Figure 2. A model for the modulation of CP activity and dynamic behavior of actin filament ends in response to PA. CP binds to the barbed ends of actin filaments and prevents profilin-actin complex addition onto this end; i.e., actin polymerization is inhibited in the presence of CP. (A, B and C represent individual actin filaments). Reduced CP levels results in more available dynamic ends, which leads to new actin filament assembly ③ and enhanced filament-filament annealing ④. The end-capping activity of CP is negatively regulated by PA ①. When cellular PA levels increase, direct binding to PA dissociates CP from barbed ends ②, causes uncapping of the actin filaments, and leads to enhanced dynamic behavior of filament ends (followed by processes ③ and ④).