RECEPTOR RECOGNITION, PROTEIN-CARBOHYDRATE INTERACTIONS AND A SEARCH FOR RECEPTOR ANTAGONISTS OF THE CHOLERA TOXIN FAMILY. Wim G.J. Hol, Ethan Merritt, Focco van den Akker, Ingeborg Feil, Steve Sarfaty, Wendy Minke, and Christophe Verlinde, Howard Hughes Medical Institute, Biomolecular Structure Center, Departments of Biological Structure and Biochemistry, University of Washington, Box 357742, Seattle, Washington 98195-7742.

Cholera toxin and the closely related heat-labile enterotoxin of E. coli are the prime virulence factors secreted by Vibrio cholerae and enterotoxigenic E.coli. These pathogens are responsible for significant mortality during epidemics as well as occurring endemically in third world countries. The toxins recognize as receptor the pentasaccharide head group of the glycolipid ganglioside GM1 on the outer surface of epithelial cells. In collaboration with three other groups, this recognition process has been studied crystallographically by determining the structures of:

1. The cholera toxin B-pentamercomplexed with GM1 pentasaccharide;

2. The heat labile enterotoxin in complex with galactose, lactose, and galactose-ß 1,3-N-acetylgalactosamine (the Thomsen-Friedenreich `T-antigen' disaccharide used in cancer diagnosis);

3. The crystal structures of mutants of the two toxins which have impaired receptor recognition properties. In particular position Gly33 is intriguing since this residue does not interact directly with the receptor and yet some, but not all, amino acid substitutions at this position affect receptor binding.

In three of the mutant structures it was discovered that the imidazole ring of a histidine of a neighboring B-pentamer is positioned above the indole ring of Trp88. This indole ring is the prime hydrophobic interaction in the GMl:cholera toxin complex. So we have two starting points for our attempts to arrive at molecules which might interfere with receptor binding: X-ray structures with sugars bound, and the mutant structures with imidazole rings in the sugar binding site.