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Actin polymerization can occur spontaneously or through actin-binding proteins.

Spontaneous actin polymerization has three phases: nucleation, elongation, and steady-state.

During nucleation, three actin monomers assemble into an actin nucleus.

In elongation, monomers are rapidly added to either end of the nucleus forming a filament. The end with faster addition of monomers is termed the plus-end or barbed-end, and the other is the minus-end or pointed-end.

At steady-state, the rates of monomer addition and filament disassembly are equal.

Actin-binding protein-mediated polymerization requires two proteins, profilin, and formin.

Profilin binds ADP-bound G-actin monomers and catalyzes ADP/ATP exchange forming ATP-G-actin-profilin complexes.

Formin has a thread-like FH1 domain that binds ATP-G-actin-profilin complexes and a donut-shaped FH2 domain that binds F-actin.

FH2 removes the capping protein from the plus-end of F-actin to allow the ATP-G-actin-profilin complexes from the FH1 domain to bind the uncapped-F-actin.

Upon binding, the ATP-G-Actin-profilin complex undergoes ATP hydrolysis, releasing profilin and allowing rapid actin polymerization at the plus-end of the filament.