7.1 Microfilaments and actin dynamics
3 min read•july 22, 2024
are thin, flexible structures made of actin proteins. They're key players in the cell's cytoskeleton, helping maintain shape, enable movement, and assist in cell division. These dynamic filaments are constantly changing, with new actin units being added and removed.
Actin dynamics are tightly controlled by various proteins. Some, like , promote growth, while others, like , encourage breakdown. This balance allows cells to quickly respond to their environment, changing shape or moving as needed. Microfilaments also serve as tracks for intracellular transport.
Microfilament Structure and Composition
Structure and role of microfilaments
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- Microfilaments are thin, flexible filaments with a diameter of about 7 nm composed of globular actin () monomers that polymerize to form filamentous actin ()
- G-actin monomers have a molecular weight of approximately 42 kDa (actin, tubulin)
- Microfilaments are polarized structures with a barbed (+) end where actin predominantly occurs and a pointed (-) end where actin predominantly occurs
- Microfilaments are a major component of the cytoskeleton that provide mechanical support, maintain cell shape, enable (, ) and intracellular transport, and are involved in cell division, specifically in the formation of the contractile ring during
Actin Dynamics and Regulation
Actin polymerization and regulation
- Actin polymerization involves the addition of G-actin monomers to the barbed (+) end of the microfilament
- G-actin monomers bind ATP, which is hydrolyzed to ADP after incorporation into the filament
- ATP-bound G-actin has a higher affinity for the barbed end, promoting polymerization
- Actin depolymerization occurs at the pointed (-) end of the microfilament where ADP-bound G-actin dissociates, resulting in filament shortening
- Actin dynamics are regulated by various actin-binding proteins:
- Profilin promotes actin polymerization by facilitating the exchange of ADP for ATP on G-actin monomers
- Cofilin promotes actin depolymerization by severing and disassembling F-
- , such as CapZ, bind to the barbed end and prevent further polymerization
- nucleates new actin filaments and creates branched actin networks (lamellipodia)
Functions of microfilaments in cells
- Cell shape: Microfilaments form a cortical actin network beneath the plasma membrane, providing mechanical support and maintaining cell shape through actin-myosin interactions that generate contractile forces enabling cells to change shape and respond to external stimuli ()
- Cell motility: Microfilaments are essential for and locomotion with actin polymerization at the leading edge of the cell creating protrusive structures (lamellipodia, filopodia) and actin-myosin interactions in stress fibers generating contractile forces for movement
- Intracellular transport: Microfilaments serve as tracks for the movement of cargo (organelles, vesicles) by myosin motor proteins (, ) involved in the distribution of organelles, vesicles, and macromolecules within the cell
Key proteins in actin dynamics
- Profilin: Binds to G-actin monomers, promotes the exchange of ADP for ATP, and facilitates the addition of ATP-bound G-actin to the barbed end of the filament, enhancing polymerization
- Cofilin: Severs and depolymerizes F-actin filaments by binding to ADP-bound actin subunits, increases the pool of available G-actin monomers for polymerization, and plays a role in actin filament turnover and remodeling
- Arp2/3 complex: Nucleates new actin filaments by mimicking the barbed end, binds to the side of an existing filament and promotes the formation of branched actin networks, playing a crucial role in the formation of lamellipodia and other actin-based structures involved in cell motility
- Other important actin-binding proteins:
- Capping proteins (CapZ) bind to the barbed end and prevent further polymerization
- stabilizes F-actin filaments and regulates their interaction with myosin
- severs and caps actin filaments in a calcium-dependent manner (calcium signaling)
Key Terms to Review (26)
Actin cortex: The actin cortex is a dense network of actin filaments located just beneath the plasma membrane of eukaryotic cells, playing a crucial role in maintaining cell shape, enabling cell motility, and facilitating various cellular processes. This structure is essential for processes like cytokinesis, endocytosis, and maintaining mechanical integrity by providing resistance against external forces. Its dynamic nature allows for rapid remodeling in response to changes in the cellular environment.
Actin filaments: Actin filaments, also known as microfilaments, are thin protein fibers made of actin monomers that play a crucial role in various cellular processes, including motility, shape maintenance, and division. They are part of the cytoskeleton, providing structural support and enabling cellular movements by interacting with myosin and other motor proteins. Their dynamic nature allows them to rapidly grow and shrink, facilitating changes in cell shape and movement.
Arp2/3 complex: The arp2/3 complex is a multi-protein assembly that plays a crucial role in the regulation of actin filament dynamics by nucleating new actin filaments and promoting the branching of existing ones. This complex is essential for the formation of dendritic networks of actin filaments, which are important for various cellular processes such as cell motility, shape changes, and intracellular transport.
Capping proteins: Capping proteins are regulatory molecules that bind to the ends of actin filaments, specifically the barbed (+) end, and stabilize them while preventing further polymerization. These proteins play a crucial role in controlling the dynamics of microfilaments by influencing their assembly and disassembly, ultimately affecting cellular processes like movement, shape, and division.
Cell migration: Cell migration is the process by which cells move from one location to another, often in response to specific signals or environmental cues. This dynamic movement is crucial for various biological processes, including tissue development, wound healing, and immune responses. The ability of cells to migrate relies on the remodeling of the cytoskeleton, particularly microfilaments, interactions with the extracellular matrix, and the regulation of adhesion molecules that facilitate connections between cells and their surroundings.
Cell motility: Cell motility refers to the ability of cells to move and navigate through their environment, a process that is essential for various biological functions such as tissue development, immune response, and wound healing. This movement is largely driven by the dynamics of the cytoskeleton, especially microfilaments composed of actin, which polymerize and depolymerize to facilitate changes in cell shape and movement. Understanding cell motility is crucial as it involves intricate mechanisms like cell adhesion, signaling pathways, and mechanical forces.
Cofilin: Cofilin is an actin-binding protein that plays a critical role in the dynamics of microfilaments by promoting the disassembly of actin filaments. It binds to ADP-actin, facilitating filament severing and enhancing turnover, which is essential for processes like cell motility and shape changes. Cofilin's activity is tightly regulated by phosphorylation, impacting its ability to interact with actin and thus modulating actin dynamics.
Cytokinesis: Cytokinesis is the process during cell division where the cytoplasm of a parental cell divides into two daughter cells. This crucial phase follows mitosis or meiosis and involves various cellular structures that ensure the proper distribution of organelles and cytoplasmic content between the two new cells, linking it closely to cellular regulation, reproductive processes, and the architecture of the cytoskeleton.
Depolymerization: Depolymerization is the process through which polymers are broken down into their monomeric units or smaller oligomers, often involving the cleavage of chemical bonds. This process is crucial in various cellular dynamics, especially regarding the assembly and disassembly of structural components, impacting cellular shape and function.
Electron microscopy: Electron microscopy is a powerful imaging technique that uses electrons instead of light to visualize the fine details of biological specimens at a much higher resolution. This technique allows scientists to observe structures within cells, such as organelles, membranes, and cytoskeletal components, enabling a deeper understanding of cellular organization and function.
F-actin: F-actin, or filamentous actin, is a polymerized form of actin that forms long, thin filaments crucial for various cellular functions. This structure is essential for maintaining cell shape, enabling motility, and facilitating intracellular transport. F-actin is dynamic, continuously undergoing polymerization and depolymerization, allowing cells to adapt their shape and respond to environmental cues.
Filopodia: Filopodia are slender, actin-rich projections that extend from the surface of a cell, playing a crucial role in sensing the environment and facilitating cell movement. These dynamic structures are composed primarily of bundled microfilaments, which are essential for their growth and retraction, allowing cells to explore their surroundings and make connections with other cells or surfaces.
Fluorescence microscopy: Fluorescence microscopy is a powerful imaging technique that uses fluorescent probes to visualize specific structures and processes within cells and tissues. By illuminating samples with specific wavelengths of light, this method allows scientists to observe the spatial distribution and dynamics of molecules in real-time, providing insights into cellular functions and interactions.
G-actin: G-actin, or globular actin, is a monomeric protein that serves as the building block for filamentous actin (F-actin) in cells. It plays a crucial role in the dynamics of microfilaments, contributing to various cellular processes such as motility, shape maintenance, and intracellular transport. G-actin's ability to polymerize into F-actin is essential for forming the cytoskeletal structures that are vital for cell function and integrity.
Gelsolin: Gelsolin is a cytoskeletal protein that plays a vital role in regulating the dynamics of actin filaments, essential components of microfilaments. This protein severs actin filaments and caps their barbed ends, facilitating the rapid assembly and disassembly of actin networks. Through its actions, gelsolin influences various cellular processes, including motility, shape, and signaling pathways.
Lamellipodia: Lamellipodia are flat, sheet-like extensions of the cell membrane that play a critical role in cell movement and migration. These structures are rich in actin filaments and are formed by the dynamic polymerization of actin, allowing cells to crawl on surfaces. Their formation is essential for processes such as wound healing, immune response, and cancer metastasis.
Microfilaments: Microfilaments are the thinnest filaments of the cytoskeleton, primarily composed of actin protein, and are crucial for maintaining cell shape, enabling movement, and facilitating intracellular transport. They interact with other cytoskeletal elements and motor proteins, playing a vital role in various cellular processes including muscle contraction and cell division.
Muscle contraction: Muscle contraction is the physiological process where muscle fibers generate force and shorten, leading to movement. This process is primarily driven by the interaction of actin and myosin filaments within muscle cells, which relies heavily on microfilaments and actin dynamics to facilitate movement and support various functions throughout the body.
Myosin V: Myosin V is a motor protein that plays a crucial role in cellular transport by interacting with actin filaments, facilitating the movement of cargo within cells. This protein is known for its ability to move along actin filaments in a process that requires ATP, making it essential for various cellular functions such as organelle transport, vesicle movement, and cell division.
Myosin vi: Myosin VI is a type of motor protein that moves along actin filaments, playing a vital role in various cellular processes such as intracellular transport and cell signaling. It is unique among myosins due to its ability to move towards the minus end of actin filaments, which is the opposite direction compared to most myosins. This distinctive movement allows myosin VI to function effectively in transporting cargo within cells and participating in cellular dynamics involving actin microfilaments.
Phosphoinositides: Phosphoinositides are a group of phospholipids that play crucial roles in cellular signaling and membrane dynamics. These lipids are derivatives of phosphatidylinositol and contain phosphate groups that can be phosphorylated or dephosphorylated, leading to various signaling pathways, particularly those involved in cytoskeletal organization and actin dynamics.
Polymerization: Polymerization is the process by which individual monomers, small repeating units, chemically bond together to form a larger, more complex structure known as a polymer. This process is essential for the assembly and dynamic regulation of various cellular components, particularly in forming structural proteins and filaments that are vital for cell shape, movement, and overall function.
Profilin: Profilin is a vital actin-binding protein that plays a crucial role in the dynamics of microfilaments by promoting the polymerization of actin filaments. It facilitates the exchange of ADP for ATP on actin monomers, thereby increasing the availability of ATP-actin for filament formation. This activity helps to regulate the growth and organization of the actin cytoskeleton, which is essential for various cellular processes such as motility, shape, and division.
Rho gtpases: Rho GTPases are a family of small GTP-binding proteins that regulate various cellular processes, including cytoskeletal dynamics, cell motility, and cell cycle progression. They act as molecular switches, cycling between an active GTP-bound state and an inactive GDP-bound state, thereby controlling signaling pathways that influence the organization of actin filaments and microfilaments. Their activity is critical for the processes that allow cells to move and change shape.
Stress fibers: Stress fibers are contractile bundles of actin filaments found in various cell types, playing a critical role in maintaining cellular tension and shape. These structures are important for processes such as cell adhesion, migration, and mechanotransduction, where cells respond to mechanical stimuli from their environment.
Tropomyosin: Tropomyosin is a coiled-coil protein that binds to actin filaments in muscle and non-muscle cells, playing a crucial role in regulating muscle contraction and other cellular processes. By binding along the length of actin filaments, tropomyosin helps stabilize these structures and prevents unwanted interactions with other proteins, such as myosin, thereby controlling muscle contraction dynamics and cytoskeletal organization.