Protein expression and purification. Recombinant TbPTR1, LmPTR1, TbDHFR-TS, LmDHFR-TS, hDHFR and hTS were expressed and purified according to previously reported procedures. Crystallization of TbPTR1 and LmPTR1. Well-ordered monoclinic crystals of histidinetagged TbPTR1 were obtained by the vapor diffusion hanging drop technique at rt. Drops were prepared by mixing equal volumes of protein and precipitant solution (2 -2.5 M sodium acetate and 0.1 M sodium citrate pH 5) according to a previously described procedure. The TbPTR1-cofactor-inhibitor ternary complexes were obtained by the soaking technique. The compounds, solubilized in DMSO, were diluted in the cryoprotectant solution (30% vol/vol glycerol added to the precipitant solution) to a final concentration of 2-4 mM (keeping the DMSO concentration below 10% vol/vol).