Myosin and Actin: How They Power Cell Movement

Within nearly every cell, a partnership between two proteins, actin and myosin, powers movement on a microscopic scale. Actin provides a structural track, while myosin acts as a motor protein that travels along this track to generate force. Their interaction is a widespread form of biological motion, responsible for activities from flexing a bicep to the internal reorganization of a cell’s contents. They work together, turning chemical energy into mechanical work.

The Key Players and Their Structure

The effectiveness of the actin-myosin system begins with their distinct structures. Actin starts as a globular protein, G-actin, which can polymerize into long, thin filaments known as F-actin. This filament creates a double helical structure that serves as a track for cellular activities. F-actin provides the structural backbone for myosin to perform its function.

Myosin is a more complex motor protein composed of a long tail domain and a globular head region. The tail allows myosin molecules to assemble into thick filaments. The head protrudes from the thick filament to physically interact with the actin filament. This head region contains a binding site for actin and an enzyme that breaks down ATP to release energy.

The Sliding Filament Mechanism

The most well-known function of actin and myosin is muscle contraction, a process explained by the sliding filament mechanism. This activity occurs within structures inside muscle cells called sarcomeres. A sarcomere is an organized arrangement of parallel actin (thin) and myosin (thick) filaments. When a muscle contracts, these filaments do not shorten; instead, they slide past one another, causing the entire sarcomere to shorten. This collective shortening of sarcomeres results in the contraction of the muscle.

The sliding process is driven by a repeating sequence of events known as the cross-bridge cycle. It begins when the myosin head, loaded with energy, attaches to an exposed binding site on the actin filament, forming a cross-bridge. This is followed by the power stroke, where the myosin head pivots, pulling the actin filament along with it. This action generates the force of contraction as the actin filament slides toward the center of the sarcomere.

Following the power stroke, the myosin head remains attached to actin. The cycle can only continue when a new molecule of adenosine triphosphate (ATP) binds to the myosin head, causing it to detach. Once detached, the enzyme on the myosin head hydrolyzes the ATP. The energy released from this reaction re-cocks the myosin head, returning it to its high-energy position, ready to repeat the cycle.

The Role of ATP and Calcium

The sliding filament mechanism is regulated by ATP and calcium ions. ATP provides the energy for the power stroke when it is hydrolyzed, priming the myosin head for action. ATP is also necessary for the detachment of myosin from actin. Without ATP, the cross-bridges remain locked, a state observed in rigor mortis, where muscle stiffness occurs after death.

Calcium ions act as the on-off switch for contraction. In a resting muscle, binding sites on the actin filament are blocked by the regulatory proteins tropomyosin and troponin. Tropomyosin is a long protein that lies in the groove of the actin filament, covering the myosin-binding sites. Troponin is a complex of proteins attached to tropomyosin.

When a nerve impulse reaches a muscle cell, it triggers the release of calcium ions from the sarcoplasmic reticulum. These ions bind to troponin, causing it to change shape, which in turn pulls tropomyosin away from the actin binding sites. With the sites exposed, myosin heads can attach to actin, and the cross-bridge cycle begins. When the nerve signal ceases, calcium is pumped back into storage, and tropomyosin again blocks the binding sites, causing the muscle to relax.

Beyond Muscle Contraction

The actin and myosin partnership extends beyond muscle contraction to other cellular processes, such as cytokinesis, the final stage of cell division. During this process, a contractile ring of actin and myosin filaments assembles at the cell’s equator. Myosin motors pull on the actin filaments, constricting the ring like a purse string. This action pinches the cell membrane inward, eventually cleaving the single cell into two.

The actin-myosin system also serves as an internal transport network. Actin filaments act as tracks throughout the cell’s cytoplasm for moving organelles and vesicles. Myosin motors function as cargo transporters, “walking” along these actin tracks. They bind to cargo and use energy from ATP to move step-by-step, delivering the payload to its destination.