Generation of Straight or Branched Actin Filaments

Formin is a dimeric protein that generates straight actin filaments through its characteristic FH1 and FH2 domains. The FH1 domain binds the profilin-ATP-actin complex, while the donut-shaped FH2 domain initiates nucleation with two actin monomers. Formin remains processively associated with the growing barbed end to protect it from capping proteins.

The Arp2/3 complex, a seven-subunit protein assembly, binds to nucleation-promoting factors (NPFs) with attached actin monomers. This binding alters the complex's conformation, allowing it to attach to the sides of pre-existing actin filaments. NPFs then add actin monomers to the activated complex before dissociating, with elongation proceeding at a seventy-degree angle to form Y-shaped networks.

Straight actin filaments are linear structures generated by formin proteins, while branched filaments form Y-shaped networks created by the Arp2/3 complex. Branched filaments extend at a seventy-degree angle from existing actin filaments, creating a more complex three-dimensional architecture compared to the linear organization of straight filaments.

Nucleation-promoting factors (NPFs) activate the Arp2/3 complex by binding to it while carrying an attached actin monomer. This interaction triggers conformational changes in the complex, enabling it to recognize and bind to the sides of pre-existing actin filaments. NPFs facilitate the addition of actin monomers to the complex before dissociating from the growing branch.

Formin isoforms are function-specific variants that cannot be interchanged. In fission yeast, three separate isoforms nucleate actin filaments for distinct cellular processes: the contractile ring during cytokinesis, actin cables for cell structure, and mating-related filaments. Formins are also responsible for forming actin filament bundles in filopodia.

The FH2 domain forms a donut-shaped dimer that nucleates straight actin filament formation at specific cellular sites. This domain receives actin monomers one at a time from the FH1 domain, generating controlled linear filament growth. The FH2 domain's processivity ensures continuous filament elongation while protecting the barbed end from regulatory proteins.

Arp2 and Arp3 are actin-like proteins within the Arp2/3 complex that form the pointed end of branched filaments. Their conserved surface resembles actin, promoting the binding of actin monomers to the complex and initiating branch formation. These two subunits are the first components of the daughter filament, establishing the foundation for the Y-shaped network structure.