GALACTOSE-BINDING SITE IN ESCHERICHIA-COLI HEAT-LABILE ENTEROTOXIN (LT) AND CHOLERA-TOXIN (CT)

The galactose-binding site in cholera toxin and the closely related heat-labile enterotoxin (LT) from Escherichia coil is an attractive target for the rational design of potential anti-cholera drugs. In this paper we analyse the molecular structure of this binding site as seen in several crystal structures, including that of an LT:galactose complex which we report here at 2.2 Angstrom resolution. The binding surface on the free toxin contains several tightly associated water molecules and a relatively flexible loop consisting of residues 51-60 of the B subunit. During receptor binding this loop becomes tightly ordered by forming hydrogen bonds jointly to the G(M1) pentasaccharide and to a set of water molecules which stabilize the toxin:receptor complex

STRUCTURE OF PARTIALLY-ACTIVATED ESCHERICHIA-COLI HEAT-LABILE ENTEROTOXIN (LT) AT 2.6-ANGSTROM RESOLUTION

Biological toxicity of E. coli heat-labile enterotoxin and the closely related cholera toxin requires that the assembled toxin be activated by proteolytic cleavage of the A subunit and reduction of a disulfide bond internal to the A subunit. The structural role served by this reduction and cleavage is not known, however. We have crystallographically determined the structure of the E. coli heat-labile enterotoxin AB, hexamer in which the A subunit has been cleaved by trypsin between residues 192 and 195. The toxin is thus partially activated, in that it has been cleaved but the disulfide bond has not been reduced. The structure of the A subunit in the cleaved toxin is substantially the same as that previously observed for the uncleaved AB, structure, suggesting that although such cleavage is required for biological activity of the toxin it does not by itself cause a conformational change