FIGURE 17-11 Regulation of filament turnover by actin-binding proteins. Actin-binding proteins regulate the rate of assembly and disassembly of actin filaments as well as the availability of G-actin for polymerization. In the profilin cycle 1, profilin binds ADP–G-actin and catalyzes the exchange of ADP for ATP. The ATP–G-actin–profilin complex can deliver actin to the (+) end of a filament with dissociation and recycling of profilin. In the cofilin cycle 2, cofilin binds preferentially to filaments containing ADP-actin, inducing them to fragment and thus enhancing depolymerization by making more filament ends. In the thymosin-β₄ cycle 3, ATP–G-actin made available by the profilin cycle is bound by thymosin-β₄, which sequesters it from polymerization. As the free G-actin concentration is lowered by polymerization, G-actin–thymosin-β₄ dissociates to make free G-actin available for association with profilin and further polymerization.