Figure 2.

Accelerated dynamics of actin filaments. (A) Polymerization of the actin filament preferentially occurs at the barbed end from ATP-actin–profilin complexes; after incorporation of an actin monomer into the filament, profilin dissociates. Polymerization-triggered ATP hydrolysis and subsequent release of inorganic phosphate from actin subunits make filaments more susceptible to depolymerization and increase their affinity for actin-depolymerizing factor (ADF)/cofilin. ADF/cofilin severs filaments, promoting their depolymerization. Released actin subunits bind to profilin, which competes off ADF/cofilin and promotes nucleotide exchange in the actin monomer, thus producing new ATP-actin–profilin complexes. (B) Formins and (C) Ena/VASP (vasodilator-stimulated phosphoprotein) proteins associate with actin filament barbed ends, promote their elongation by recruiting actin–profilin complexes and protecting barbed ends from capping; they also anchor barbed ends to the membrane.