Myosin
What is the primary function of myosin?
How does myosin convert chemical energy into mechanical work?
What structural components are common in myosin molecules?
What diseases are associated with mutations or other defects in myosin proteins?
Myosin, any of a group of proteins that plays a central role as a molecular motor in cellular movement and force generation. Myosin proteins are distinct from other molecular motors in having a specialized structural system that converts chemical energy from adenosine triphosphate (ATP) into mechanical work, characterized specifically by a “converter” region and a leverlike arm, which work together to amplify motion and to generate force.
Found in nearly all eukaryotic cells, myosin is best known for its involvement in muscle contraction, but it also participates in a wide range of intracellular processes, including the transport of organelles, cell division, and the maintenance of cell form. Myosins interact with actin filaments, which act as tracks along which molecules of myosin slide to produce movement and force.
Distribution and diversity
In humans myosin proteins are widely distributed throughout the body, with different classes being expressed in specific tissues; for example, certain isoforms are abundant in skeletal muscle and cardiac muscle, while others are found in epithelial cells, neurons, and immune cells. In muscle cells they are concentrated within specialized structures known as sarcomeres, where they form thick filaments that interact with actin to produce contraction. In nonmuscle cells myosin is found in the cytoplasm, often associated with actin networks beneath the cell membrane or within the cell interior.
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More than 30 distinct classes of myosin proteins have been identified, and their distribution varies widely across eukaryotic life. Some myosin classes, such as I and II, are widely conserved across many organisms, whereas others are more specialized and restricted to particular lineages—for example, certain classes are found mainly in animals and others in plants. This distribution reflects the evolutionary diversification of myosins to meet the specific functional needs of different organisms.
- Myosin I: Involved in vesicle transport and cell surface dynamics
- Myosin II: The best-characterized class, responsible for muscle contraction and contractile functions in nonmuscle cells
- Myosin IV: Atypical myosin found in certain lower eukaryotes and thought to have specialized intracellular roles
- Myosin V: Facilitates the transport of vesicles and organelles along actin filaments over long distances
- Myosin VI: Distinguished by its movement in the direction of the minus end of actin filaments (counter to most myosins)
- Myosin VIII: Found primarily in plants, involved in cell division and cellular organization
- Myosin IX: Contains signaling domains; involved in organizing the cytoskeleton and influencing cell motility
- Myosin X: Plays a role in filopodia formation and cellular extensions
- Myosin XI: Found in plants, responsible for rapid cytoplasmic streaming
- Myosin XII–XVIII: Specialized classes with diverse roles in cell signaling, cargo transport, and cell organization
Structure and function
Despite variation among the many types of myosin, they share a common structural organization. A typical myosin molecule consists of a head domain, a neck region, and a tail domain. The head contains the ATP-binding site, a small converter region, and the actin-binding site. When it binds to ATP, it functions as an ATPase enzyme, hydrolyzing ATP and thereby releasing energy for the mechanical power stroke to move along the actin filament. The neck region functions as a lever arm, amplifying movement generated by the head. This motion is driven by the converter region, which acts as a molecular hinge, translating small conformational changes in the motor domain into larger mechanical movements of the lever arm. In muscle contraction the repeated binding of the myosin head and sliding of the myosin protein along an actin filament is known as the crossbridge cycle; the sliding of actin and myosin filaments past one another shortens the muscle fiber and generates tension.
The tail region varies in length and composition depending on the myosin type and is responsible for dimerization or for binding to cargo such as vesicles or other cellular components. In muscle cells myosin molecules assemble into thick filaments through interactions among their tail regions. In contrast, many nonmuscle myosins function as single molecules or small complexes that transport materials along actin filaments. This movement may be accomplished by myosin “walking,” a processive, single-molecule action in which myosin takes “steps” along actin via repeated ATP-dependent cycles of binding, movement, and release. Both muscle and nonmuscle myosins may also function in cytokinesis, the final stage of cell division, where they help constrict the cell membrane to separate daughter cells.
Diseases and disorders
Anomalies in myosin function are associated with several diseases in humans. Mutations in genes encoding cardiac myosin, for instance, can lead to cardiomyopathies, including hypertrophic and dilated forms, which affect the ability of the heart to pump blood effectively. Abnormalities in skeletal muscle myosin may result in myopathies, characterized by muscle weakness and impaired movement.
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Certain nonmuscle myosins are also linked to disease. For example, mutations in myosin genes involved in inner ear function can cause hereditary hearing loss, as these proteins are essential for the proper operation of sensory hair cells. In addition, disruptions in myosin-mediated transport and cell division have been implicated in some forms of cancer.