Myosin is a protein that acts as a molecular motor, converting the chemical energy in adenosine triphosphate (ATP) into mechanical force. This fundamental action powers nearly all forms of movement within living cells. It is part of a large superfamily of proteins, with different classes specialized for distinct tasks. While its most famous function is generating the forces required for muscle contraction, myosin’s purpose extends far beyond the muscle fiber, participating in cellular processes from division to the transport of internal components.
Myosin’s Core Role in Muscle Tissue
The defining function of myosin involves its conventional form, Myosin II, the primary molecular engine of muscle contraction. In skeletal and cardiac muscle, Myosin II forms thick filaments organized into repeating structures called sarcomeres. These thick filaments lie parallel to thin filaments, which are primarily composed of actin, forming the basis of the sliding filament theory. During contraction, the globular heads of Myosin II attach to the actin filaments, pivot, and detach in a rapid sequence powered by ATP hydrolysis. This “ratcheting” movement pulls the thin actin filaments past the thick myosin filaments, causing the sarcomere to shorten.
In skeletal muscle, this mechanism is regulated by nerve signals that trigger calcium ion release, allowing Myosin II access to binding sites on actin. Cardiac muscle uses the same machinery, but its rhythmic, involuntary contractions are controlled by internal pacemaker cells. Myosin II in these striated muscles generates the forceful, rapid contractions necessary for locomotion and pumping blood.
Smooth muscle, which lines the walls of internal organs and blood vessels, relies on Myosin II for contraction, but its organization is less structured than striated muscle. Instead of organized sarcomeres, Myosin II thick filaments are scattered throughout the cell, anchored to the membrane and internal structures. Contraction occurs when calcium signaling activates an enzyme that phosphorylates the Myosin II regulatory light chain. This allows the myosin heads to interact with actin and generate sustained, slower contractile force, ideal for regulating blood pressure or moving food through the digestive tract.
Distribution of Non-Muscle Myosins
Outside of muscle tissue, a highly diverse group of proteins known as unconventional myosins is found in nearly every eukaryotic cell. These non-muscle myosins are classified into over a dozen classes, such as Myosin I, V, VI, and VII, each possessing unique structural features, particularly in their tail domains. Unlike the filament-forming Myosin II, these unconventional forms often function as single molecules or dimers specialized for specific cellular tasks.
Myosin I is widely distributed, lining the plasma membrane where it links the actin cytoskeleton and the cell membrane. It is located in the microvilli of intestinal epithelial cells, connecting the actin core bundle to the membrane, contributing to the structure of the brush border. Myosin V is found in specialized cells like neurons and melanocytes, distributed along actin networks in the cell periphery. Myosin VI is unique because it is the only known myosin that moves toward the minus end of the actin filament, the opposite direction of all other myosins.
Myosin VI is found in clathrin-coated pits and at the base of the stereocilia—the sensory hair bundles—of the inner ear. Its presence in these auditory structures maintains the organization and function of the stereocilia, which convert sound vibrations into electrical signals. Myosin VII is also inner ear-specific, concentrated near the tips and bases of the stereocilia, where it maintains bundle integrity. The diverse distribution of these proteins highlights that myosin’s function includes scaffolding and transport, not just force generation.
Essential Functions Beyond Contraction
Unconventional myosins perform numerous actions that maintain cellular life. One fundamental role is in cytokinesis, the final physical separation of a dividing cell into two daughter cells. This process relies on non-muscle Myosin II assembling into a contractile ring beneath the cell membrane at the equator. The motor activity of Myosin II constricts this ring, pinching the parent cell in two via the active sliding of actin filaments.
Myosin V is a processive motor, meaning it can take many steps along an actin filament without detaching, making it ideal for long-distance intracellular transport. This motor protein hauls cargo, such as vesicles, organelles, and messenger RNA molecules, across actin-rich areas of the cell. In neurons, Myosin V is important for transporting materials to the tips of growing axons and dendrites.
Other myosins contribute to cell shape and migration, which are collectively known as morphodynamics. Myosin I and Myosin VI are involved in endocytosis, where the cell membrane invaginates to engulf material. Myosin VI, with its unique directionality, internalizes vesicles from the cell surface and moves them deeper into the cell. The coordinated action of Myosin II and other non-muscle myosins helps cells crawl across surfaces and change shape by remodeling the actin cytoskeleton.