Actin and myosin are fundamental proteins found in nearly all eukaryotic cells. While well-known for their cooperative roles in muscle contraction and enabling movement, they also contribute to many other cellular processes. These proteins are important for maintaining cell shape, facilitating cell division, and enabling the transport of materials within the cell. Their widespread presence highlights their significance in basic cellular function.
The Composition of Actin
Actin is a protein built from amino acids linked in a specific sequence. Its basic building block is a globular monomer called G-actin, which has a molecular weight of approximately 42 kDa and consists of 375 amino acids. Each G-actin monomer contains binding sites for ATP (adenosine triphosphate) or ADP (adenosine diphosphate) and a magnesium ion, which are important for its stability and function.
These individual G-actin units then polymerize, or link together, to form longer, filamentous structures called F-actin. This polymerization involves the head-to-tail interaction of G-actin monomers, forming a double-stranded helix. F-actin filaments, also known as microfilaments, are thin, flexible fibers about 7 nanometers in diameter and can extend for several micrometers. The helical F-actin provides a structural framework that serves as a track along which other proteins, like myosin, can move. This dynamic assembly and disassembly of G-actin into F-actin, driven by ATP hydrolysis, allows cells to rapidly reorganize their internal structures and generate force.
The Composition of Myosin
Myosin is another protein, also assembled from amino acids, that possesses a distinct structure enabling its role as a molecular motor. Myosin molecules, particularly Myosin II which is prevalent in muscle, are composed of two heavy chains and two pairs of light chains. Each heavy chain, approximately 2000 amino acids long, forms three primary domains: a globular head, a neck region, and a long tail.
The globular head is the motor domain where ATP binds and is hydrolyzed, generating the energy required for force production. This head also contains the binding site for actin. The neck domain acts as a lever arm, linking the head to the tail and amplifying the small movements generated by the head’s activity. The light chains bind to this neck region, regulating myosin activity and stabilizing the lever arm. The tail region of the heavy chains often exhibits a coiled-coil structure, enabling two heavy chains to wrap around each other, forming a dimer. Multiple myosin molecules then assemble via their tail regions to form thick filaments, with the heads protruding outwards in a regular pattern. This organized arrangement allows them to interact with actin and generate coordinated movement.
How Actin and Myosin Interact
The compositions of actin and myosin allow them to work together in a dynamic process, most notably in muscle contraction, described by the “sliding filament model.” In this model, the actin and myosin filaments do not shorten themselves; instead, they slide past one another, causing the overall structure, such as a muscle sarcomere, to shorten. This interaction is powered by the chemical energy released from ATP.
The process begins with the myosin head binding to an actin filament, forming a cross-bridge. Following this attachment, a “power stroke” occurs, where the myosin head undergoes a conformational change, causing it to tilt and pull the actin filament along. This movement is accompanied by the release of ADP and inorganic phosphate from the myosin head. A new ATP molecule then binds to the myosin head, which causes it to detach from the actin filament. The ATP is subsequently hydrolyzed into ADP and inorganic phosphate, re-energizing the myosin head and returning it to its original “cocked” position, ready to bind to another site further along the actin filament. This cycle of attachment, power stroke, detachment, and re-cocking repeats continuously as long as ATP and regulatory signals, such as calcium ions, are present, leading to sustained cellular movement or muscle contraction.