lecture 17

Basic properties of G-actin and F-actin:

  • G-actin (Globular actin): The monomeric form of actin, a globular protein that binds ATP or ADP. In its ATP-bound form, it polymerizes to form microfilaments.

  • F-actin (Filamentous actin): The polymerized, helical form of actin made of G-actin subunits. F-actin has inherent polarity with a plus (barbed) end, where growth is more rapid, and a minus (pointed) end, where dissociation is more likely.

Nucleation of microfilaments:

  • Spontaneous nucleation: Occurs when three G-actin monomers come together to form a stable nucleus, which then grows by adding more monomers.

  • Arp2/3 complex-mediated nucleation: The Arp2/3 complex nucleates new filaments by binding to the side of an existing filament (forming a branch) and providing a scaffold for new actin polymerization. WASp (Wiskott-Aldrich Syndrome Protein) activates the Arp2/3 complex, binding to both the complex and actin monomers, enhancing filament branching and nucleation.

ATP-actin concentration and filament polymerization:

  • The critical concentration (Cc) is the minimum concentration of ATP-actin required for polymerization. The Cc at the plus end (barbed end) is lower than at the minus end (pointed end).

  • If ATP-actin concentrations are above the Cc at the plus end, polymerization occurs; if below, depolymerization happens. Similarly, for the minus end, polymerization occurs if ATP-actin is above the Cc, but this is less common as the Cc is higher.

Profilin and Formin in actin polymerization:

  • Profilin: Binds to G-actin and promotes exchange of ADP for ATP, increasing the pool of ATP-actin available for polymerization. Profilin also delivers actin monomers to formins and other elongation factors.

  • Formin: A protein that nucleates and promotes the elongation of unbranched actin filaments by binding to the barbed end, allowing the addition of new monomers while protecting against capping proteins.

 Bacterial manipulation of eukaryotic actin: Some bacterial pathogens, like Listeria monocytogenes, hijack the host's actin machinery by expressing proteins (e.g., ActA) that mimic host nucleation factors, recruiting the Arp2/3 complex to promote actin polymerization, thereby propelling the bacteria through the host cell.

Cofilin and Tropomyosin effects on actin polymerization:

  • Cofilin: Binds to ADP-actin regions in F-actin, promoting disassembly and severing of older filaments, which accelerates actin turnover and the availability of G-actin monomers.

  • Tropomyosin: Binds along the length of F-actin filaments, stabilizing them and protecting against severing and depolymerization by cofilin and other disassembly factors. It can regulate interactions with other actin-binding proteins and modulate filament dynamics.