Myosin and actin are fundamental proteins found in nearly all living cells, playing a central role in various cellular processes. These proteins enable cells to move, change shape, and maintain their internal organization. Their interactions orchestrate a wide range of movements, from the smallest cellular activities to muscle contractions.
Actin: The Cellular Framework
Actin is a highly abundant protein that exists in two primary forms: globular actin (G-actin) and filamentous actin (F-actin). G-actin monomers polymerize to form F-actin, which are thin, flexible fibers. These F-actin filaments, also known as microfilaments, are a major component of the cell’s cytoskeleton, providing structural support.
Actin filaments organize into complex networks and bundles within the cell, particularly concentrated just beneath the cell’s outer membrane. This network helps determine cell shape and enables dynamic changes. As part of the cytoskeleton, actin filaments serve as cellular “tracks” or “scaffolds” that guide the movement of various cellular components and facilitate cell movement.
Myosin: The Molecular Motor
Myosin is a family of motor proteins that convert chemical energy into mechanical force, driving movement along actin filaments. Most myosin molecules are composed of a head, neck, and tail domain. The globular head domain binds to actin and contains the site for ATP hydrolysis, which powers its movement.
The neck domain acts as a linker and a lever arm, helping to transmit the force generated by the head. The tail domain varies among different myosin types and is often involved in interacting with other cellular structures or cargo. Numerous classes of myosin exist, such as Myosin II, known for its role in muscle contraction, and Myosin V, involved in transporting vesicles and organelles within the cell.
The Dynamic Duo: Muscle Contraction
The most recognized collaboration between myosin and actin occurs in muscle contraction, explained by the “sliding filament model.” In this process, thick myosin filaments and thin actin filaments are organized into repeating units called sarcomeres within muscle cells. Muscle contraction begins when myosin-binding sites on the actin filaments become exposed.
Myosin heads then attach to these exposed sites on the actin, forming cross-bridges. The energy released from the hydrolysis of adenosine triphosphate (ATP) causes the myosin head to change its shape, performing a “power stroke” that pulls the actin filament. This action slides the actin filaments past the myosin filaments, drawing them closer to the center of the sarcomere.
After the power stroke, a new ATP molecule binds to the myosin head, causing it to detach from the actin filament. The ATP is then hydrolyzed, re-cocking the myosin head for another cycle. This repeated attachment, pulling, and detachment causes the sarcomere to shorten, leading to muscle contraction.
Beyond Muscle: Diverse Cellular Roles
Beyond their well-known role in muscle contraction, myosin and actin perform many other important functions throughout the body. In cell division, for instance, a contractile ring made of actin filaments and Myosin II forms around the middle of a dividing cell. The contraction of this ring pinches the cell into two daughter cells, a process called cytokinesis.
These proteins also drive cell migration, enabling cells to move and navigate their environment. Myosin-driven contraction of actin networks provides the necessary force for cells to extend protrusions, adhere to surfaces, and then pull their trailing edges forward. This coordinated action is important for processes like wound healing and immune responses.
Myosins also play an important role in intracellular transport, acting as molecular couriers that move various cellular components. Different types of myosin, such as Myosin V, transport vesicles, organelles, and other cargo along actin filament tracks within the cell. This transport ensures that cellular materials are delivered to their correct destinations, maintaining cellular organization and function.