Actin and myosin are fundamental proteins in nearly all eukaryotic cells. Actin, a globular protein, forms thin filaments, serving as a primary structural component. Myosin is a motor protein, converting chemical energy into mechanical force. Together, these proteins drive cellular processes, from maintaining cell shape to enabling movement.
The Fundamental Role in Muscle Contraction
Actin and myosin’s most recognized function is their coordinated action in muscle contraction, explained by the “sliding filament model.” Muscle cells contain specialized sarcomeres, the basic contractile units. Within a sarcomere, thin actin filaments interdigitate with thicker myosin filaments.
Muscle contraction begins with an electrical signal, releasing calcium ions within the muscle cell. These ions bind to troponin, a protein associated with tropomyosin, which blocks myosin-binding sites on actin filaments. Calcium binding to troponin causes a conformational change, moving tropomyosin and exposing the myosin-binding sites. This allows myosin heads to attach to actin filaments, forming cross-bridges.
Cross-bridge formation is followed by a power stroke, where the myosin head pivots, pulling the actin filament towards the sarcomere’s center. This action is powered by ATP hydrolysis, converting chemical energy into mechanical force. A new ATP molecule then binds to the myosin head, causing detachment. The myosin head re-cocks, ready to bind to another site, repeating the cycle. This continuous binding, pulling, and detaching causes actin filaments to slide past myosin filaments, shortening the sarcomere and causing muscle contraction.
Actin’s Versatile Contributions to Cell Structure and Movement
Beyond muscle contraction, actin plays a role in maintaining the structural integrity and dynamic capabilities of non-muscle cells. Actin filaments form a significant part of the cytoskeleton, a complex network that provides mechanical support and helps cells maintain their shapes.
Actin is also central to various forms of cell motility, allowing cells to move and change shape. Cells can extend protrusions like lamellipodia and filopodia, which are driven by the controlled assembly and disassembly of actin filaments. This dynamic restructuring of the actin cytoskeleton facilitates processes such as cell crawling and amoeboid movement, enabling cells to navigate their environment. Furthermore, actin contributes to intracellular transport, serving as “tracks” along which other proteins, including myosins, can move to transport cellular components.
Myosin’s Role in Intracellular Transport and Cell Division
Myosin functions as a molecular motor outside of muscle tissue, facilitating critical processes like intracellular transport. Various types of myosin act as transporters, moving vesicles, organelles, and other cellular cargo along actin filament tracks. Myosin’s ability to convert ATP’s chemical energy into mechanical force makes it a driving force for much of the active transport within the cytoplasm.
Myosin also plays a specialized role in cell division, particularly during cytokinesis, the final stage where the cell divides into two daughter cells. Non-muscle myosin II is responsible for forming the contractile ring, a structure composed of actin and myosin filaments that constricts the cell membrane. This constriction creates a cleavage furrow, ultimately pinching the cell in half. Additionally, myosin contributes to maintaining overall cell tension and can influence the protrusion of actin-rich structures at the cell surface in non-muscle cells.