Victor Forge
Myosin is a family of motor proteins that are best known for their role in muscle contraction. Myosin II, also known as conventional myosin, is the type of myosin that is responsible for producing muscle contractions in most animal cell types. It is composed of thick filaments that are about 15 nm in diameter. Myosin II has two heads and is present in striated muscles, which are skeletal and cardiac muscles. These muscles are composed of bundles of single large cells called muscle fibers that are formed by cell fusion and contain multiple nuclei. The striations in skeletal muscle are created by the organization of actin and myosin filaments, resulting in the banding pattern of myofibrils. These actin and myosin filaments slide over each other to cause the shortening of sarcomeres and the cells to produce force.
| Characteristics | Values |
|---|---|
| Myosin type | Myosin II (conventional myosin) |
| Myosin structure | Head, neck, and tail domain |
| Myosin function | Muscle contraction |
| Myosin composition | Thick filaments |
| Myosin location | Sarcomere |
| Myosin interaction | Interacts with actin to generate muscle tension |
| Striated muscle type | Skeletal and cardiac muscle |
| Striated muscle structure | Alternating strands of actin and myosin filaments |
| Striated muscle function | Voluntary and involuntary movement |
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What You'll Learn
Myosin is a dimeric mechanoenzyme that uses the free-energy change
The myosin heads contain binding sites for actin and ATP, which are essential for their function. During muscle contraction, myosin interacts with actin filaments, resulting in fibre contraction. This interaction is powered by the energy released from ATP hydrolysis, which causes conformational changes in the myosin molecule, leading to muscle contraction. The thick filaments of muscle cells are composed of several hundred myosin molecules, arranged in parallel arrays called sarcomeres, which give the muscle its striated appearance under microscopy.
In striated muscle, the troponin complex, consisting of troponin C, troponin I, and troponin T, plays a crucial role in regulating muscle contraction. Calcium ions (Ca2+) released from the sarcoplasmic reticulum increase the concentration of Ca2+ in the cytosol, triggering muscle contraction. The increased Ca2+ concentration signals the two accessory proteins, tropomyosin and troponin, bound to the actin filaments to initiate the process. The troponin-tropomyosin complex blocks the interaction between actin and myosin at low Ca2+ concentrations, preventing muscle contraction. However, at high Ca2+ concentrations, the binding of Ca2+ to troponin C shifts the position of the complex, allowing contraction to proceed.
Myosin also plays a vital role in cell movement and cargo transport. Different isoforms of myosin, such as Myosin V and Myosin VI, facilitate the transport of cargo, including RNA, vesicles, organelles, and mitochondria, within cells. Myosin X, for example, is involved in wound healing, filopodia formation, and angiogenesis. Myosin I, despite lacking the ability to form dimers, can move along actin filaments, transporting its attached cargo. The functions of unconventional myosins, such as those involved in sensory functions like vision and hearing, are still being elucidated.
Overall, myosin is a versatile mechanoenzyme that utilizes the free-energy change from ATP hydrolysis to perform various biological functions, including muscle contraction, cell movement, and cargo transport, highlighting its significance in maintaining cellular processes and overall physiological homeostasis.
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Myosin II molecules generate force in skeletal muscle
Myosin is a viscous protein found in skeletal muscle, which was first discovered in 1864 by Wilhelm Kühne. Myosin II molecules generate force in skeletal muscle through a power stroke. The power stroke occurs at the release of phosphate from the myosin molecule after ATP hydrolysis while myosin is tightly bound to actin. The effect of this release is a conformational change in the molecule that pulls against actin. The release of the ADP molecule leads to the rigor state of myosin. The binding of a new ATP molecule will release myosin from actin.
Skeletal muscles are bundles of muscle fibers, which are single large cells formed by the fusion of many individual cells during development. Most of the cytoplasm consists of myofibrils, which are cylindrical bundles of two types of filaments: thick filaments of myosin and thin filaments of actin. Each myofibril is organized as a chain of contractile units called sarcomeres, which are responsible for the striated appearance of skeletal and cardiac muscle. The thick filaments are made from the protein myosin, which has one pair of heavy chains and two pairs of light chains. The two heavy and light chains differ from the thin and thick filaments of myofibrils. At the tail of the thick filament, the two heavy chains are intertwined in a helical formation. At the other end of the thick filament, each heavy chain is paired with two light chains, giving rise to two heads. The myosin heads have an actin-binding site that helps them attach to the thin filaments.
The sliding filament theory states that the sliding of actin past myosin generates muscle tension. Because actin is tethered to structures located at the lateral ends of each sarcomere called z discs or "z bands," any shortening of the actin filament length would result in a shortening of the sarcomere and thus the muscle. The contraction of skeletal muscle is triggered by nerve impulses, which stimulate the release of Ca2+ from the sarcoplasmic reticulum. The release of Ca2+ from the sarcoplasmic reticulum increases the concentration of Ca2+ in the cytosol. The increased Ca2+ concentration signals muscle contraction via the action of two accessory proteins bound to the actin filaments: tropomyosin and troponin.
In smooth muscle, a single gene (MYH11) codes for the heavy chains of myosin II, but splice variants of this gene result in four distinct isoforms. It also contains four myosin light chains (MLC), resulting in two per head. These bind the heavy chains in the "neck" region between the head and tail. The MLC20 is also known as the regulatory light chain and actively participates in muscle contraction. The MLC17 is also known as the essential light chain. Its exact function is unclear, but it is believed to contribute to the structural stability of the myosin head along with MLC20.
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Myosin filaments remain centred during muscle contraction
Myosin is a motor protein that is essential for muscle contraction. It is one of the components of muscle fibres, along with actin, tropomyosin, and troponin. These muscle fibres are present in striated muscles, which are characterised by their appearance under a microscope, featuring alternating dark and light bands. Striated muscles include skeletal muscles, which are responsible for voluntary movements, and cardiac muscles, which facilitate involuntary movements such as pumping blood from the heart.
The sliding filament theory, proposed by Huxley and Niedergerke, and Huxley and Hanson in 1954, explains the process of muscle contraction. According to this theory, myosin filaments remain centred during muscle contraction, while the actin filaments surrounding them change in length. This sliding movement of actin past myosin generates muscle tension, resulting in the shortening of the sarcomere and, consequently, the muscle.
The sarcomere is a contractile unit within each muscle fibre, consisting of alternating dark "A bands" and light "I bands". The A bands contain thick filaments of myosin, while the I bands are composed of thinner actin filaments. During muscle contraction, the A bands remain relatively constant in length, suggesting that the myosin filaments at their core maintain their central position.
The mechanism of muscle contraction involves the interaction of actin and myosin filaments, along with calcium ions and accessory proteins like tropomyosin and troponin. Calcium ions (Ca2+) are released from the sarcoplasmic reticulum, a specialised network of internal membranes. The increased concentration of Ca2+ ions enhances muscle contraction by facilitating the interaction between actin and myosin filaments. The binding of Ca2+ to troponin C, a component of the troponin complex, shifts the position of the troponin-tropomyosin complex, enabling the interaction between actin and myosin.
The actual contraction process involves the cyclical movement of myosin and actin filaments. The myosin head binds to actin, contracts, releases actin, and then reaches forward to bind to actin again in a new cycle. This cyclical process is powered by ATP hydrolysis, which provides the energy for myosin to release actin, change its conformation, contract, and repeat the cycle.
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Myosin is a family of motor proteins
Myosin is a superfamily of actin motor proteins that convert chemical energy in the form of ATP to mechanical energy, thus generating force and movement. Myosin was first discovered in 1864 by Wilhelm Kühne, who extracted a viscous protein from skeletal muscle that he believed was responsible for maintaining the tension state in muscles. He named this protein myosin.
Myosin is responsible for muscle contraction in muscle cells in most animal cell types. Myosin II, the first identified myosin, is an elongated protein formed from two heavy chains with motor heads and two light chains. Each myosin head contains an actin and ATP binding site. The myosin heads bind and hydrolyze ATP, providing the energy to walk toward the plus end of an actin filament. Myosin II is also vital in the process of cell division, providing the force of contraction needed to divide the cell into two daughter cells during cytokinesis.
Myosin is also involved in a variety of movements of non-muscle cells, such as intracellular organization and the protrusion of actin-rich structures at the cell surface. Myosin V, for example, is involved in vesicle and organelle transport, while myosin XI is involved in cytoplasmic streaming, allowing organelles and cytoplasm to stream in a particular direction. Myosin IX is a group of single-headed motor proteins that walk towards the barbed ends of filaments, while myosin X is an unconventional myosin motor that functions as a dimer.
Myosin is an essential component of muscle contraction in striated muscle fibers, which include skeletal and cardiac muscle tissues. These muscle fibers contain actin and myosin filaments that power contraction and are organized into repeating arrays called sarcomeres, giving them a striated appearance under microscopy. The thick filaments are made from the protein myosin, which interacts with actin to produce fiber contraction. The contraction of skeletal muscle is triggered by nerve impulses that stimulate the release of Ca2+ from the sarcoplasmic reticulum, increasing the concentration of Ca2+ in the cytosol. This increased concentration signals muscle contraction via the action of two accessory proteins bound to the actin filaments: tropomyosin and troponin.
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Myosin and actin filaments slide over each other to cause shortening of sarcomeres
Myosin is a motor protein that drives the sliding of actin filaments, resulting in muscle contraction. This sliding mechanism is known as the sliding filament theory, which was first proposed in 1954 by scientists who observed the interaction of myosin and actin filaments during muscle contraction.
The sliding filament theory explains that muscle tension is generated by the sliding of actin filaments past myosin filaments within the sarcomere. Sarcomeres are the contractile units within muscle fibres that give striated muscles their characteristic appearance. During muscle contraction, the sarcomeres shorten, bringing the Z discs closer together. This shortening is facilitated by the myosin filaments, which remain centred and act as anchors, while the actin filaments slide towards the centre of the sarcomere.
The thick filaments are composed of myosin, a large protein with two identical heavy chains and two pairs of light chains. The heavy chains have a globular head region and a long alpha-helical tail. The light chains play a regulatory role, while the heavy chain head region interacts with actin to generate force. The actin filaments, on the other hand, are thinner and composed of two filamentous actin chains (F-actin), with each globular actin monomer (G-actin) containing a myosin-binding site.
For the sliding mechanism to occur, the myosin-binding sites on the actin filaments must be exposed. This exposure is regulated by the proteins tropomyosin and troponin. In a low calcium state, tropomyosin blocks the myosin-binding sites, preventing contraction. However, when calcium levels increase, it binds to troponin, shifting the position of the tropomyosin-troponin complex and exposing the myosin-binding sites. This allows myosin to bind to actin and initiate the sliding process, resulting in sarcomere shortening and muscle contraction.
The sliding of actin and myosin filaments past each other is analogous to a person standing between two bookcases and pulling them together using ropes. The person represents myosin, the ropes represent the actin filaments, and the bookcases represent the Z discs at the lateral ends of the sarcomere. As the person pulls on the ropes, the bookcases move closer together, similar to the shortening of the sarcomere during muscle contraction.
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Frequently asked questions
Myosin is a family of motor proteins that are best known for their role in muscle contraction. It is a component of muscle cells and is responsible for their contractile nature.
Striated muscles are muscles that exhibit a striped or banded pattern under a microscope. This pattern is due to the arrangement of actin and myosin filaments into sarcomeres.
Myosin is essential for the contraction of striated muscles. It interacts with actin filaments to generate muscle tension and contraction. This sliding of actin past myosin is known as the sliding filament theory.
Skeletal muscles and cardiac muscles are examples of striated muscles. They are composed of long muscle fibers that contain sarcomeres, which give them their striated appearance.
