Activation in isolation: Exposure of the actin-binding site in the C-terminal half of gelsolin does not require actin

Gelsolin requires activation to carry out its severing and capping activities on F-actin. Here, we present the structure of the isolated C-terminal half of gelsolin (G4–G6) at 2.0 A resolution in the presence of Ca2+ ions. This structure completes a triptych of the states of activation of G4–G6 that illuminates its role in the function of gelsolin. Activated G4–G6 displays an open conformation, with the actin-binding site on G4 fully exposed and all three type-2 Ca2+ sites occupied. Neither actin nor the type-l Ca2+, which normally is sandwiched between actin and G4, is required to achieve this conformation.