Actin is an abundant protein found within eukaryotic cells, which are cells containing a nucleus. It serves as a fundamental building block for the cell’s internal framework. This protein plays a widespread role across various cellular activities, reflecting its conserved importance in cell biology.
The Two Forms of Actin
Actin exists in two forms: globular actin (G-actin) and filamentous actin (F-actin). G-actin represents the individual protein monomer, much like a single LEGO brick ready for assembly. Each monomer has binding sites that facilitate interactions with other actin monomers.
G-actin monomers can polymerize, or link together, to form long structures called F-actin filaments. This process involves G-actin units assembling in a head-to-tail fashion, creating a double-stranded helical filament. Imagine connecting many LEGO bricks to construct a long, flexible beam; this illustrates how G-actin forms F-actin. This assembly is a dynamic process, meaning filaments can grow and shrink rapidly by adding or removing G-actin units.
F-actin filaments exhibit polarity, with a “plus end” and a “minus end.” All G-actin monomers within the filament are oriented in the same direction, making the ends distinguishable. G-actin addition occurs more rapidly at the plus end than at the minus end. This polarity is important for how actin interacts with other proteins and generates directional force within the cell.
Cellular Structure and Support
F-actin filaments are a component of the cytoskeleton, the cell’s internal scaffolding system. This intricate network provides mechanical support and helps cells maintain their shapes. Think of it as the poles and ropes of a tent, providing structure and stability to the fabric.
An actin network is the “cell cortex,” a specialized layer of cytoplasmic proteins located just beneath the cell membrane. This cortex is a thin layer. It forms a complex meshwork of actin filaments, often cross-linked by various proteins like spectrin, which contributes to the cell’s mechanical rigidity and elasticity. This underlying network modulates membrane behavior and cell surface properties.
Actin’s Role in Movement
Actin’s dynamic capabilities are displayed in various forms of cellular movement. One example is muscle contraction. In muscle cells, actin filaments, known as thin filaments, are arranged alongside thicker filaments composed of another protein called myosin. These filaments are organized into repeating units called sarcomeres, which shorten during contraction.
Muscle contraction occurs through the sliding filament model, where actin and myosin filaments slide past each other. Myosin molecules have projections, often called myosin heads, which bind to specific sites on the actin filaments. The binding of ATP to myosin causes the myosin head to detach from actin, and subsequent ATP hydrolysis reorients the myosin head into a cocked position. This re-cocked myosin head then reattaches to a new site on the actin filament and performs a “power stroke,” pulling the actin filament towards the center of the sarcomere.
This repetitive cycle of myosin binding, pulling, and detaching, powered by ATP, causes the muscle sarcomeres to shorten, resulting in muscle contraction. In non-muscle cells, actin also drives cell motility, enabling cells to “crawl” or migrate. This process involves the dynamic assembly of actin filaments at the cell’s leading edge, forming structures like lamellipodia. Lamellipodia are thin, sheet-like protrusions of the cell membrane, supported by a dense, branched network of actin filaments.
As G-actin monomers polymerize at the plus ends of existing filaments within the lamellipodium, they generate force that pushes the cell membrane forward. This continuous polymerization at the leading edge, coupled with the disassembly of filaments at the rear, creates a “treadmilling” effect, enabling the entire actin network to advance. This coordinated action of actin assembly and disassembly, along with interactions with adhesion molecules that anchor the cell to its environment, allows cells to extend, adhere, contract, and retract, thereby moving across surfaces.
Actin’s Involvement in Cell Division
Actin plays a role in the final stage of cell division, a process known as cytokinesis. During cytokinesis, a single parent cell divides into two distinct daughter cells. This separation is facilitated by the formation of a temporary structure called the contractile ring.
The contractile ring is composed of actin filaments and myosin II, a motor protein similar to the one found in muscles. This ring assembles at the cell’s equator, perpendicular to the axis where the chromosomes separated. As the ring forms, it attaches to the inner face of the plasma membrane.
Once assembled, the actin and myosin filaments within the contractile ring interact to generate a constricting force. This force causes the ring to tighten, much like a purse string being pulled closed. As the ring constricts, it pulls the cell membrane inward, creating a deepening indentation known as the cleavage furrow. This continuous tightening eventually pinches the parent cell completely into two separate, genetically identical daughter cells.