Actin filament

Actin filaments, also called microfilaments, are thin protein fibers found in the cytoplasm of eukaryotic cells. They serve as a vital part of the cytoskeleton, which is the internal framework of a cell. These filaments are incredibly small, measuring approximately 7 nanometers in diameter. They are primarily made of polymers of actin, which are repeating units of protein. While actin is the main building block, many other proteins modify and interact with these filaments to control their behavior.

To understand how these filaments work, we must look at their structure and growth. Each microfilament consists of two helical strands of subunits that are interlaced together. These subunits are known as globular actin, or G-actin. When they are joined into a long fiber, they are called filamentous actin, or F-actin. A key feature of these filaments is their polarity, meaning they have two distinct ends. One end is the fast-growing barbed end, also called the plus (+) end. The other is the slow-growing pointed end, or the minus (−) end.

The process of building these filaments is called polymerization. In a laboratory setting, this starts when three G-actin monomers join to form a trimer. An ATP-bound actin then attaches to the barbed end. Following this, ATP hydrolysis occurs, which is a chemical reaction that breaks down ATP. This hydrolysis happens with a half-time of about 2 seconds. Later, the inorganic phosphate dissociates with a half-time of about 6 minutes. This process reduces the binding strength between subunits, which can destabilize the filament. In living cells, this growth is managed by molecular motors called actoclampins.

Because the barbed end grows much faster than the pointed end shrinks, a unique process occurs called treadmilling. At a steady state, the rate of polymerization at the barbed end matches the rate of depolymerization at the pointed end. This results in the filament appearing to move or "treadmill" through the cell. This constant turnover is essential for cell movement. To keep the cycle going, the protein cofilin helps by severing ADP-rich regions near the pointed end. This releases actin monomers back into the cytosol, where they can bind to new ATP and be reused for growth.

Actin filaments are organized into different shapes by various actin-binding proteins. They can form dense bundles or wide networks. Bundles can be polar, where all barbed ends point the same way, or non-polar. Cross-linking proteins are responsible for determining the orientation and spacing of these structures. Other proteins serve specific roles, such as branching proteins, severing proteins, and capping proteins. For example, the Arp2/3 complex binds to an existing "mother filament" to create a new "daughter filament" at a 70-degree angle. This creates a beautiful, fan-like branched network.

The history of actin research reveals how much we have learned about cellular life. F.B. Straub first discovered actin in rabbit skeletal muscle during the mid-1940s. About twenty years later, H.E. Huxley demonstrated that actin is essential for muscle contraction. In the mid-1980s, scientists finally described the mechanism of how actin creates long filaments. Since then, studies have expanded to show how actin influences cell shape, motility, and cytokinesis, which is the division of a cell.

Actin also acts as a track for molecular motors called myosin. These enzymes use energy from ATP to move along the filaments. In muscle cells, actin and myosin work together as actomyosin-driven contractile motors. The thin actin filaments serve as platforms for myosin's pulling action. This interaction is what allows for muscle contraction and the advancement of pseudopods. Beyond muscles, these motors help transport cargo vesicles and generate the force needed for cells to crawl or change shape.

Different types of cells use actin to create specialized structures. Red blood cells use a spectrin-actin hexagonal lattice to maintain their shape. In human embryonic kidney cells, the cortical actin forms a scale-free fractal structure. Neurons use actin to form periodic rings that are stabilized by proteins like spectrin and adducin. This ring structure has been found in many different animals, including fruit flies and mice. Even mammalian sperm use actin to form a helical structure in the midpiece of their flagellum. These diverse examples show how fundamental actin is to life.