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

Abstract The sequence of 96 amino acid residues from the COOH-terminus of the active subunit of cholera toxin, A 1 , has been determined as PheAsnValAsnAspVal LeuGlyAlaTyrAlaProHisProAsxGluGlu GluValSerAlaLeuGlyGly IleProTyrSerGluIleTyrGlyTrpTyrArg ValHisPheGlyValLeuAsp GluGluLeuHisArgGlyTyrArgAspArgTyr TyrSerAsnLeuAspIleAla ProAlaAlaAspGlyTyrGlyLeuAlaGlyPhe ProProGluHisArgAlaTrp ArgGluGluProTrpIleHisHisAlaPro ProGlyCysGlyAsnAlaProArg(OH). This is the largest fragment obtained by BrCN cleavage of the subunit A 1 ( M r 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 A 1 polypeptide. The site of self ADP-ribosylation in the A 1 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 A 2 subunit in the holotoxin is at position 91.