Data collection and structure determination
X-ray diffraction data were collected at the synchrotron beamline BL19U1 in SSRF, Shanghai, China. The crystals were maintained at 100 K during data collection. Data were indexed, integrated and scaled using HKL3000 (HKL Research, Charlottesville, VA). The structure of the complex was determined by molecular replacement using molrep (CCP4) and PDB entry 4JTU as an initial model. The resulting model was re-builded and refined in Coot. The images were generated using the software PyMOL (Molecular Graphics System, Version 2.2.3; Schrödinger, LLC). A summary of the data collection and processing statistics was given in Table S1. The coordinates and structure factors of the final model of DHODH-piprine were deposited in the Protein Data Bank (PDB, www.rcsb.org, PDB ID: 6IWU).
Mouse spleen cell proliferation
Single-cell suspensions of spleens from individual C57BL/6 mice were harvested as described previously(Wanke et al., 2017). Briefly, the spleen was aseptically taken from mice, cut into small pieces and crush gently through the cell strainers in 5 ml 1×HBSS. Cells were suspended in Ammonium-Chloride-Potassium (ACK) lysing buffer to remove the erythrocytes and then collected by centrifugation at 2000 rpm for 10 min. After washing twice with 1×HBSS, cells were re-suspended in RPMI-1640 with 10% FBS for culture. Spleen cells were seeded into 96-wells cell culture plates at a density of 5×105cells/well in RPMI-1640 with 10% FBS and subsequently incubated with various concentrations of PIP in the presence or absence of uridine (100 μM). Cell proliferation was stimulated by adding 10 μg/ml of Con A. After 72 h treatment, cells growth was measured by MTT assay with EnVision Multimode Plate Reader (Perkin Elmer, ‎Waltham, MA).