Actin and myosin are fundamental proteins that work together to enable various forms of movement and maintain structural integrity within cells. Actin forms filaments, often called microfilaments, which are long, repetitive structures serving as cellular tracks or frameworks. Myosin functions as a molecular motor, converting chemical energy from ATP into mechanical force and movement. This partnership orchestrates a diverse array of cellular processes, from large-scale muscle contractions to the subtle movements of internal cellular components.
The Sarcomere in Muscle Contraction
The most recognized structure involving actin and myosin is the sarcomere, which serves as the basic functional unit of striated muscle. Skeletal and cardiac muscle cells exhibit a distinctive striped appearance due to the highly organized, repeating arrangement of these proteins within structures called myofibrils. Each sarcomere is defined by two Z-discs, which anchor the thin actin filaments.
Within the sarcomere, thick myosin filaments are centrally located, forming the A band, with a region called the H zone containing only myosin. Thin actin filaments extend inward from the Z-discs, overlapping with the myosin filaments in the peripheral regions of the A band, while the I bands contain only thin actin filaments. This precise arrangement allows for efficient muscle contraction through a process known as the “sliding filament model”.
Muscle contraction begins when myosin heads bind to specific sites on the actin filaments, forming cross-bridges. The myosin heads then undergo a conformational change, pulling the actin filaments inward towards the center of the sarcomere.
This pulling action shortens the sarcomere, and the cumulative shortening of many sarcomeres results in muscle contraction. ATP molecules quickly bind to the myosin heads, causing them to detach from actin, allowing the cycle to repeat.
Calcium ions play a regulatory role in initiating this process. In resting muscle, strands of tropomyosin block the myosin-binding sites on actin, preventing interaction. When calcium ions are released, they bind to troponin, a protein associated with tropomyosin. This binding causes a shift in the tropomyosin, exposing the myosin-binding sites on the actin filaments and allowing contraction to proceed.
The Contractile Ring in Cell Division
Beyond muscle, actin and myosin are also responsible for the precise division of a cell during cytokinesis in animal cells. Following the separation of chromosomes, a temporary structure called the contractile ring forms around the equator of the dividing cell. This ring is composed of interdigitated actin filaments and bipolar filaments of myosin II.
Myosin II molecules within the ring act as motors, sliding the actin filaments past one another. This contractile force causes the ring to constrict, much like pulling a drawstring. The constriction of the contractile ring pinches the plasma membrane inward, forming a cleavage furrow. This inward constriction ultimately divides the single parent cell into two distinct daughter cells, each with its own nucleus and cytoplasm.
The contractile ring is a transient assembly, formed for cell division. Once cytokinesis is complete, the components of the ring disassemble, highlighting its role as a dynamic structure. This mechanism ensures that genetic material is evenly distributed between the new cells.
Cytoskeletal Roles in Cell Shape and Motility
Actin filaments are integral components of the cytoskeleton in non-muscle cells, contributing to cell shape. These filaments form complex networks, including stress fibers, which provide internal tension and help maintain cellular integrity. The dynamic assembly and disassembly of these actin networks allow cells to adapt their shape in response to various internal and external cues.
Cell motility, or the crawling movement of cells across a surface, is another function heavily reliant on actin and myosin. At the leading edge of a moving cell, actin filaments rapidly polymerize to form dynamic protrusions, such as broad, sheet-like lamellipodia or slender, finger-like filopodia. These extensions push the cell membrane forward, allowing the cell to explore its environment.
Myosin motors, particularly myosin II, are located further back from these leading-edge protrusions. These myosins provide the contractile force needed to pull the trailing edge of the cell body forward. The coordinated action of actin polymerization at the front and myosin-driven contraction at the rear enables the entire cell to move directionally. This mechanism differs from muscle contraction as it involves the movement of the entire cell rather than the shortening of a fixed structure.
Intracellular Transport and Organization
The actin-myosin system also plays a logistical role in moving components within the cell, acting like an internal railway system. Actin filaments serve as tracks throughout the cytoplasm, along which specific types of myosin proteins can travel. These myosins utilize ATP energy to “walk” along the actin filaments.
Various types of myosin, such as myosin I and myosin V, specialize in transporting different cellular cargo. These motor proteins can carry vesicles from one part of the cell to another. They also transport larger organelles, such as mitochondria or lysosomes, ensuring their proper distribution and localization within the cell.
This intracellular transport system is important for maintaining cellular organization and delivering materials to their correct destinations. For example, myosin XI facilitates organelle and cytoplasmic streaming, contributing to the directed movement of cellular contents. The actin-myosin partnership thus enables the precise and efficient trafficking of a wide range of cellular components, supporting numerous cellular functions.