The solution structures of isolated monomeric actins in their Mg 2+-ATP and Ca2+-ATP bound forms and in complexes with gelsolin segment-1 have been probed using hydroxyl radicals (·OH) generated by synchrotron X-ray radiolysis. Proteolysis and mass spectrometry analysis of 28 peptides containing 58 distinct reactive probe sites within actin were used to monitor conformational variations linked to divalent cation and gelsolin segment-1 binding. The solvent accessibilities of the probe sites, as measured by footprinting in solution for the Ca2+-G-actin and Mg2+-G-actin complexes with gelsolin segment-1, were consistent with available crystallographic data. This included a specific protection at the contact interface between the partners, as revealed by reduced reactivity of peptide 337-359 in the complex. Aside from the specific protection indicated previously, the oxidation rates for the reactive residues of the isolated Ca2+-G-actin were similar to those of the actin gelsolin segment-1 complexes; however, the reactivity of numerous residues in the isolated Mg 2+-G-actin form was significantly reduced. Specifically, Mg 2+-G-actin has a set of protected sites relative to Ca 2+-G-actin that suggest a structural reorganization in subdomains 4 and 2 and a C-terminus more closely packed onto subdomain 1. These conformational variations for isolated Mg2+-G-actin provide a structural basis for its greater tendency to polymerize into filaments as compared to Ca2+-G-actin.
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