Oligomerization of Escherichia coli enterotoxin b through its C-terminal hydrophobic alpha-helix

Using a chemical cross-linker and gel electrophoresis or a dot blot overlay assay, we studied protein-protein interaction of STb toxin, a 48-residue amphiphilic polypeptide causing intestinal disorders. For the first time, we report on the oligomerization property of STb. This enterotoxin forms hexamers and heptamers in a temperature-independent fashion in presence or absence of its receptor (sulfatide) anchored in a 50-nm liposome or as a free molecule. Full STb structure integrity is necessary for its oligomerization as this process is not observed under reducing conditions in the presence of beta-mercaptoethanol. STb treatment with tetramethylurea (TMU) and different detergents prevented oligomerization. Site-directed mutagenesis decreasing overall STb hydrophobicity in the hydrophobic alpha-helix resulted in the incapacity to form oligomers. Taken together, these data suggest that the C-terminal hydrophobic alpha-helix corresponds to the domain of STb-STb inter-binding where hydrophobic interaction is involved.