The structural basis of receptor-binding by Escherichia coli associated with diarrhea and septicemia.

GafD in Escherichia coli G (F17) fimbriae is associated with diarrheal disease, and the structure of the ligand-binding domain, GafD1-178, has been determined at 1.7A resolution in the presence of the receptor sugar N-acetyl-D-glucosamine. The overall fold is a beta-barrel jelly-roll fold. The ligand-binding site was identified and localized to the side of the molecule. Receptor binding is mediated by side-chain as well main-chain interactions. Ala43-Asn44, Ser116-Thr117 form the sugar acetamide specificity pocket, while Asp88 confers tight binding and Trp109 appears to position the ligand. There is a disulfide bond that rigidifies the acetamide specificity pocket. The three fimbrial lectins, GafD, FimH and PapG share similar beta-barrel folds but display different ligand-binding regions and disulfide-bond patterns. We suggest an evolutionary path for the evolution of the very diverse fimbrial lectins from a common ancestral fold.