The actin cytoskeleton is a dynamic internal framework within cells. This adaptable network of protein filaments constantly reorganizes in response to cellular needs. It functions much like a cell’s combined “bones and muscles,” providing structural support and enabling a wide range of movements. This system allows cells to adopt diverse shapes and perform numerous tasks fundamental for life.
The Building Blocks and Structure of Actin
The actin cytoskeleton is constructed from a protein called actin. Individual, globular actin proteins are known as G-actin monomers, serving as the basic “bricks” for assembly. These G-actin units can reversibly join together through a process called polymerization, forming long, thread-like structures known as filamentous actin, or F-actin. F-actin filaments resemble a double-stranded helix, approximately 7 nanometers in diameter.
The dynamic assembly and disassembly of these filaments are precisely controlled, allowing rapid structural adaptation. F-actin can break down into G-actin through depolymerization, making units available for reuse. This reversible process, where monomers are added to one end (often called the “barbed” or “plus” end) and removed from the other (“pointed” or “minus” end), is coupled with ATP hydrolysis, influencing the filament’s stability and behavior. Actin filaments are organized into various higher-order structures within the cell, including parallel bundles that offer strength or branched, mesh-like networks that provide broad support and flexibility.
Maintaining Cellular Shape and Driving Movement
The actin network provides internal scaffolding that helps cells maintain their shapes and resist external forces. A dense meshwork of actin filaments is often located just beneath the cell membrane, providing mechanical integrity. This network can reorganize quickly, allowing the cell to change shape in response to environmental cues.
Beyond maintaining shape, actin directly drives cell movement, such as migration. This movement is often initiated by the rapid assembly of actin filaments at the leading edge of the cell, forming broad, flattened protrusions called lamellipodia. As new actin monomers are added to the growing ends of filaments, they push the cell membrane forward, similar to how a tank tread propels a vehicle. Older actin filaments at the rear of these protrusions simultaneously depolymerize, recycling the actin monomers for continued forward movement. This coordinated assembly and disassembly, known as treadmilling, allows the cell to move across surfaces, a process regulated by various proteins and signaling pathways.
Powering Muscle Contraction and Internal Transport
Actin filaments interact with motor proteins, particularly myosin, to generate forces. In muscle cells, thick filaments of myosin are arranged alongside thin actin filaments within structures called sarcomeres. Muscle contraction occurs when myosin heads bind to actin and, powered by the hydrolysis of ATP, pull the actin filaments past the myosin filaments in a “sliding filament” mechanism. This action shortens the sarcomere, generating force for movement.
This actin-myosin interaction also extends to intracellular transport, where myosin motors function as cellular “trucks.” Myosin motors “walk” along actin filaments, which serve as intracellular “highways,” carrying cargo. This cargo can include vesicles containing proteins, organelles like mitochondria, or even ribonucleoprotein particles. This directed transport is important for distributing materials and maintaining cellular organization.
The Role of Actin in Cell Division
Actin plays a fundamental role in the final stage of cell division, known as cytokinesis. After a cell has duplicated its genetic material and separated the chromosomes into two new nuclei, the actin cytoskeleton forms a structure called the contractile ring. This ring assembles at the cell’s equator, perpendicular to the axis of chromosome separation.
Composed of actin filaments and myosin II, the contractile ring functions like a drawstring. Myosin motors within the ring pull on the actin filaments, causing the ring to constrict. This tightening action pinches the cell membrane inward, forming a cleavage furrow that deepens progressively. The continuous contraction of this ring ultimately divides the cell into two daughter cells, a process important for growth, development, and tissue repair.
Actin Dysfunction in Human Disease
When the functions of the actin cytoskeleton are disrupted, it can contribute to human diseases. For instance, malfunctions in the actin-myosin interactions that power muscle contraction can lead to muscular dystrophies, characterized by progressive muscle weakness and degeneration. These conditions often involve defects in proteins that link the actin cytoskeleton to the muscle cell membrane or regulate actin-myosin force generation.
Unregulated cell motility, a process normally driven by actin, is a hallmark of cancer metastasis. Cancer cells can hijack their actin machinery to become migratory and invasive, using structures like lamellipodia and filopodia to break away from a primary tumor and invade new tissues, spreading the disease. Some pathogens and toxins target the actin cytoskeleton. For example, phalloidin, a toxin from poisonous mushrooms, binds tightly to F-actin, stabilizing it and preventing its depolymerization, which disrupts normal cellular processes and can be lethal. Understanding these dysfunctions is important for developing therapeutic strategies.