Actin Polymerization

Spontaneous actin polymerization occurs in three phases: nucleation, elongation, and steady-state. During nucleation, three actin monomers assemble into a stable nucleus. In elongation, monomers rapidly add to either end of the nucleus, forming a filament. At steady-state, the rates of monomer addition and filament disassembly become equal.

The plus-end, also called the barbed-end, has faster monomer addition rates compared to the minus-end or pointed-end. This directional difference in polymerization rates creates filament polarity. The plus-end is the primary site for rapid actin monomer incorporation during the elongation phase.

Profilin binds ADP-bound G-actin monomers and catalyzes ADP/ATP exchange, forming ATP-G-actin-profilin complexes. These complexes are then captured by formin's FH1 domain, delivering actin monomers to the growing filament. Profilin essentially prepares monomers for rapid polymerization by exchanging their nucleotide cofactor.

Formin's donut-shaped FH2 domain binds to F-actin and removes capping proteins from the plus-end, exposing binding sites for new monomers. The FH2 domain forms a sleeve around actin subunits, stabilizing the filament. This removal of capping proteins is essential for enabling rapid polymerization at the plus-end.

Upon binding to uncapped F-actin, the ATP-G-actin-profilin complex undergoes ATP hydrolysis. This hydrolysis releases profilin from the complex, allowing the actin monomer to integrate into the filament. The energy from ATP hydrolysis drives rapid polymerization at the plus-end of the filament.

Formins generate straight actin filaments by removing capping proteins and promoting linear monomer addition through their FH1 and FH2 domains. Unlike other actin-binding proteins, formins create unbranched filaments through their sleeve-like FH2 structure. This mechanism contrasts with other pathways that produce branched filament networks.

Formins can form clusters near the plasma membrane to initiate new actin filament formation. In the cytoplasm, they participate in the elongation phase of pre-existing actin filaments. This dual localization allows formins to both nucleate new filaments and extend existing ones depending on cellular location and signaling.