25.11: Actin Polymerization
Actin polymerization occurs through the head-to-tail association of binding sites on monomeric actin or G-actin to form filamentous or F-actin. The polymerization can be divided into three phases ̶ nucleation, elongation, and steady-state phase.
The nucleation phase involves forming a stable nucleus consisting of three actin monomers to form a new actin filament. Actin-binding proteins such as formins and Arp2/3 complex help filament growth post-nucleation. The Formins form straight actin filaments with the help of actin-profilin complexes. They can form clusters near the plasma membrane to initiate actin filament formation. While in the cytoplasm, they may participate in the elongation phase of pre-existing actin filaments. During elongation of actin filaments, formins remove the capping proteins and form a sleeve around the actin subunits. The removal of capping proteins involves the dimerization of formin in the shape of a donut and binding to a pre-existing filament through the formin homology 2 or FH2 domain. Additionally, the FH1 domains of formin monomers also capture the profilin-G-actin complexes, thereby promoting the addition of G-actin to the filament.
