Gelsolin regulates the dynamic assembly and disassembly of the actin-based cytoskeleton in non-muscle cells and clears the circulation of filaments released following cell death. Gelsolin is a six-domain (G1-G6) protein activated by calcium via a multistep process that involves unfolding from a compact form to a more open form in which the three actin-binding sites (on the G1, G2, and G4 subdomains) become exposed. To follow the global structural changes that accompany calcium activation of gelsolin, small-angle x-ray scattering (SAXS) data were collected for full-length human plasma gelsolin at nanomolar to millimolar concentrations of free Ca2+. Analysis of these data showed that, upon increasing free Ca2+ levels, the radius of gyration (Rg) increased nearly 12 Å, from 31.1 ± 0.3 to 43 ± 2Å, and the maximum linear dimension (Dmax) of the gelsolin molecule increased 55 Å, from 100 to 155 Å. Structural reconstruction of gelsolin from these data provided a striking visual tracking of the gradual Ca2+-induced opening of the gelsolin molecule and highlighted the critical role played by the flexible linkers between homologous domains. The tightly packed architecture of calcium-free gelsolin, seen from both SAXS and x-ray crystallographic models, is already partially opened up in as low as 0.5 nM Ca2+. Our data confirm that, although the molecule springs open from 0 to 1 μM free Ca2+, even higher calcium concentrations help to stabilize a more open structure, with increases in R g and Dmax of ∼2 and ∼15 Å, respectively. At these higher calcium levels, the SAXS-based models provide a molecular shape that is compatible with that of the crystal structures solved for Ca 2+/gelsolin C-terminal and N-terminal halves ± monomeric G-actin. Placement of these crystal structures within the boundaries of the SAXS-based model suggests a movement of the G1/G2 subunits that would be required upon binding to actin.
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