Random circular permutation leading to chain disruption within and near α helices in the catalytic chains of aspartate transcarbamoylase: Effects on assembly, stability, and function

• Published in: Protein science Vol. 10; no. 3; pp. 528 - 537
• Main Authors: Beernink, Peter T., Yang, Ying R., Graf, Roney, King, David S., Shah, Shaival S., Schachman, Howard K.
• Format: Journal Article
• Language: English
• Published: Bristol Cold Spring Harbor Laboratory Press 01.03.2001
• Subjects: Amino Acid Sequence; Asp, aspartate; Aspartate Carbamoyltransferase - chemistry; Aspartate Carbamoyltransferase - genetics; Aspartate Carbamoyltransferase - isolation & purification; Aspartic Acid - analogs & derivatives; ATCase, aspartate transcarbamoylase; c, catalytic polypeptide chain; Catalytic Domain - genetics; Catalytic Domain - physiology; CbmP, carbamoyl phosphate; Circular permutation; cooperativity; DSC, differential scanning calorimetry; EDTA, ethylenediaminetetraacetic acid; Enzyme Activation - physiology; Enzyme Stability - genetics; Enzyme Stability - physiology; Escherichia coli - genetics; Escherichia coli - metabolism; Escherichia coli Proteins; folding; H6 as subscript, hexa‐His sequence at the N terminus of the regulatory chain; Holoenzymes - chemistry; Holoenzymes - genetics; Holoenzymes - metabolism; IPTG, isopropyl β‐d‐thiogalactopyranoside; K0.5 (Asp), apparent dissociation constant for aspartate; Kinetics; Met, methionine; MOPS, 3‐(N‐morpholino)propanesulfonic acid; Mutagenesis - genetics; OAc, acetate; PAGE, polyacrylamide gel electrophoresis; PALA, N‐(phosphonacetyl)‐l‐aspartate; Phosphonoacetic Acid - analogs & derivatives; Protein Conformation; protein engineering; Protein Structure, Quaternary; Protein Structure, Secondary - genetics; Protein Structure, Secondary - physiology; r, regulatory polypeptide chain; stability; wt as subscript, wild type; βME, β‐mercaptoethanol
• ISSN: 0961-8368; 1469-896X
• DOI: 10.1110/ps.39001

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 α 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 α 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.