title: The quaternary structure of the HisZ-HisG N-1-(5'-phosphoribosyl)-ATP transferase from Lactococcus lactis.
abstract: The N-1-(5'-phosphoribosyl)-ATP transferase (ATP-PRTase) encoded by the hisG locus catalyzes the condensation of ATP with PRPP, the first reaction in the biosynthesis of histidine. Unlike the homohexameric forms of the enzyme found in Escherichia coli and Salmonella typhimurium, the ATP-PRTase from Lactococcus lactis and a number of other bacterial species consists of two different polypeptides, both of which are required for catalytic activity (Sissler et al. (1999) Proc. Natl. Acad. Sci. 96, 8985-8990). The first of these is a truncated version of HisG that is approximately 100 amino acids shorter than the canonical versions. The second, HisZ, is a 328-residue version of a class II aminoacyl-tRNA synthetase catalytic domain that possesses no aminoacylation function. Here, the molecular mass and subunit composition of the L. lactis HisZ-HisG heteromeric ATP-PRTase is investigated using liquid chromatography, analytical ultracentrifugation, and quantitative protein sequencing. Individually, HisZ and HisG form inactive but stable dimers with association constants in the range of 2.5-3.3 x 10(5) M(-1). When both types of subunits are present, a quaternary octamer complex is formed with a sedimentation coefficient of 10.1 S. Incubation of this complex with ATP promotes a shift to 10.7 S. By contrast, incubation with the allosteric modulators AMP and histidine destabilizes the complex, resulting in a shift to multiple species in equilibrium with an average of 9.3 S. While this octameric structure is unique to both the phosphoribosyl transferases and the aminoacyl-tRNA synthetases, the change in sedimentation behavior elicited by substrates and inhibitors suggests the presence of allosteric regulatory mechanisms reminiscent of other multisubunit enzymes of metabolic importance.
authors.affiliation: Department of Biochemistry, The University of Vermont Health Sciences Complex, Burlington, Vermont, 05405-0068, USA.
authors.forename: Michael L
authors.initials: ML
authors.lastname: Bovee
authors.forename: Karen S
authors.initials: KS
authors.lastname: Champagne
authors.forename: Borries
authors.initials: B
authors.lastname: Demeler
authors.forename: Christopher S
authors.initials: CS
authors.lastname: Francklyn
mesh_terms: D000604:Amino Acyl-tRNA Synthetases / Q000737:chemistry* / Q000302:isolation & purification; D001426:Bacterial Proteins; D001665:Binding Sites; D002850:Chromatography, Gel; D019281:Dimerization; D004795:Enzyme Stability; D005782:Gels; D006882:Hydroxyapatites; D013294:Lactococcus lactis / Q000201:enzymology*; D046911:Macromolecular Substances; D009004:Monosaccharide Transport Proteins / Q000737:chemistry* / Q000302:isolation & purification; D020836:Protein Structure, Quaternary; D021122:Protein Subunits; D020539:Sequence Analysis, Protein / Q000379:methods; D013379:Substrate Specificity; D014461:Ultracentrifugation
chemical_list: D001426:Bacterial Proteins; D005782:Gels; C106638:HisG protein, bacteria; D006882:Hydroxyapatites; D046911:Macromolecular Substances; D009004:Monosaccharide Transport Proteins; D021122:Protein Subunits; D000604:Amino Acyl-tRNA Synthetases; C120928:HisZ protein, Lactococcus lactis