Purification of the ST_B enterotoxin of Escherichia coli and the role of selected amino acids on its secretion, stability and toxicity

The methanol‐insolouble heat‐stable enterotoxin of Escherichia coli (ST_B) was purified and characterized by automated Edman degradation and tryptic peptide analysis. The amino‐terminal residue, Ser‐24, confirmed that the first 23 amino acids inferred from the gene sequence were removed during translocation through the E. coli inner membrane. Tryptic peptide analysis coupled with automated Edman degradation revealed that disulphide bonds are formed between residues Cys‐33 and Cys‐71 and between Cys‐44 and Cys‐59. Oligonucleotide‐directed mutagenesis performed on the ST_B gene demonstrated that disulphide bond formation does not precede translocation of the polypeptide through the inner membrane and that disulphide bridge formation is a periplasmic event; apparently, elimination of either of two disulphides of ST_B renders the molecule susceptible to periplasmic proteolysis. In addition, a loop defined by the Cys‐44—Cys‐59 bond contains at least two amino acids (Arg‐52 and Asp‐53) required for STQ toxic activity.