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 A₂ (M_r 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% α-helix, 25% β-structure, 12% β-turn, and 36% random coil. The majority of the β-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 β-sheet domain led immediately into the active site region characterized by the alternating structures of β-pleated sheet and α-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. Ganrier 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 α-helix configuration.