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 a mphiphilic polypeptide causing intestinal disorders. For the first time, we report on the oligomerization property of STb. This enterotoxin forms hexa mers and heptamers in a temperature-independent fashion in presence or abse nce of its receptor (sulfatide) anchored in a 50-nm liposome or as a free m olecule. Full STb structure integrity is necessary for its oligomerization as this process is not observed under reducing conditions in the presence o f beta -mercaptoethanol. STb treatment with tetramethylurea (TMU) and diffe rent detergents prevented oligomerization. Site-directed mutagenesis decrea sing 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-ST b inter-binding where hydrophobic interaction is involved. (C) 2001 Elsevie r Science B.V. All rights reserved.