Microfilaments are dynamic protein filaments integral to the cytoskeleton of eukaryotic cells. They provide structural support and facilitate a variety of cellular processes. These filaments are relatively thin, typically measuring about 7 nanometers in diameter. They are flexible yet strong, able to withstand significant forces while contributing to the cell’s overall architecture.
Actin: The Primary Component
Microfilaments are primarily composed of actin, a highly abundant, globular protein found in virtually all eukaryotic cells. The individual, unpolymerized form is known as globular actin (G-actin). Each G-actin monomer has a mass of approximately 42 kilodaltons and features a binding site for an ATP or ADP molecule, necessary for its stability. The high concentration of G-actin within cells, often exceeding 100 micromolar, is essential for the rapid assembly and disassembly of microfilaments, allowing cells to quickly adapt their internal structures.
Assembling the Microfilament Structure
Individual G-actin monomers come together through actin polymerization to form the microfilament structure. This process begins with three G-actin monomers forming a trimer, which serves as a nucleation site. More G-actin monomers then add to this seed, forming a long, filamentous polymer known as F-actin.
The resulting F-actin is a double-stranded helix of two intertwined actin chains. Each G-actin monomer within the filament is oriented in the same direction, giving the microfilament a distinct structural polarity. This polarity means the two ends are structurally different: a “plus” or barbed end and a “minus” or pointed end. Monomers add more rapidly to the plus end and dissociate more slowly from the minus end, a dynamic behavior known as treadmilling.
Microfilament Functions in the Cell
Microfilaments play multiple roles within the cell, leveraging their dynamic assembly and unique structure. They maintain cell shape and provide mechanical support. This network of filaments, often concentrated just beneath the plasma membrane, helps cells resist tension and provides rigidity.
Microfilaments are also instrumental in various forms of cell movement. They enable cells to change shape and move, as seen in processes like amoeboid movement, where cells crawl across surfaces. This movement involves the dynamic polymerization of actin at the leading edge, forming structures like lamellipodia and filopodia that push the cell forward. White blood cells utilize this ability to move towards and engulf pathogens.
During cell division, microfilaments are essential for cytokinesis, the process where a single cell divides into two daughter cells. They form a contractile ring, composed of actin and myosin, which pinches the cell in two. In muscle cells, microfilaments, also called thin filaments, work with myosin proteins to facilitate muscle contraction. The sliding of actin and myosin filaments past each other generates the force needed for muscle contraction.