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

¹ Department of Molecular and Cell Biology and Virus Laboratory, University of California at Berkeley, Berkeley, California 94720, USA
² Howard Hughes Medical Institute and Department of Molecular and Cell Biology, University of California at Berkeley, Berkeley, California 94720, USA

Reprint requests to: Howard K. Schachman, University of California at Berkeley, Department of Molecular and Cell Biology, 229 Stanley Hall #3206, Berkeley, California 94720-3206, USA; e-mail: schach{at}socrates.berkeley.edu; fax: (510) 642-8699.

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 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 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.

Keywords: Circular permutation; protein engineering; cooperativity; folding; stability

Abbreviations: Asp, aspartate • ATCase, aspartate transcarbamoylase • ßME, ß-mercaptoethanol • c, catalytic polypeptide chain • CbmP, carbamoyl phosphate • DSC, differential scanning calorimetry • EDTA, ethylenediaminetetraacetic acid • IPTG, isopropyl ß-D-thiogalactopyranoside • K0.5 (Asp), apparent dissociation constant for aspartate • Met, methionine • MOPS, 3-(N-morpholino)propanesulfonic acid • OAc, acetate • PAGE, polyacrylamide gel electrophoresis • PALA, N-(phosphonacetyl)-L-aspartate • r, regulatory polypeptide chain • H6 as subscript, hexa-His sequence at the N terminus of the regulatory chain • wt as subscript, wild type

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