SAXS Analysis of SALM3-PTPσ Complex

SAXS data on the SALM3 LRR-Ig and PTPσ complex was measured by the SEC-SAXS method at Diamond Lightsource on the B21 beamline. For SALM3 LRR-Ig and PTPσ complex preparation, SALM3 LRR-Ig and PTPσ were mixed in the molar ratio of 1:1.2 of SALM3 LRR-Ig (60 µM) and PTPσ (80 µM) in total volume of 50 µl, followed by incubation at 4 °C for 1 h. Sample compartment and exposure cell were cooled to 4 °C. Superdex 200 PC 3.2/30 (GE Healthcare) column was used with flow rate of 0.5 ml/min in 30 mM Tris HCl pH7.5, 150 mM NaCl. Data processing was performed using ScÅtter³⁸ and DATASW^{39 (link)}. Particle shapes at low resolution were reconstructed ab initio by DAMMIN^{40 (link)} in P1 with P(r) function generated from the data truncated to 0.25 Å⁻¹. A total of 10 independent reconstructions were performed, and the models were averaged with the program DAMAVER^{41 (link)}. Rigid body modeling was performed using CORAL^{42 (link)}, where available partial crystal structures were treated as rigid bodies while missing residues were modelled ab initio as a Cα-chain by the software. The final model was converted to a full all-atom model using FULCHER tool (Shkumatov A. et al., unpublished). The modeling was done with either P1 or P2 symmetry. Porod and dimensionless Kratky plots were calculated for estimation of D_max and the overall compactness of the models (Supplementary Fig. S3).