Covalent structure of the gamma chain of the A subunit of cholera toxin.

The complete amino acid sequence of the gamma chain of the A subunit of cholera enterotoxin was determined: NH2-Met-Ser-Asn-Thr-Cys-Asp-Glu-Lys-Thr-Gln-Ser-Leu-Gly-Val-Lys-Phe-Leu-Asp-Glu -Tyr-Gln-Ser-Lys-Val-Lys-Arg-Gln-Ile-Phe-Ser-Gly-Tyr-Gln-Ser-Ile-Asp-Asp-Thr- His-Asn-Arg-Ile-Lys-Asn-Glu-Leu-COOH. This amino acid sequence was established in part by automated sequence analysis of intact gamma chain as well as of clostripain fragments and tryptic fragments generated from succinylated gamma chain. Smaller peptides from chymotryptic and Staphylococcus aureus protease hydrolyses provided the necessary overlaps. The gamma chain represents a segment of 46 amino acid residues (Mr = 5,395) from the carboxyl-terminal region of the precursor single-chain A subunit of cholera toxin (Duffy, L. K., Peterson, J. W., and Kurosky, A. (1981) FEBS Lett. 126, 187-190). Alignment of the gamma chain region of cholera toxin to a comparable chain region of 36 residues previously reported for heat-labile enterotoxin of Escherichia coli revealed an identity of 31%. The chemical similarity between the gamma chain regions of these two enterotoxins was thus considerably less than that of their beta chains which were shown to be 79% identical.