Actin and myosin are proteins, fundamental components in biology. They are responsible for a wide range of cellular movements and forces, involved in processes from muscle contraction to cell division. They are found across all muscle tissue and are highly conserved.
Defining Actin and Myosin
Actin and myosin are distinct proteins with specialized structures and functions. Actin is a globular protein that assembles into thin filaments, approximately 7 nanometers in diameter. It is among the most abundant proteins in eukaryotic cells, existing as monomeric (G-actin) or filamentous (F-actin), with transitions controlled by ions and other proteins.
Myosin is a superfamily of motor proteins. It forms thicker filaments, about 15 nanometers in diameter, converting chemical energy from ATP into mechanical energy. Myosin molecules are composed of one or two heavy chains and several light chains, with a globular head region that binds to actin.
Their Role in Muscle Contraction
The primary function of actin and myosin is their cooperative action in muscle contraction, explained by the “sliding filament theory.” This theory describes how muscle shortens as thin actin filaments slide past thick myosin filaments, without the individual filaments changing length.
The process begins when a nerve impulse signals a muscle cell, leading to the release of calcium ions from the sarcoplasmic reticulum. These calcium ions bind to troponin, a protein associated with actin, which causes tropomyosin to move away from the myosin-binding sites on the actin filament. This allows the myosin heads to attach to actin, forming cross-bridges.
With ATP providing the necessary energy, the myosin heads undergo a conformational change, performing a “power stroke” that pulls the actin filaments toward the center of the sarcomere. After the power stroke, ATP binds to the myosin head, causing it to detach from the actin. The myosin head then re-cocks, ready for another cycle of attachment and pulling. This repeated cycle of attachment, pulling, and detachment causes the sarcomere, the basic contractile unit of muscle, to shorten, leading to overall muscle contraction.
Beyond Muscle: Other Cellular Functions
Beyond muscle contraction, actin and myosin contribute to other fundamental cellular processes in non-muscle cells. Actin is a primary component of the cytoskeleton, a dynamic network providing structural support and maintaining cell shape. It is involved in cell movement, such as crawling across surfaces, driven by actin polymerization and actin-myosin interactions.
Myosin’s motor activity extends to intracellular transport, moving vesicles, organelles, and other cellular cargo along actin filaments. For example, myosin I transports membrane vesicles, and myosin V moves vesicles and organelles. Both proteins are involved in cell division, specifically in cytokinesis, where actin filaments and myosin II form a contractile ring that pinches the cell into two daughter cells.