The primary structure of the COOH-terminal half of cholera toxin subunit A1 containing the ADP-ribosylation site.

The sequence of 96 amino acid residues from the COOH-terminus of the active subunit of cholera toxin, A1, has been determined as (sequence; see text) This is the largest fragment obtained by BrCN cleavage of the subunit A1 (Mr 23,000), and has previously been indicated to contain the active site for the adenylate cyclase-stimulating activity. Unequivocal identification of the COOH-terminal structure was achieved by separation and analysis of the terminal peptide after the specific chemical cleavage at the only cysteine residue in A1 polypeptide. The site of self ADP-ribosylation in the A1 subunit [C. Y. Lai, Q.-C. Xia, and P.T. Salotra (1983) Biochem. Biophys. Res. Commun. 116, 341-348] has now been identified as Arg-50 of this peptide, 46 residues removed from the COOH-terminus. The cysteine that forms disulfide bridge to A2 subunit in the holotoxin is at position 91.