Observation by 13C NMR of interactions between cholera toxin and the oligosaccharide of ganglioside GM1.

The initial event in the action of cholera toxin on intact cells is its recognition of cell-surface receptors, molecules of ganglioside GM1. We have studied details of this interaction by 13C NMR, which enables us to examine simultaneously both the protein and the ganglioside or, as in the present instance, its oligosaccharide portion. 13C NMR spectra of the toxin are consistent with the long correlation times expected for this 84,000-dalton protein. They show, however, some resolved resonances (including one tentatively assigned to the epsilon 2 carbon of tryptophan, 138.3 ppm downfield from tetramethylsilane). When oligosaccharide is added to the toxin this resonance broadens or moves further upfield to reside under phenylalanine resonances at 136.7 ppm. Of the seven tryptophan residues in cholera toxin, five are in the B subunits which bind GM1, so that the data are consistent with a resonance shift for the epsilon 2 carbons of these residues on binding the oligosaccharide. Resonances arising from the anomeric and methylene carbons of the sialic acid moiety of the oligosaccharide are also shifted. Comparison with corresponding chemical shifts in a series of model compounds suggest that the latter effects may originate in a toxin-induced conformational change in the oligosaccharide. At high resolution, anomeric carbon resonances of terminal galactose residues in free and bound oligosaccharide are also resolved.