Three crystal forms of the bifunctional enzyme proline utilization A (PutA) from Bradyrhizobium japonicum

The first diffraction-quality crystals of a PutA protein are reported. One of the three crystal forms described here exhibits pseudo-merohedral twinning. Removal of the N-terminal histidine tag aided the crystallization of another form. Proline utilization A proteins (PutAs) are large (1000–1300 residues) membrane-associated bifunctional flavoenzymes that catalyze the two-step oxidation of proline to glutamate by the sequential action of proline dehydrogenase and Δ¹-pyrroline-5-carboxylate dehydrogenase domains. Here, the first successful crystallization efforts for a PutA protein are described. Three crystal forms of PutA from Bradyrhizobium japonicum are reported: apparent tetragonal, hexagonal and centered monoclinic. The apparent tetragonal and hexagonal crystals were grown in the presence of PEG 3350 and sodium formate near pH 7. The apparent tetragonal form diffracted to 2.7 Å resolution and exhibited pseudo-merohedral twinning such that the true space group is P2₁2₁2₁ with four molecules in the asymmetric unit. The hexagonal form diffracted to 2.3 Å resolution and belonged to space group P6₂22 with one molecule in the asymmetric unit. Centered monoclinic crystals were grown in ammonium sulfate, diffracted to 2.3 Å resolution and had two molecules in the asymmetric unit. Removing the histidine tag was important in order to obtain the C2 crystal form