Isolation and characterization of a precursor form of the 'A' subunit of cholera toxin

Isolation of high-specific-activity subunits of cholera toxin by reversed-phase high-performance liquid chromatography

Facile, rapid procedures for the separation of active cholera toxin subunits were developed, based on high-performance liquid chromatography (HPLC) with a Nucleosil C8 reversed-phase column. These procedures were capable of completely resolving subunits A and B as well as S-carboxymethylated or reduced alpha-, gamma-, and beta-chains. The binding of HPLC-purified B subunit to GM1 ganglioside was essentially identical to that of cholera toxin when compared on a molar basis. The adenosine 5'-diphosphate-ribosyltransferase activity of HPLC-purified A subunit, reduced alpha-chain, or carboxymethyl alpha-chain was also determined to be reasonably high compared to that of cholera toxin or commercially prepared A subunit.

Cholera toxin A subunit: functional sites correlated with regions of secondary structure

The A subunit of cholera toxin contains the ADP-ribosyltransferase activity in its major constituent polypeptide A1 (Mr 23,000) which is responsible for the elevation of cAMP typically observed with most mammalian cell types after exposure to the toxin. The primary structure of the A subunit, recently established by sequence analyses, is presented and used as the basis for the secondary structure prediction according to the method of Chou and Fasman. The results indicated the presence of 27% alpha-helix, 25% beta-structure, 12% beta-turn, and 36% random coil. The majority of the beta-structure consisted of six strands located in the NH2-terminal portion of the molecule (residues 33-106) covering one-half of the region corresponding to the A1 polypeptide portion. The beta-sheet domain led immediately into the active site region characterized by the alternating structures of beta-pleated sheet and alpha-helix (residues 95-140) similar to that reported for other NAD+ binding proteins. The presence of this structural feature in the region was confirmed by the use of another predictive method (J. Garnier et al., J. Mol. Biol. 1978, 120, 97-120). In addition, two regions (residues 14-18 and 200-214), previously identified to contain binding sites for the B subunit as evidenced by chemical modification and monoclonal antibody studies, were found to be in alpha-helix configuration.

Vibrio cholerae enterotoxin genes: nucleotide sequence analysis of DNA encoding ADP-ribosyltransferase

The nucleotide sequence of the DNA encoding the ADP-ribosyltransferase (A1) fragment of cholera enterotoxin was determined. A putative precursor of the A1 peptide contains an 18-amino acid leader peptide, and the mature A1 peptide contains 194 amino acids. The primary structure of the A1 fragment from cholera enterotoxin is more related to that from a human enterotoxigenic Escherichia coli than to that from a porcine enterotoxigenic E. coli.