A note on the predicted secondary structures of the active chains of cholera and diphtheria toxins

Both the A regions of diphtheria and cholera toxins contain the site of ADP-ribosyl transferase activity which is responsible for the modification of specific target proteins in mammalian cell types. The secondary structure prediction for these A regions has been made on the basis of their recently reported primary structures. In the center of both toxin A chains, the beta-structure and alpha-helix regions alternate in a manner similar to that reported for some NAD binding proteins. Other regions of alpha-helix in the A chains may be involved in the interactions with the toxin B chains. The lack of primary structure homology between these toxins indicates that the secondary structure homology is the result of convergent evolution of a NAD binding domain in each protein.