SAXS data collection, analysis and molecular dynamic simulation

Small angle X-ray scattering (SAXS) data sets in either 10 mM Tris-HCl pH8, 300 mM NaCl or 0.5% TBE buffer were collected at the synchrotron BioSAXS beamline BM29 of the ESRF, Grenoble, France (27). The dark state of the protein was handled under safe light (λ_max ≈ 650 nm) and measured in darkness. For the lit state samples were illuminated for at least 10 min using blue light (λ_max ≈ 470 nm) and measured under white light. SAXS data sets were analyzed using the ATSAS software package (28). Details of data collection and analysis can be found in Supplementary Methods.

Molecular dynamic simulations were performed with the AMBER14 package, using the amber ff14SB force field (29), TIP3P water model and the corresponding monovalent ion parameters (30). After minimization and temperature and pressure equilibration, self-guided Langevin dynamics were run for 1 ns with moving, rigid body fitted EMAP (31,32) restraints based on an a priori coarse SAXS model, followed by 1 ns of relaxation dynamics. Next, production simulations were run for 145 ns. Details of analysis can be found in Supplementary Methods.