title: Random circular permutation leading to chain disruption within and near alpha helices in the catalytic chains of aspartate transcarbamoylase: effects on assembly, stability, and function.
abstract: A collection of circularly permuted catalytic chains of aspartate transcarbamoylase (ATCase) has been generated by random circular permutation of the pyrB gene. From the library of ATCases containing permuted polypeptide chains, we have chosen for further investigation nine ATCase variants whose catalytic chains have termini located within or close to an alpha helix. All of the variants fold and assemble into dodecameric holoenzymes with similar sedimentation coefficients and slightly reduced thermal stabilities. Those variants disrupted within three different helical regions in the wild-type structure show no detectable enzyme activity and no apparent binding of the bisubstrate analog N:-phosphonacetyl-L-aspartate. In contrast, two variants whose termini are just within or adjacent to other alpha helices are catalytically active and allosteric. As expected, helical disruptions are more destabilizing than loop disruptions. Nonetheless, some catalytic chains lacking continuity within helical regions can assemble into stable holoenzymes comprising six catalytic and six regulatory chains. For seven of the variants, continuity within the helices in the catalytic chains is important for enzyme activity but not necessary for proper folding, assembly, and stability of the holoenzyme.
authors.affiliation: Department of Molecular and Cell Biology, University of California at Berkeley, Berkeley, CA 94720, USA.
authors.forename: P T
authors.initials: PT
authors.lastname: Beernink
authors.forename: Y R
authors.initials: YR
authors.lastname: Yang
authors.forename: R
authors.initials: R
authors.lastname: Graf
authors.forename: D S
authors.initials: DS
authors.lastname: King
authors.forename: S S
authors.initials: SS
authors.lastname: Shah
authors.forename: H K
authors.initials: HK
authors.lastname: Schachman
mesh_terms: D000595:Amino Acid Sequence; D001221:Aspartate Carbamoyltransferase / Q000737:chemistry* / Q000235:genetics* / Q000302:isolation & purification; D001224:Aspartic Acid / Q000031:analogs & derivatives; D020134:Catalytic Domain / Q000235:genetics* / Q000502:physiology; D004789:Enzyme Activation / Q000502:physiology; D004795:Enzyme Stability / Q000235:genetics / Q000502:physiology; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism; D029968:Escherichia coli Proteins; D020035:Holoenzymes / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D007700:Kinetics; D016296:Mutagenesis / Q000235:genetics*; D010746:Phosphonoacetic Acid / Q000031:analogs & derivatives; D011487:Protein Conformation; D020836:Protein Structure, Quaternary; D017433:Protein Structure, Secondary / Q000235:genetics* / Q000502:physiology
chemical_list: D029968:Escherichia coli Proteins; D020035:Holoenzymes; D001224:Aspartic Acid; C013195:sparfosic acid; D001221:Aspartate Carbamoyltransferase; C107142:pyrB protein, E coli; D010746:Phosphonoacetic Acid