Actin and myosin are fundamental proteins found in nearly all eukaryotic cells, playing a central role in generating movement and force within biological systems. These molecular components collaborate in diverse cellular activities, from muscle contraction to cell division. Their widespread presence underscores their importance in maintaining cellular structure and enabling dynamic cellular functions. Understanding how these proteins interact provides insight into the mechanics of life at a microscopic level.
The Structure of Actin
Actin is a globular, multi-functional protein that exists in two primary forms: globular actin (G-actin) and filamentous actin (F-actin). G-actin monomers polymerize to form F-actin, a double-helical structure composed of two intertwined strands of actin subunits. These strands form microfilaments, which are thin, flexible fibers and a component of the cell’s cytoskeleton.
F-actin has a distinct polarity, with a faster-growing “barbed” or plus (+) end and a slower-growing “pointed” or minus (-) end. This polarity is crucial for directing cellular movements and for the interaction with other proteins. The dynamic assembly and disassembly of these filaments allow cells to rapidly change shape and move.
The Structure of Myosin
Myosin is a motor protein characterized by its distinct structural domains: a head, neck, and tail. The head domain is a globular region that contains binding sites for both actin and adenosine triphosphate (ATP). This head region is responsible for hydrolyzing ATP, which provides the energy for movement and force generation. The neck domain acts as a lever arm to amplify movements generated by the head. It also serves as a binding site for regulatory light chains that influence myosin activity.
The tail domain of myosin varies significantly among different myosin types, dictating their specific functions. For instance, Myosin II, known for its role in muscle contraction, has a long, coiled-coil tail that allows it to self-assemble into thick filaments. Other myosin classes, like Myosin I, have shorter tails that interact with cargo molecules for intracellular transport.
How Actin and Myosin Drive Muscle Contraction
Muscle contraction occurs through the “sliding filament theory,” where actin and myosin filaments slide past one another. This mechanism relies on the cyclical interaction between the myosin heads and the actin filaments. The process begins when a myosin head binds to an active site on the actin filament, forming a cross-bridge.
Upon binding, the myosin head undergoes a conformational change, known as the “power stroke,” which pulls the actin filament towards the center of the sarcomere, the basic contractile unit of muscle. This movement is powered by the release of inorganic phosphate and adenosine diphosphate (ADP) from the myosin head. Following the power stroke, a new ATP molecule binds to the myosin head, causing it to detach from the actin filament.
The bound ATP is then hydrolyzed into ADP and inorganic phosphate, which re-energizes the myosin head, returning it to a “cocked” position. This cycle of binding, pivoting, detaching, and re-cocking repeats rapidly, causing continuous sliding. The coordinated action of numerous myosin heads along actin filaments leads to the shortening of muscle fibers and, consequently, muscle contraction. This cyclical interaction continues as long as ATP and the necessary regulatory signals are present.
Roles Beyond Muscle: Actin and Myosin in Cell Function
Beyond their well-known function in muscle contraction, actin and myosin play diverse and important roles in the functioning of non-muscle cells. These proteins are fundamental components of the cytoskeleton, providing structural support and enabling dynamic cellular processes. Their interaction is central to cell division, specifically during cytokinesis, the final stage where a cell divides into two daughter cells.
During cytokinesis, a contractile ring composed of actin filaments and Myosin II assembles beneath the plasma membrane, constricting to pinch the cell in two. Actin and myosin also facilitate cell migration, a process important for wound healing, immune responses, and embryonic development. Cells extend protrusions like lamellipodia, driven by actin polymerization, and then retract their trailing edges through actin-myosin contraction, allowing the cell to “crawl” along surfaces.
These proteins also help maintain the cell’s shape and structural integrity, adapting to mechanical stresses and enabling changes in cell morphology. Their partnership is also important for intracellular transport, moving vesicles, organelles, and other cellular components within the cytoplasm. Myosin motor proteins “walk” along actin tracks, carrying their cargo to specific destinations within the cell.
Regulating the Actin-Myosin Partnership
The precise control of actin-myosin interactions is important for coordinated cellular activities, relying on molecules like ATP and calcium ions. ATP (adenosine triphosphate) serves as the direct energy source for myosin’s mechanical work. Myosin’s ability to bind and hydrolyze ATP drives the conformational changes necessary for its movement along actin filaments. Without ATP, myosin remains rigidly bound to actin, a state observed in rigor mortis.
Calcium ions (Ca2+) are primary regulators, especially in muscle contraction. In a resting muscle, regulatory proteins, tropomyosin and troponin, prevent myosin from binding to actin. Tropomyosin blocks the myosin-binding sites on actin, while troponin is associated with tropomyosin and binds calcium.
When a nerve impulse triggers muscle contraction, calcium ions are released into the muscle cell. Calcium then binds to troponin, causing a conformational change that shifts tropomyosin away from the myosin-binding sites on the actin filament. This uncovers the binding sites, allowing myosin heads to attach to actin and initiate the contraction cycle. Once the calcium signal subsides, calcium is pumped away, and tropomyosin returns to its blocking position, leading to muscle relaxation. This interplay of ATP, calcium, troponin, and tropomyosin ensures precise control of the actin-myosin partnership.