SAXS data were collected at beamline BM29 of the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, using the automatic sample changer (32). The protein was dialyzed either against buffer A [20 mM Mops (pH 7.0), 100 mM NaCl, and 5 mM CaCl2] (calcium-bound state) or against buffer B [20 mM Mops (pH 7.0), 100 mM NaCl, and 10 mM EDTA] (calcium-free state). Before measurement, the protein was centrifuged to remove any larger particles. Samples were measured at concentrations of 2.5, 10, and 20 mg/ml. The dialysis buffer was used for reference buffer correction. Data were collected at 293 K using a wavelength of 0.995 Å and a sample-to-detector distance of 2.867 m. One hundred microliters was loaded of each sample concentration, and 10 frames of were collected and merged. Samples were flowed constantly into the cell to minimize radiation damage effects. The detector images were integrated and reduced to one-dimensional scattering curves, and buffer contributions to scattering were subtracted using the beamline software BsxCuBE. Further data processing was performed automatically using the online EDNA pipeline (33) to assess sample quality and radiation damage effects. No protein aggregation or radiation damage was observed.

Data were further analyzed with the ATSAS software package (34). Briefly, primary data reduction and analysis were performed using the programs PRIMUS (35) and GNOM (36).

The theoretical scattering from the crystal structure was calculated using the program CRYSOL (37) (fig. S2), and the molecular weights were calculated using a bovine serum albumin sample as the standard.

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