaments in muscle cells, with approximately six actin filaments for every one myosin filament, reflecting their relative abundance.
Myosin
Definition and Structure
Myosin is a motor protein characterized by its ability to convert chemical energy from ATP hydrolysis into mechanical energy. It forms thick filaments in muscle cells and plays a central role in muscle contraction. Myosin proteins are composed of heavy and light chains, where the heavy chains form a long rod-like tail and a globular head, which interact with actin filaments.
Myosin filaments are thicker and longer than actin filaments, with diameters of about 15 nanometers and lengths of 4 to 5 micrometers. Filaments consist of multiple myosin molecules organized into bipolar structures with outward-projecting heads.
Regulatory Proteins
Myosin filaments are associated with meromyosin, which contains a light chain and a head domain that interacts with actin. The head region of myosin contains the ATPase activity essential for converting ATP into mechanical energy.
position
Myosin filaments are primarily present in the A-band of sarcomeres. Myosin filaments overlap with actin filaments in the A-band but are absent from the I-band.
Appearance and Surface
Myosin filaments have a rough surface due to their prominent myosin heads. Compared to actin filaments, myosin filaments have dark stripes, which explains the dark appearance of the A band.
function
During contraction, myosin filaments do not slide into the H-zone, but instead form cross-bridges with actin filaments, facilitating their sliding towards the center of the sarcomere, a mechanism essential for muscle contraction.
Abundant
There are fewer myosin filaments than actin filaments: Typically, one myosin filament interacts with multiple actin filaments, reflecting the low number of actin filaments in muscle cells.
Muscle contraction is driven by the sliding filament model, which describes how actin and myosin interact to generate movement. When a muscle is stimulated by a motor neuron, calcium ions are released from the sarcoplasmic reticulum. These ions bind to troponin and cause changes in tropomyosin to expose binding sites for actin.
Energized by the hydrolysis of ATP, the myosin head binds to exposed sites of actin and forms cross-bridges. The myosin head then rotates, pulling the actin filaments toward the center of the sarcomere. This action shortens the sarcomere, resulting in muscle contraction. This process is repeated as long as ATP and calcium ions are available.
Key Differences between Actin and Myosin
| side | Actin | Myosin |
| Definition and Function | A globular protein that forms thin filaments and is involved in muscle contraction and cell shape, movement, and division. | Motor proteins that convert ATP hydrolysis into mechanical energy and form thick filaments that interact with actin to contract muscles. |
| structure | Thin filaments, about 7 nm in diameter, are helical structures made of G-actin. | A thick filament with a diameter of about 15 nm, consisting of a long rod-like tail and a spherical head. |
| size | They are short (2-2.6 µm in length) and thin (0.005 µm in diameter). | It is long (4-5 µm in length) and thick (0.01 µm in diameter). |
| Surface properties | Smooth surface. | The surface is rough due to the protruding myosin heads. |
| Regulatory Proteins | Tropomyosin (blocks myosin binding sites); troponin (binds calcium and regulates the position of tropomyosin). | Meromyosin (contains head and tail domains involved in cross-bridge formation). |
| Location within the sarcomere | It is present in both the A and I bands and is anchored in the Z disc. | It is mainly present in the A band and is anchored to the M line. |
| Abundant | More abundant. Typically 6 actin filaments per myosin filament. | Few in number; 1 myosin filament for every 6 actin filaments. |
| Cross-bridge formation | They do not form direct bridges but provide binding sites for myosin. | It cross-links actin filaments during contraction. |
| Association with ATP | It is not directly related to ATP. | It is directly related to ATP, and its movement is driven by ATPase activity. |
| Sliding mechanism during contraction | It slides into the H-zone during contractions. | It remains stationary while pulling the actin filaments towards the center of the sarcomere. |
| End and Conclusion | One end is free (the barbed or positive end) and the other end is bound to the Z-disc (the pointed or negative end). | Both ends are free, and the heads remain bound to ATP. |
| Appearance under a microscope | It appears as lighter stripes (I bands). | It appears as darker stripes (A bands). |
| Additional Roles | They form microfilaments in the cytoskeleton and are involved in cell division and motility. | Depending on the type of myosin, they function as molecular motors in muscle contraction and other cellular processes. |
Conclusion
Actin and myosin are essential for muscle function and various cellular processes. Actin forms thin filaments that provide structural support and interact with myosin during contraction. Myosin forms thick filaments as a motor protein and is essential for generating the force required for muscle movement. Understanding the differences between these two proteins will shed light on their distinct roles in muscle physiology and their coordinated function in the mechanism of muscle contraction.
