Actin is the primary component of the thin filament, a fundamental structural element within muscle tissue. This filament is located in the contractile units of skeletal and cardiac muscle cells, known as myofibrils. Myofibrils are composed of repeating segments called sarcomeres, which are the smallest functional unit of muscle contraction. Within the sarcomere, the thin filament works with the thick filament to generate force and shorten the muscle.
Actin: The Primary Component of the Thin Filament
The thin filament’s backbone is formed by the protein actin, which exists in two forms. The individual, spherical protein molecules are known as globular actin, or G-actin, and each contains a binding site for the thick filament protein, myosin.
G-actin molecules polymerize end-to-end to create a long, fibrous strand called filamentous actin, or F-actin. Two F-actin strands then twist around each other in a double-helical formation, creating the core structure of the thin filament, which is roughly 7 nanometers in diameter.
Accessory Proteins That Complete the Thin Filament Structure
The thin filament is a complex structure that includes two regulatory proteins controlling muscle contraction. The first is tropomyosin, a long, rod-shaped protein that spirals along the groove of the double-helical F-actin strand. In a relaxed muscle, tropomyosin physically covers the myosin-binding sites located on the actin molecules.
The second accessory protein is the troponin complex, which is attached to the tropomyosin strand at regular intervals. Troponin is a complex of three subunits: Troponin-T (TnT) binds to tropomyosin, Troponin-I (TnI) binds to actin, and Troponin-C (TnC) binds calcium ions. This complex acts as a molecular switch, regulating the interaction between the thin and thick filaments.
Muscle contraction is initiated when calcium ions are released into the muscle cell cytoplasm. The calcium binds to the Troponin-C subunit, causing a conformational change in the troponin complex. This change pulls the attached tropomyosin molecule away from the myosin-binding sites on the actin. The repositioning of tropomyosin exposes the binding sites, making them available for interaction with the thick filament and starting the contraction cycle.
Myosin: The Thick Filament Counterpart
The thick filament, which is roughly 15 nanometers in diameter, is primarily composed of the motor protein myosin. Each myosin molecule is shaped like a golf club, featuring a long tail and two globular heads. The myosin heads contain a binding site for actin and possess ATPase activity, meaning they can hydrolyze adenosine triphosphate (ATP) to release energy.
Hundreds of myosin molecules aggregate to form the thick filament bundle, with their tails intertwined to form the central shaft. The globular heads project outward in a helical pattern along the length of the filament, except for a central region called the bare zone. These heads interact with the surrounding thin actin filaments, acting as the molecular motors that drive muscle shortening.
The Sliding Mechanism of Muscle Contraction
Muscle contraction is explained by the sliding filament theory, where the thin and thick filaments slide past one another without changing length. This movement is powered by the cyclical interaction between the myosin heads and the actin binding sites, known as the cross-bridge cycle. The cycle begins when the energized myosin head, having already hydrolyzed ATP into adenosine diphosphate (ADP) and inorganic phosphate (\(\text{P}_{\text{i}}\)), binds to the exposed actin site.
The binding of the myosin head to actin triggers a release of \(\text{P}_{\text{i}}\) and then ADP, causing a conformational change in the myosin head. This change results in the power stroke, where the myosin head pivots and forcefully pulls the attached thin filament toward the sarcomere’s center. This movement generates the force required for muscle shortening.
A new ATP molecule must bind to the myosin head, causing the head to detach from the actin filament. The myosin head hydrolyzes the new ATP, returning to its high-energy, “cocked” position, ready to bind to a new site if calcium is still present. This asynchronous, repeated cycling allows the filaments to continually slide past each other, resulting in the overall shortening of the sarcomere.