Myosin is a fundamental motor protein in nearly all eukaryotic cells. It acts as a molecular machine, transforming chemical energy from adenosine triphosphate (ATP) into mechanical force and movement. This ability makes it indispensable for various cellular functions, from muscle contraction to intricate intracellular activities.
Myosin’s Role in Muscle Contraction
Myosin’s most recognized role is in muscle contraction, forming the basis for all bodily movements. Muscle fibers contain sarcomeres, the fundamental contractile units. Each sarcomere has thick filaments of myosin and thin filaments of actin, arranged in an overlapping pattern that gives muscle its striated appearance.
Muscle contraction follows the “sliding filament theory.” Myosin (thick) filaments slide past actin (thin) filaments, shortening the sarcomere without changing filament length. This sliding is powered by the globular heads of myosin molecules, which extend from thick filaments and interact with actin.
The mechanism involves a cyclical series of events called the cross-bridge cycle. A myosin head first binds to an actin filament, forming a cross-bridge. ATP then binds to the myosin head, causing it to detach.
After detachment, ATP hydrolyzes into adenosine diphosphate (ADP) and inorganic phosphate (Pi), re-energizing the myosin head into a “cocked” position. When calcium ions are present, binding sites on actin are exposed, allowing the myosin head to bind to a new position. The release of Pi initiates the “power stroke,” where the myosin head pivots and pulls the actin filament toward the sarcomere’s center. ADP is then released, and the myosin head remains bound until a new ATP molecule arrives. This cycle of attachment, pivoting, and detachment, fueled by ATP, shortens sarcomeres, leading to muscle contraction and force generation.
Beyond Muscle: Myosin’s Diverse Functions
While myosin is prominently known for its role in muscle contraction, various types of myosin proteins perform a multitude of functions in non-muscle cells throughout the body. These diverse myosins, often referred to as unconventional myosins, are involved in a range of cellular processes, highlighting their fundamental importance beyond generating large-scale force. Myosin-II, for instance, is a widespread conventional myosin found in non-muscle cells.
One important function of myosin is in cell division, specifically during cytokinesis, the final stage where a single cell divides into two daughter cells. Non-muscle myosin II plays a central role in forming the contractile ring, a structure composed of actin and myosin filaments that pinches the cell in half. This ring forms at the cell’s equator and constricts, effectively cleaving the cell.
Myosin also contributes significantly to cell migration, enabling cells to move and change shape within tissues. Class I myosins, for example, are involved in regulating membrane tension and facilitating the movement of various cell types, including immune cells. Myosin-driven contraction of actin filaments provides the necessary forces for cellular propulsion and adaptation to their environment.
Additionally, certain myosins act as “cargo carriers” for intracellular transport, moving vesicles, organelles, and other cellular components along actin filaments. Myosin V is a double-headed molecular motor that transports intracellular cargo by stepping along actin tracks. Myosin VI, another type, moves cargo in the opposite direction, demonstrating the precise and varied transport capabilities of these proteins. These transport myosins ensure that cellular materials are delivered to specific locations, supporting processes like organelle positioning and the growth of cellular protrusions. Myosin also helps maintain cell adhesion and overall cell shape.
Myosin’s Impact on Health
Dysfunction of myosin due to genetic mutations or other issues can have significant health consequences, affecting various systems in the body. When myosin proteins do not function correctly, the fundamental cellular processes they govern can be disrupted, leading to disease.
One notable condition linked to myosin dysfunction is Hypertrophic Cardiomyopathy (HCM), a genetic disorder characterized by the thickening of the heart muscle. Mutations in the MYH7 gene, which encodes the beta-myosin heavy chain found in cardiac muscle, are a common cause of HCM. These mutations can impair the heart’s ability to pump blood effectively, increasing the risk of heart failure and sudden cardiac death.
Myosin mutations are also implicated in various forms of hearing loss. For instance, mutations in Myosin VIIA (MYO7A) can lead to sensorineural hearing loss, affecting the function of hair cells in the inner ear. These hair cells contain specialized actin-rich projections called stereocilia, which are essential for converting sound waves into nerve impulses. Disruption of Myosin VIIA can lead to disorganization of these stereocilia, resulting in profound deafness.
Usher syndrome, a condition characterized by a combination of hearing loss, vision loss, and balance problems, is also associated with MYO7A gene mutations. Specifically, Usher syndrome type 1B (USH1B) is caused by mutations in MYO7A, leading to profound congenital deafness and progressive vision loss due to retinitis pigmentosa. The dysfunction of Myosin VIIA in both the inner ear and retina underscores its critical role in the development and maintenance of these sensory organs.