Actin and myosin are two fundamental protein types that form the molecular machinery responsible for biological movement across nearly all forms of life. They are the primary components of the contractile system in muscle tissue, generating the force required for movement, posture, and circulation. Beyond muscle, they play a foundational role within the cytoskeleton of every cell, providing structural support and enabling crucial cellular processes. The dynamic interaction between actin and myosin converts chemical energy into mechanical work, making them the motor proteins of the cellular world.
Molecular Architecture of Actin and Myosin
The thin filament is constructed primarily from actin, which exists in two forms: globular actin (G-actin) and filamentous actin (F-actin). G-actin monomers polymerize in a head-to-tail fashion to create the long, helical double-stranded structure of F-actin. This filamentous structure provides the track along which the motor protein moves.
Myosin forms the thick filament, a larger structure composed of several hundred bundled myosin molecules. Each myosin molecule has two distinct regions: a long, rod-like tail and two globular heads. The tails intertwine to form the central backbone of the thick filament.
The globular head region, or motor domain, projects outward from the thick filament’s backbone and contains two specialized binding sites. One site attaches to the actin filament, forming a cross-bridge. The other is an active ATPase site where adenosine triphosphate (ATP) binds and is hydrolyzed, providing the chemical energy necessary for mechanical motion.
The Sliding Filament Theory of Muscle Contraction
The sliding filament theory describes how actin and myosin generate force. It posits that muscle shortening occurs when thin filaments slide past stationary thick filaments, rather than the proteins themselves contracting. This action takes place within the sarcomere, the functional unit of muscle fibers, and is achieved through the repetitive cross-bridge cycle fueled by ATP.
The cycle begins when an energized myosin head, resulting from the hydrolysis of ATP into ADP and inorganic phosphate (Pi), attaches to an exposed binding site on the actin filament, forming the cross-bridge. Next, the release of Pi triggers a conformational change in the protein structure. This change results in the “power stroke,” a forceful pivot of the myosin head that pulls the attached actin filament toward the center of the sarcomere.
Following the power stroke, ADP is released, leaving the myosin and actin in a tightly bound state known as rigor. This state is broken when a new ATP molecule binds to the myosin head’s ATPase site, causing the myosin head to detach from the actin filament. The newly bound ATP is then hydrolyzed into ADP and Pi, which re-energizes the myosin head and moves it back into its cocked position, ready to bind further along the actin filament. This continuous cycling causes the thin filaments to slide inward, shortening the sarcomere and generating force.
Regulatory Mechanisms: Calcium, Troponin, and Tropomyosin
The interaction between actin and myosin is tightly controlled by a regulatory system acting as a molecular switch. The primary components are tropomyosin and troponin, integrated into the thin actin filament structure. In a resting state, the elongated protein tropomyosin physically wraps around the actin helix, covering the binding sites where myosin heads attach. This blockade prevents cross-bridge formation, ensuring the muscle remains relaxed.
Muscle contraction is initiated by an increase in the intracellular concentration of calcium ions (\(\text{Ca}^{2+}\)), released from the sarcoplasmic reticulum. These calcium ions bind to troponin, which is positioned along the tropomyosin molecule. Calcium binding causes a change in the shape of the troponin complex.
This conformational change in troponin acts as a lever, pulling the associated tropomyosin strand away from the myosin-binding sites on the actin filament. With the binding sites uncovered, energized myosin heads are free to attach to the actin and begin the cross-bridge cycle. Contraction continues as long as the high calcium concentration maintains the conformational shift in troponin and tropomyosin.
Actin and Myosin in Non-Muscle Cells
While their contractile role in muscle is most widely recognized, actin and myosin are fundamental components of the cytoskeleton in virtually every non-muscle cell. Actin filaments form a dynamic network that helps determine and maintain cell shape, acting like the cell’s internal scaffolding. This network is constantly assembled and disassembled, allowing cells to change their form and respond to external cues.
Myosin motors in non-muscle cells, often non-muscle myosin II (NMII), perform similar motor functions but are adapted for different tasks than those in striated muscle. They assemble into smaller, less organized contractile bundles that generate tension within the cell cortex, a specialized layer beneath the cell membrane. This tension is crucial for processes like cell migration, where coordinated pushing and pulling allows a cell to crawl across a surface.
A primary non-muscle function is cytokinesis, the physical division of a parent cell into two daughter cells following mitosis. During this process, a specialized contractile ring forms around the cell’s equator. This ring is composed of circumferentially arranged actin filaments and NMII motors that contract, pinching the cell membrane inward until the cell separates. Actin and myosin also facilitate intracellular transport, moving organelles and vesicles through the cytoplasm.