Actin Filaments: Videos & Practice Problems

Actin filaments, also known as microfilaments, are crucial components of the cytoskeleton, primarily responsible for cell movement and shape changes. These filaments are located near the plasma membrane, which plays a significant role in determining the cell's shape. Actin filaments are formed from monomers called G-actin, which polymerize to create F-actin filaments. Each F-actin filament consists of two strands of G-actin monomers twisted together, resembling a rope.

There are different types of actin, including alpha, beta, and gamma actin, which are found in various tissues, particularly muscle and non-muscle tissues. While it's not necessary to memorize the specific types, it's important to understand that actin exists in multiple forms depending on its location in the body.

Actin filaments exhibit polarity, meaning they have distinct ends: a plus end (barbed) and a minus end (pointed). This polarity is essential for the dynamic behavior of actin filaments, as it influences how monomers are added. G-actin monomers can be added to either end, but the plus end is favored for faster growth. The addition of G-actin to the filament is facilitated by ATP, which binds to G-actin and is hydrolyzed during the polymerization process. This hydrolysis releases energy, allowing the filament to grow.

The dynamics of actin filament growth and shrinkage can be described by two key concepts: dynamic instability and treadmilling. Dynamic instability occurs at the plus end, where the filament can switch between growth and shrinkage based on the rate of ATP hydrolysis. In contrast, treadmilling describes a situation where the filament grows at the plus end while simultaneously shrinking at the minus end, maintaining a constant length despite the turnover of monomers.

The initial formation of actin filaments begins with a process called nucleation, where G-actin monomers come together to form a stable structure. Once nucleation occurs, additional monomers can easily add to the growing filament. As ATP is hydrolyzed, it converts to ADP and inorganic phosphate, which contributes to the stability and dynamics of the filament.

Actin is a crucial protein in cellular structure and function, and it interacts with various associated proteins that facilitate its roles in nucleation, polymerization, and stabilization. Nucleation is the initial step in filament formation, primarily assisted by the Arp2/3 complex and formins. These proteins help initiate the assembly of actin filaments, which are essential for various cellular processes.

Other proteins, such as ADF (Actin Depolymerizing Factor) and cofilin, play a significant role in enhancing the disassociation of ADP-actin. This disassociation is vital because it prepares actin for the binding of ATP, which is necessary for filament stability. Conversely, profilin stimulates the addition of actin monomers to growing filaments, reversing the action of ADF and cofilin. Understanding the dynamics between ADP and ATP is crucial, as rapid hydrolysis of ATP can destabilize actin filaments, particularly at the minus end.

Actin-regulating proteins have diverse functions, including regulating polymerization, capping filament ends to prevent growth, cross-linking filaments, severing them, and bundling them together. Actin can be organized into distinct structures, such as actin bundles and networks. Actin bundles consist of closely packed, parallel arrays of filaments, resembling a bundle of sticks, while actin networks form orthogonal arrays that create a three-dimensional meshwork, similar to a gel-like structure.

The cell cortex is a specialized structure composed of actin filaments and associated proteins located beneath the plasma membrane. Additionally, microvilli are finger-like extensions of the plasma membrane that increase surface area for absorption. Cells with numerous microvilli are described as having a "brush border," akin to the bristles of a hairbrush, enhancing their absorptive capacity. These structures are integral to the cell's functionality and are primarily composed of actin, demonstrating the protein's versatility in cellular architecture.