The (beta)-Thymosin/WH2 Domain: Structural Basis for the Switch from Inhibition to Promotion of Actin Assembly

The widespread (beta)-thymosin/WH2 actin binding domain has versatile regulatory properties in actin dynamics and motility. (beta)-thymosins (isolated WH2 domain) maintain monomeric actin in a “sequestered” nonpolymerizable form. In contrast, when repeated in tandem or inserted in modular proteins, the (beta)-thymosin/WH2 domain promotes actin assembly at filament barbed ends, like profilin. The structural basis for these opposite functions is addressed using ciboulot, a three (beta)thymosin repeat protein. Only the first repeat binds actin and possesses the function of ciboulot. The region that shows the strongest interaction with actin is an amphipathic N-terminal (alpha) helix, present in all (beta)-thymosin/WH2 domains, which recognizes the ATP bound actin structure and uses the shear motion of actin linked to ATP hydrolysis to control polymerization. Crystallographic (1H, 15N), NMR, and mutagenetic data reveal that the weaker interaction of the C-terminal region of (beta)-thymosin/WH2 domain with actin accounts for the switch in function from inhibition to promotion of actin assembly.