7.1 Microfilaments and actin dynamics

Microfilaments 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 profilin, promote growth, while others, like cofilin, 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 (G-actin) monomers that polymerize to form filamentous actin (F-actin)
  • G-actin monomers have a molecular weight of approximately 42 kDa (actin, tubulin)
• Microfilaments are polarized structures with a barbed (+) end where actin polymerization predominantly occurs and a pointed (-) end where actin depolymerization predominantly occurs
• Microfilaments are a major component of the cytoskeleton that provide mechanical support, maintain cell shape, enable cell motility (lamellipodia, filopodia) and intracellular transport, and are involved in cell division, specifically in the formation of the contractile ring during cytokinesis

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:
  1. Profilin promotes actin polymerization by facilitating the exchange of ADP for ATP on G-actin monomers
  2. Cofilin promotes actin depolymerization by severing and disassembling F-actin filaments
  3. Capping proteins, such as CapZ, bind to the barbed end and prevent further polymerization
  4. Arp2/3 complex 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 (stress fibers)
• Cell motility: Microfilaments are essential for cell migration 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 (myosin V, myosin VI) 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
  • Tropomyosin stabilizes F-actin filaments and regulates their interaction with myosin
  • Gelsolin severs and caps actin filaments in a calcium-dependent manner (calcium signaling)