A Model for the Bacillus subtilis Formylglycinamide Ribonucleotide Amidotransferase Multiprotein Complex^†,‡

Formylglycinamide ribonucleotide amidotransferase (FGAR-AT) catalyzes the conversion of formylglycinamide ribonucleotide (FGAR), ATP, and glutamine to formylglycinamidine ribonucleotide (FGAM), ADP, P_i, and glutamate in the fourth step of the purine biosynthetic pathway. PurL exists in two forms:  large PurL (lgPurL) is a single chain, multidomain enzyme of about 1300 amino acids, whereas small PurL (smPurL) contains about 800 amino acids but requires two additional gene products, PurS and PurQ, for activity. smPurL contains the ATP and FGAR binding sites, PurQ is a glutaminase, and the function of PurS is just now becoming understood. We determined the structure of Bacillus subtilis PurS in two different crystal forms P2₁ and C2 at 2.5 and 2.0 Å resolution, respectively. PurS forms a tight dimer with a central six-stranded β-sheet flanked by four helices. In both the P2₁ and the C2 crystal forms, the quaternary structure of PurS is a tetramer. The concave faces of the PurS dimers interact via the C-terminal region to form a twelve-stranded β-barrel with a hydrophilic core. We used the structure of PurS together with the structure of lgPurL from Salmonella typhimurium to construct a model of the PurS/smPurL/PurQ complex. The HisH (glutaminase) domain of imidazole glycerol phosphate synthetase was used as an additional model of PurQ. The model shows stoichiometry of 2PurS/smPurL/PurQ using a PurS dimer or 4PurS/2smPurL/2PurQ using a PurS tetramer. Both models place key conserved residues at the ATP/FGAR binding site and at a structural ADP binding site. The homology model is consistent with biochemical studies on the reconstituted complex.