Actin and myosin are fundamental proteins in biology, working together to create movement and maintain structure inside virtually all eukaryotic cells. This partnership is a universal mechanism that underlies a wide range of cellular activities, from cell division to the crawling motion of immune cells. These dynamic elements convert chemical energy into mechanical work through a highly regulated cycle. Understanding their interaction provides insight into the mechanical forces that shape a cell and allow for large-scale biological functions, such as organ function.
Actin: The Thin Filament Structure
Actin is the protein that forms microfilaments, the thinnest components of the cell’s internal skeleton, or cytoskeleton. It begins as globular actin (G-actin) monomers, which link together through polymerization to form filamentous actin (F-actin). The resulting microfilament is a long, flexible structure composed of two twisted helical strands. The microfilament exhibits polarity, meaning its two ends are chemically and structurally different. The “barbed” (+) end drives filament growth, while the opposite “pointed” (-) end is where depolymerization tends to occur, establishing the directionality essential for myosin movement.
Myosin: The Molecular Motor
Myosin is a family of motor proteins known for converting the chemical energy stored in adenosine triphosphate (ATP) into mechanical force and movement. A myosin molecule consists of three main parts: the head, the neck, and the tail. The globular head is the functional core, containing binding sites for both actin and ATP, and possesses ATPase activity to break down ATP for energy. The neck domain acts as a rigid lever arm, amplifying the structural changes that occur in the head during ATP hydrolysis. The tail domain is highly varied, determining if the molecule functions as a single unit, transports cargo, or assembles into a thick filament, such as Myosin II in muscle tissue.
The Sliding Filament Model of Movement
The interaction between actin and myosin is best understood through the sliding filament model, which explains how these proteins generate force and shorten a biological structure. This movement is powered by the cross-bridge cycle, a sequence of molecular events driven by ATP hydrolysis. The cycle begins with the attachment phase, where the energized myosin head, having broken down ATP into ADP and inorganic phosphate (P\(_{\text{i}}\)), strongly binds to a site on the actin filament.
Next, the power stroke occurs, initiated by the release of P\(_{\text{i}}\), which triggers a conformational change in the neck region. This rotation causes the myosin head to pivot, pulling the attached actin filament a short distance. Following the power stroke, ADP is released, leaving the myosin head tightly bound to the actin (the rigor state). The cycle repeats when a new ATP molecule binds, causing the myosin head to detach and become re-energized for the next binding site.
The continuous repetition of this cycle by numerous myosin heads causes the actin filaments to slide past the myosin filaments, resulting in the overall shortening of the structure. In muscle cells, this sliding action is precisely regulated by troponin and tropomyosin. Tropomyosin wraps around the actin filament, physically blocking the myosin-binding sites when the muscle is at rest. Contraction is initiated when calcium ions bind to troponin, causing a shift in tropomyosin that exposes the binding sites and allows the cross-bridge cycle to begin.
Roles Outside of Muscle Tissue
While the sliding filament model is most dramatically observed in muscle contraction, actin and myosin perform a variety of functions necessary for the survival of non-muscle cells. One fundamental role is in cytokinesis, the physical division of a parent cell into two daughter cells following mitosis. During this process, a contractile ring of actin filaments and non-muscle myosin II assembles beneath the cell membrane. The motor activity of myosin II causes the ring to constrict, cinching the cell membrane inward until the cell is pinched into two separate entities.
Actin and myosin are also essential for cell migration, enabling cells to move and change shape within tissues. This movement involves the extension of cellular protrusions at the leading edge and myosin-powered contraction at the cell’s rear to pull the body forward. Single myosin molecules, such as Myosin V, also act as transport vehicles, carrying vesicles and organelles along the actin microfilaments.