Immunological cross-reactivity of Escherichia coli anthranilate synthetase, glutamate synthase, and other proteins

Abstract

A radioimmunoassay has been used to search for proteins that cross-react with Escherichia coli anti-anthranilate synthetase or with antibodies to the subunits of the enzyme, anthranilate synthetase components I (AS I) and II (AS II).Cross-reactivity was examined in crude extracts of an E. coli strain with a complete deletion of all tryptophan operon structural genes.Extract proteins competed with ly51-labeled anthranilate synthetase for binding to anti-anthranilate synthetase.anti-AS I and anti-AS II.Using anti-AS II competition by extract was 75% compared to 90% by pure anthranilate synthetase.Thus most antigenic determinants on AS II, the glutamine-binding subunit of the enzyme, appear to be present in other E. coli proteins.Fractionation of extract by ion exchange chromatography indicates that a number of proteins cross-react with anti-AS II; fewer proteins cross-react with anti-AS I.Several glutamine amidotransferases have been examined for cross-reactivity with anti-AS II.Reciprocal cross-reactivity of anthranilate synthetase and glutamate synthase was shown by: (a) homogeneous glutamate synthase competes with 1251-labeled anthranilate synthetase for precipitation by anti-AS II.(b) Homogeneous anthranilate synthetase competes with lz51-labeled glutamate synthase for precipitation by anti-glutamate synthase.(c) lZ51-Labeled glutamate synthase is immunoprecipitated by anti-AS II in a double antibody reaction.(d) Microcomplement fixation was used to demonstrate binding of anthranilate synthetase to anti-glutamate synthase.These results provide evidence for limited structural similarity between anthranilate synthetase and glutamate synthase.Cross-reactivity was not detected between several other glutamine amidotransferases and anti-anthranilate synthetase.Thus amidophosphoribosyltransferase, carbamyl phosphate synthetase, CTP synthetase, formylglycinamide ribonucleotide amidotransferase,