Actin Binding Proteins (ABPs) are a diverse group of molecules that associate with the structural framework of the cell known as the cytoskeleton. They are the master regulators that dictate the shape, organization, and movement of a cell by directly interacting with actin. These proteins manage the assembly and disassembly of actin filaments, ensuring the cell can respond rapidly to its environment and perform essential functions like division and migration. ABPs transform the passive components of the cytoskeleton into a dynamic and highly functional system.
Actin Filaments: The Foundation for ABPs
The structure ABPs regulate is built from actin, a highly conserved globular protein referred to as G-actin. G-actin monomers spontaneously polymerize to form long, thread-like structures called filamentous actin (F-actin). These F-actin filaments are double-stranded helices that form the core structural component of the cell’s internal scaffolding, or microfilaments.
F-actin is characterized by its polarity, having two distinct ends: a fast-growing, or barbed (+), end and a slow-growing, or pointed (-), end. Actin monomers preferentially add to the barbed end and dissociate from the pointed end, a dynamic process known as treadmilling. This slow turnover means that without specialized proteins, the actin network would not be dynamic enough to support the rapid changes required for cellular life.
Structural ABPs: Shaping and Stabilizing Cellular Architecture
Structural ABPs focus on organizing the actin network into specific architectures. These proteins control the initiation, length, and arrangement of filaments without generating active force. This precise management is necessary for creating the diverse shapes a cell requires, from the thin protrusions used for movement to the stable networks beneath the cell membrane.
Nucleation
Nucleation is the process of initiating the formation of a new actin filament. Actin polymerization is kinetically unfavorable, requiring a stable aggregate of three monomers before rapid growth can occur. Nucleation factors like the Arp2/3 complex overcome this hurdle by mimicking the structure of an actin trimer, kickstarting filament assembly.
The Arp2/3 complex binds to the side of an existing filament and initiates a new one at a 70-degree angle, creating branched actin networks that push the cell membrane forward during movement. Another group of nucleators, the formins, initiate the formation of long, unbranched filaments by remaining associated with the barbed end as it elongates. The activity of these nucleators determines the fundamental geometry of the actin network, dictating whether it forms a dense, branched mesh or long, parallel cables.
Capping and Severing
Once a filament has been started, its length must be regulated by capping and severing proteins. Capping proteins, such as CapZ, bind tightly to the fast-growing barbed end of a filament, preventing the further addition or loss of actin monomers. This action stabilizes the filament and ensures that the cell’s limited supply of G-actin monomers is not used up by unchecked growth at one site.
Other ABPs actively break down existing filaments to maintain the necessary pool of actin monomers for new construction. Proteins like cofilin bind to F-actin and promote the severing of older, less stable filaments, which increases the rate of depolymerization from the pointed end. Gelsolin is another severing protein whose activity is often regulated by calcium levels, allowing the cell to rapidly slice through actin networks to fluidize the cytoplasm or prepare for major structural changes.
Bundling and Cross-Linking
The individual filaments must be organized into stable, higher-order structures by bundling and cross-linking proteins. Bundling proteins, such as fascin and fimbrin, organize actin filaments into tightly packed, parallel arrays, like the supportive rods within microvilli or the cores of filopodia. These proteins have two actin-binding sites that are spaced precisely to hold filaments close together, forming rigid structures.
Cross-linking proteins, such as filamin, link filaments at wide angles to create a less rigid, three-dimensional gel-like network. This network forms the cell cortex, a layer just beneath the plasma membrane that provides mechanical strength and resists external forces. The specific spacing and angle of the cross-linking protein determines the stiffness and porosity of the resulting actin structure.
Motor ABPs: Generating Force and Movement
Motor ABPs use chemical energy to generate mechanical force, acting as the cell’s internal engines. The primary family is Myosin, which converts the energy stored in adenosine triphosphate (ATP) into movement along the actin filaments. Actin filaments thus serve as the tracks along which Myosin proteins “walk,” driving movement and contraction.
The general mechanism of Myosin involves a cycle of binding, pulling, and releasing the actin filament, powered by ATP hydrolysis. Myosin molecules have a head domain that binds actin and possesses ATPase activity, a neck domain that acts as a lever arm, and a tail domain that determines the cargo or structure it forms. The energy released from ATP hydrolysis causes a conformational change, or “power stroke,” in the Myosin head that pulls the actin filament past it.
Myosin proteins are categorized into numerous classes, with Myosin II being the most widely studied and responsible for contraction. Myosin II molecules assemble into thick filaments that slide actin filaments past each other within the sarcomere of muscle cells, shortening the cell and generating force. Outside of muscle, Myosin II forms contractile rings during cell division and stress fibers that generate tension.
Other Myosin classes perform functions related to intracellular transport and membrane dynamics. Myosin I and Myosin V are unconventional myosins that typically function as single molecules. Myosin V is highly processive, meaning it can take many steps without detaching, and is used to transport vesicles and organelles along actin tracks deep within the cell. Myosin I is involved in linking the actin cytoskeleton to the cell membrane, facilitating changes in cell shape and membrane protrusion.
Cellular Control Over ABP Activity
The actions of ABPs must be tightly regulated to ensure that a cell’s shape and movement are coordinated with environmental signals. ABPs are not constantly active but are switched on or off in specific locations and at precise times by signaling pathways. This system provides the cell with a rapid and localized means of remodeling its internal architecture.
A major regulatory mechanism is the activity of small GTPases, a family of proteins that act as molecular switches in intracellular signaling networks. The Rho family of GTPases, including Rho, Rac, and Cdc42, are central to controlling the actin cytoskeleton. When bound to GTP, these proteins are active and engage various ABPs to initiate specific cellular responses, such as the formation of stress fibers (Rho) or the extension of sheet-like protrusions (Rac).
Phosphorylation, the addition of a phosphate group to a protein, is another common method of controlling ABP activity. This modification can alter an ABP’s function; for example, the phosphorylation of cofilin prevents it from severing actin filaments, thereby stabilizing the existing actin network.