Toxin into Vaccine
Studies of AB5 bacterial toxins

Ethan A Merritt, Ingeborg Feil, Wim G J Hol: Dept of Biological Structure and HHMI, University of Washington, Seattle WA 98195-7742
Randall K Holmes: Dept of Microbiology, University of Colorado Health Sciences Center, Denver CO 80262
Rino Rappuoli: IRIS, Via Florentina 1, 53100 Siena, Italy
Cholera toxin and E. coli heat-labile enterotoxin are closely related AB5 hexameric assemblies secreted into the intestine during bacterial infection. Together they are responsible for over a million deaths annually. As with many other bacterial toxins the catalytic activity resides in a separate 'A' subunit, while receptor binding and delivery of the toxin to the target cell is mediated by a separate `B' fragment, in this case a pentamer which binds to the oligosaccharide of ganglioside GM1.

In addition to their deleterious biological effect as toxins, however, these molecular assemblies exhibit a remarkable ability to stimulate the immune system. In particular they are capable of evoking a strong mucosal immune response when administered orally or intranasally, and have been reported to confer the same sort of evoked response to co-administered antigens. It is therefore of great interest to determine whether these toxins can be engineered to lose toxicity while retaining their immunological properties.

We report here our recent crystallographic studies aimed at understanding both carbohydrate recognition by the receptor binding site and substrate recognition by the catalytic site. In particular we report the structures of oligosaccharide complexes with mutant toxins exhibiting altered receptor binding specificity, and also the structure of an engineered mutant at the active site with altered toxicity.