BACKGROUND Pertussis toxin is an exotoxin of the A-B class produced by Bordetella pertussis. The holotoxin comprises 952 residues forming six subunits (five different sequences, S1-S5). It plays an important role in the development of protective immunity to whooping cough, and is an essential component of new acellular vaccines. It is also widely used as a biochemical tool to ADP-ribosylate GTP-binding proteins in the study of signal transduction. RESULTS The crystal structure of pertussis toxin has been determined at 2.9 A resolution. The catalytic A-subunit (S1) shares structural homology with other ADP-ribosylating bacterial toxins, although differences in the carboxy-terminal portion explain its unique activation mechanism. Despite its heterogeneous subunit composition, the structure of the cell-binding B-oligomer (S2, S3, two copies of S4, and S5) resembles the symmetrical B-pentamers of the cholera toxin and Shiga toxin families, but it interacts differently with the A-subunit. The structural similarity is all the more surprising given that there is almost no sequence homology between B-subunits of the different toxins. Two peripheral domains that are unique to the pertussis toxin B-oligomer show unexpected structural homology with a calcium-dependent eukaryotic lectin, and reveal possible receptor-binding sites. CONCLUSIONS The structure provides insight into the pathogenic mechanisms of pertussis toxin and the evolution of bacterial toxins. Knowledge of the tertiary structure of the active site forms a rational basis for elimination of catalytic activity in recombinant molecules for vaccine use.