Crystal structures reveal that Lewis-x and fucose bind to secondary cholera toxin binding site - in contrast to fucosyl-GM1

Cholera is a life-threatening diarrhoeal disease caused by the human pathogen Vibrio cholerae. Infection occurs after ingestion of the bacteria, which colonize the human small intestine and secrete their major virulence factor, the cholera toxin (CT). Recent studies suggest that the GM1 receptor may not be the only target of the CT, and that fucosylated receptors such as Lewis-x (LeX) and histo-blood group antigens may also be important for cellular uptake and toxicity. However, where and how LeX binds to the CT remains unclear. Here we report the high-resolution crystal structure (1.5 A) of the receptor-binding B-subunit of the CT bound to the LeX trisaccharide, and present matching SPR data for CT holotoxins. LeX, and also L-fucose alone (at 500-fold molar excess), bind to the secondary binding site of the toxin, distinct from the GM1 binding site. In contrast, fucosyl-GM1 binds to the primary binding sites due to high-affinity interactions of its GM1 core. The two binding sites are likely connected by allosteric cross-talk.