Myosin and actin are proteins that work together to enable cellular movements. Myosin functions as a motor protein, while actin forms filamentous structures. Their coordinated interaction is important for many cellular processes.
Meet the Proteins: Myosin and Actin
Actin is a globular protein that can polymerize to form long, thin actin filaments. These filaments are a major component of the cytoskeleton, providing structural support and helping maintain cell shape. Actin filaments are approximately 7 nanometers in diameter and can exist as single units (G-actin) or as assembled filaments (F-actin).
Myosin is a motor protein that interacts with actin filaments to generate force and movement. A typical myosin molecule has distinct regions: a head, a neck, and a tail. The head region is where adenosine triphosphate (ATP) binds and interacts with actin. Myosin II is known for its role in muscle contraction. The tail region allows myosin molecules to self-assemble into larger structures or bind to cellular cargo.
The Sliding Filament Theory: How Muscles Contract
Muscle contraction is a key example of myosin and actin interaction, explained by the sliding filament theory. Muscle fibers are organized into repeating units called sarcomeres, the basic contractile units. Within a sarcomere, thin actin filaments overlap with thick myosin filaments.
Muscle contraction occurs when these actin filaments slide past the myosin filaments, causing the sarcomere to shorten. This sliding motion reduces the distance between the Z-discs, which mark the boundaries of a sarcomere. The myosin heads repeatedly bind to actin, pull the actin inward, and then detach, driving this sliding action. The lengths of the actin and myosin filaments themselves do not change during this process; rather, it is their relative positions that shift.
The Molecular Dance: The Cross-Bridge Cycle
The mechanism of myosin binding to actin and generating movement is described by the cross-bridge cycle. This cycle begins with the myosin head attached to an actin filament in a state known as rigor. The binding of an ATP molecule to the myosin head causes a conformational change, reducing its affinity for actin and leading to its detachment from the actin filament.
Once detached, the ATP is hydrolyzed into adenosine diphosphate (ADP) and an inorganic phosphate (Pi) by an enzyme called ATPase located on the myosin head. This hydrolysis releases energy, which “cocks” the myosin head into a high-energy position, ready for the next interaction. The myosin head then weakly reattaches to a new position on the actin filament. The release of the inorganic phosphate strengthens this binding and triggers the “power stroke”.
During the power stroke, the myosin head pivots, pulling the actin filament along with it. Following the power stroke, ADP is released from the myosin head, leaving it tightly bound to the actin filament in the rigor state once more, completing the cycle. This cycle repeats as long as ATP is available and calcium ions are present.
Calcium ions regulate this binding by interacting with regulatory proteins, troponin and tropomyosin, associated with the actin filament. In a resting muscle, tropomyosin blocks the myosin binding sites on actin. When calcium ions are released, they bind to troponin, causing a conformational change that moves tropomyosin away, exposing the myosin binding sites on actin and allowing the cross-bridge cycle to begin.
Beyond Muscle: Myosin-Actin in Other Cell Functions
Beyond muscle contraction, the interaction between myosin and actin is important to many other cellular processes in non-muscle cells. Their versatility highlights their role as molecular machines. For instance, during cell division, actin and myosin form a contractile ring that constricts to divide the cytoplasm, a process known as cytokinesis.
Myosin and actin also play roles in cell migration, allowing cells to move and change shape. This is relevant for processes like wound healing and the movement of immune cells. Within cells, these proteins facilitate intracellular transport, acting as tracks along which organelles and vesicles are moved. Myosin-actin interactions also contribute to maintaining the shape and integrity of cells.