title: A 70-amino acid zinc-binding polypeptide from the regulatory chain of aspartate transcarbamoylase forms a stable complex with the catalytic subunit leading to markedly altered enzyme activity.
abstract: In an effort to clarify effects of specific protein-protein interactions on the properties of the dodecameric enzyme aspartate transcarbamoylase (carbamoyl-phosphate:L-aspartate carbamoyltransferase, EC 2.1.3.2), we initiated studies of a simpler complex containing an intact catalytic trimer and three copies of a fragment from the regulatory chain. The partial regulatory chain was expressed as a soluble 9-kDa zinc-binding polypeptide comprising 11 amino acids encoded by the polylinker of pUC18 fused to the amino terminus of residues 84-153 of the regulatory chain; this polypeptide includes the zinc domain detected in crystallographic studies of the holoenzyme. In contrast to intact regulatory chains, the zinc-binding polypeptide is monomeric in solution because it lacks the second domain responsible for dimer formation and assembly of the dodecameric holoenzyme. The isolated 9-kDa protein forms a tight, zinc-dependent complex with catalytic trimer, as shown by the large shift in electrophoretic mobility of the trimer in nondenaturing polyacrylamide gels. Enzyme assays of the complex showed a hyperbolic dependence of initial velocity on aspartate concentration with Vmax and Km for aspartate approximately 50% lower than the values for free catalytic subunit. A mutant catalytic subunit containing the Lys-164----Glu substitution exhibited a striking increase in enzyme activity at low aspartate concentrations upon interaction with the zinc domain because of a large reduction in Km upon complex formation. These changes in functional properties indicate that the complex of the zinc domain and catalytic trimer is an analog of the high-affinity R ("relaxed") state of aspartate transcarbamoylase, as proposed previously for a transiently formed assembly intermediate composed of one catalytic and three regulatory subunits. Conformational changes at the active sites, resulting from binding the zinc-containing polypeptide chains, were detected by difference spectroscopy with trinitrophenylated catalytic trimers. Isolation of the zinc domain of aspartate transcarbamoylase provides a model protein for study of oligomer assembly, communication between dissimilar polypeptides, and metal-binding motifs in proteins.
authors.affiliation: Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720.
authors.forename: D W
authors.initials: DW
authors.lastname: Markby
authors.forename: B B
authors.initials: BB
authors.lastname: Zhou
authors.forename: H K
authors.initials: HK
authors.lastname: Schachman
mesh_terms: D001221:Aspartate Carbamoyltransferase / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D002850:Chromatography, Gel; D002852:Chromatography, Ion Exchange; D004591:Electrophoresis, Polyacrylamide Gel; D004926:Escherichia coli / Q000201:enzymology* / Q000235:genetics; D007700:Kinetics; D046911:Macromolecular Substances; D010957:Plasmids; D013053:Spectrophotometry; D014462:Ultrafiltration; D015032:Zinc / Q000378:metabolism*; D016335:Zinc Fingers / Q000235:genetics / Q000502:physiology*
chemical_list: D046911:Macromolecular Substances; D001221:Aspartate Carbamoyltransferase; D015032:Zinc