Common architecture of the primary galactose binding sites of Erythrina corallodendron lectin and heat-labile enterotoxin from Escherichia coli in relation to the binding of branched neolactohexaosylceramide.

The heat-labile enterotoxin from Escherichia coli (LT) is responsible for so-called traveller's diarrhea and is closely related to the cholera toxin (CT). Toxin binding to GM1 at the epithelial cell surface of the small intestine initiates the subsequent diarrheal disease. However, LT has a broader receptor specificity than CT in that it also binds to N-acetyllactosamine-terminated structures. The unrelated lectin from Erythrina corallodendron (ECorL) shares this latter binding property. The findings that both ECorL and porcine LT (pLT) bind to lactose as well as to neolactotetraosylceramide suggests a common structural theme in their respective primary binding sites. Superimposing the terminal galactose of the lactoses in the respective crystal structures of pLT and ECorL reveals striking structural similarities around the galactose despite the lack of sequence and folding homology, whereas the interactions of the penultimate GlcNAcb3 in the neolactotetraosylceramide differ. The binding of branched neolactohexaosylceramide to either protein reveals an enhanced affinity relative to neolactotetraosylceramide. The b3-linked branch is found to bind to the primary Gal binding pocket of both proteins, whereas the b6-linked branch outside this site provides additional interactions in accordance with the higher binding affinities found for this compound. While the remarkable architectural similarities of the primary galactose binding sites of pLT and ECorL point to a convergent evolution of these subsites, the distinguishing structural features determining the overall carbohydrate specificities are located in extended binding site regions. In pLT, Arg13 is thus found to play a crucial role in enhancing the affinity not only for N-acetyllactosamine-terminated structures but also for GM1 as compared to human LT (hLT) and CT. The physiological relevance of the binding of N-acetyllactosamine-containing glycoconjugates to LT and ECorL is briefly discussed.