amino acid sequence of the beta-subunit: of porcine enterotoxigenic Escherichia coli enterotoxin--analysis and comparison with literature data

The amino acid composition and sequence of the β‐subunit of heat‐labile enterotoxin (LT) purified from a porcine (LTp) strain, WT‐1, of enterotoxigenic Escherichia coli was analysed, and the result was compared with that reported by Dallas and Falkow [Nature 288 (1980) 499‐501] who deduced the amino acid sequence of LTp from data on the DNA sequence of a porcine strain, EWD299. The purified β‐subunit of the LTp of WT‐1 was carboxymethylated, succinylated, digested with chymotrypsin and subjected to high performance liquid chromatography (HPLC). The amino acid composition of the peptide peaks from the column were analysed and compared with the data reported by Dallas and Falkow. Only one fraction differed in amino acid composition from that reported, containing lysine instead of methionine. This fraction was found to consist of two peptides with the sequences Lys‐Ser‐Gly‐Glu‐Thr‐Phe and Arg‐Ile‐Thr‐Tyr. The former peptide is reported to have the sequence Met‐Ser‐Gly‐Glu‐Thr‐Phe. Thus, the amino acid at position 43 from the N terminus of the β‐subunit of LTp is lysine, not methionine as reported. This is the first report which studied the amino acid sequence of LTp analysed by protein toxin itself, not by DNA sequence analysis.