Myosin proteins are molecular machines responsible for movement within nearly all eukaryotic cells. They power processes ranging from the contraction of a bicep to the transport of materials inside a single cell. The primary function of myosin is to convert chemical energy from adenosine triphosphate (ATP) into mechanical force. This conversion allows myosin to generate movement and tension for many biological activities.
Meet the Movers: Myosin and Its Partner Actin
A typical myosin protein has a structure directly related to its function, consisting of three main parts: a head, a neck, and a tail. The head region, or motor domain, is the active part of the molecule and contains sites for binding to both ATP and a protein filament called actin. The neck region acts as a lever arm, amplifying conformational changes in the head. The tail domain’s function varies, often serving to connect with other myosins or to attach to cellular cargo.
For movement to occur, myosin requires a track provided by actin filaments. These filaments are polymers of the protein actin and form a major part of a cell’s internal scaffolding, known as the cytoskeleton. The interaction between the myosin head and an actin filament is the central event that produces force. All classes of myosin share the ability to bind actin, use ATP, and generate movement.
How Myosin Walks: The ATP-Powered Cycle
The movement of myosin along an actin filament is a cyclical process powered by ATP. The cycle begins when an ATP molecule binds to the myosin head, causing it to detach from the actin filament. The ATP is then hydrolyzed into adenosine diphosphate (ADP) and inorganic phosphate (Pi). This released energy is used to “cock” the myosin head, moving it into a high-energy position further along the actin filament.
Once in this cocked position, the myosin head re-binds to the actin filament at a new location. The binding to actin triggers the release of the inorganic phosphate (Pi), initiating the “power stroke.” During the power stroke, the myosin head swivels and pulls the actin filament along with it.
Following the power stroke, the ADP molecule is released from the myosin head. The myosin remains tightly bound to the actin in what is known as a rigor state. This state is temporary, as the cycle begins anew when another ATP molecule binds to the myosin head, causing it to release the actin. This repetitive sequence allows myosin to “walk” along the actin filament.
Myosin in Action: Powering Muscle Contraction
The most familiar function of myosin is powering muscle contraction. In muscle cells, many myosin II molecules are bundled together to form thick filaments. These thick filaments are interspersed with actin-based thin filaments in a repeating structure called a sarcomere. The sarcomere is the contractile unit of striated muscle tissue.
Muscle contraction occurs through the sliding filament theory. When a muscle is stimulated to contract, the myosin heads on the thick filaments repeatedly bind to the adjacent actin thin filaments, perform their power stroke, and pull the thin filaments toward the center of the sarcomere. This collective action causes the sarcomeres to shorten, which in turn leads to the shortening of the entire muscle fiber.
This process is tightly regulated. In a resting muscle, the proteins troponin and tropomyosin are situated on the actin filaments, blocking the sites where myosin heads bind. The arrival of a nerve signal triggers the release of calcium ions, which bind to troponin. This binding causes a shift in tropomyosin’s position, exposing the myosin-binding sites on actin and permitting the contraction cycle to begin.
Myosin’s Work Throughout the Cell
While its role in muscle is prominent, myosin performs a wide array of tasks in non-muscle cells. For instance, myosin V acts as a molecular transporter, carrying vesicles, organelles, and other cellular cargo along actin filament “highways.” This transport is much faster than simple diffusion and is important for cellular organization.
Another role for myosin is in cell division, a process known as cytokinesis. During the final stage of cell division, a contractile ring of actin and myosin II forms at the cell’s equator. The myosin pulls on the actin filaments, constricting the ring and pinching the cell membrane inward until the cell divides into two.
Myosin motors also contribute to cell migration, a process used for wound healing and immune responses. Myosins help generate the internal forces that allow a cell to change shape, extend protrusions, and pull itself forward.