Visualizing the elusive open shape of G-actin in solution by SAXS data analysis.

Though biochemical data upholds that ATP hydrolysis induces an opening of the nucleotide binding cleft, crystal structures of the G-actin in the absence of profillin represent the closed structure, regardless of the bound ATP/ADP. Analysis of small angle X-ray scattering (SAXS) intensities confirmed that ATP hydrolysis increases the radius of gyration (R(G)) and maximum linear dimension (D(max)) of G-actin molecules from 22.3 to 23.7 Ǻ and 70 to 78 Å, respectively. Kratky analysis confirmed that G-actin molecules behave like globular scattering particles regardless of the bound nucleotide state. Shape reconstruction using dummy residues and inertial axes overlay with known crystal structures confirmed that the ATP or AMP-PNP bound G-actin adopts a compact shape, and the nucleotide binding site opens up with ATP hydrolysis. Importantly, our ADP-state model resembled the open shape seen for β-actin and hexokinase.