Myosin is a motor protein that facilitates movement within cells. These proteins function like microscopic engines, converting chemical energy into mechanical force to drive cellular activities, most notably muscle contraction. Myosin proteins are dependent on ATP to power their motion, which is based on interactions with another protein called actin. While first discovered in muscle tissue, different versions of myosin exist in virtually all eukaryotic cells.
The Structure of a Myosin Filament
The myosin molecule for muscle contraction, known as myosin II, has a highly functional structure. Each molecule is composed of two identical heavy chains and four light chains. The heavy chains feature three regions: a globular head, a flexible neck, and a long tail. The head region contains the motor domain, which houses the sites for binding both ATP and actin.
The neck region acts as a lever arm, amplifying small conformational changes in the head to produce force-generating movement. The long, rod-like tails of the two heavy chains intertwine to form a coiled-coil structure. This tail portion is responsible for assembling individual myosin molecules into a thick filament.
Within a muscle cell, hundreds of myosin II molecules are bundled to create a myosin filament. Their long tails form the central backbone of the filament, while the globular heads project outwards from this core. This arrangement creates a structure with a central “bare zone” devoid of heads and bristling with heads along the rest of its length.
The Mechanism of Muscle Contraction
Muscle contraction is governed by the interaction between myosin thick filaments and adjacent actin thin filaments, a process described by the sliding filament theory. This cyclical interaction, known as the cross-bridge cycle, involves the myosin heads repeatedly binding to actin, pulling, and then releasing. The cycle converts the chemical energy from ATP into the mechanical work of muscle shortening.
The cycle begins when a myosin head that is not attached to actin binds and hydrolyzes an ATP molecule. This process energizes the myosin head, causing it to move into a “cocked” position. The trigger for this action is calcium. When a nerve impulse stimulates a muscle fiber, calcium is released, which changes regulatory proteins on the actin filament to expose myosin binding sites.
Once the binding sites are exposed, the energized myosin head attaches to the actin filament, forming a cross-bridge. The binding of myosin to actin initiates the “power stroke.” During the power stroke, the myosin head releases the products of ATP hydrolysis and pivots, pulling the actin filament and generating the force of muscle contraction.
Following the power stroke, the myosin head remains attached to actin in a state of rigor until a new ATP molecule binds to it. The binding of a new ATP molecule causes the myosin head to detach from actin. The cycle can then begin again, with the hydrolysis of the newly bound ATP re-energizing the head, provided calcium levels remain high.
Muscle Fiber Types and Myosin Variation
The performance of different muscles is determined by their fiber types, primarily categorized as Type I (slow-twitch) and Type II (fast-twitch). A defining difference between these fiber types is the specific version, or isoform, of the myosin protein they use. These isoforms have distinct properties that dictate how quickly they complete the cross-bridge cycle.
Type II fibers, for explosive movements like sprinting or weightlifting, contain fast-twitch myosin isoforms. This type of myosin hydrolyzes ATP at a very high rate. This allows the myosin heads to cycle much more quickly, generating significant force and contraction speed. The trade-off for this high power is a rapid onset of fatigue as energy reserves are depleted.
In contrast, Type I fibers are built for endurance activities like long-distance running or maintaining posture. These fibers contain slow-twitch myosin isoforms, which process ATP at a much slower rate. This slower cycle results in less powerful but more sustained contractions that are highly resistant to fatigue.
Myosin’s Role Beyond Muscle
Myosin’s function is not confined to muscle tissue, as these motor proteins perform various tasks in non-muscle cells. Different classes of myosin have evolved to carry out specialized jobs. These diverse roles range from cell division to internal transport.
One non-muscle function of myosin is in cytokinesis, the final stage of cell division. A contractile ring of actin and non-muscle myosin II assembles at the cell’s equator. The myosin motors pull on the actin filaments, tightening the ring and pinching the cell membrane inward until the cell is cleaved in two.
Myosin proteins also act as a cellular delivery system. Unconventional myosins, such as myosin V, “walk” along the actin filaments of the cell’s cytoskeleton. These myosins attach to and transport various types of cargo, including vesicles, organelles, and other cellular components to their correct destinations.