The widespread β-thymosin/WH2 actin binding domain has versatile regulatory properties in actin dynamics and motility. β-thymosins (isolated WH2 domain) maintain monomeric actin in a "sequestered" nonpolymerizable form. In contrast, when repeated in tandem or inserted in modular proteins, the β-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 β-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 α helix, present in all β-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 β-thymosin/WH2 domain with actin accounts for the switch in function from inhibition to promotion of actin assembly.
Hertzog, M., Van Heijenoort, C., Didry, D., Gaudier, M., Coutant, J., Gigant, B., … Carlier, M. F. (2004). The β-thymosin/WH2 domain: Structural basis for the switch from inhibition to promotion of actin assembly. Cell, 117(5), 611–623. https://doi.org/10.1016/S0092-8674(04)00403-9