STRUCTURAL CHARACTERIZATION OF FUNCTIONALLY IMPORTANT REGIONS OF THE ESCHERICHIA-COLI HEAT-STABLE ENTEROTOXIN STIB

The biological properties of the Escherichia coli enterotoxin STIb (ST(A-3),ST(h)) reside in a 13 amino acid C-terminal domain, abbreviated STIb(6-18). This tridecapeptide contains six cysteine residues involved in three intramolecular disulfide bridges. The solution structure of STIb(6-18) has been modeled as a series of three consecutive reverse turns [Gariepy et al. (1987) Proc. Natl. Acad. Sci. U.S.A. 83, 8907-8911]. Synthetic tridecapeptide analogues of STIb(6-18) with single amino acid substitutions at non-cysteine sites, as well as a truncated decapeptide lacking one of the three disulfide bridges, were prepared in order to examine the relationship between primary sequence and biological activity. The relative affinity of each analogue for intestinal cell receptors only partially correlates with their dose-dependent ability to cause diarrhea in suckling mice, suggesting that subsaturation doses of the enterotoxin with respect to receptor occupancy on intestinal cells may be sufficient to cause diarrhea. Two substitutions in the central-turn region of the molecule, namely, Asn12 --> Ala and Ala14 --> D-Ala, resulted in a large decrease or loss of receptor binding activity as compared to native STIb(6-18), pointing out the functional importance of this region. Analogues containing replacements at other sites showed moderate to slight reductions in biological activity. In particular, residues in the C-terminal region appear to be less important for activity, although their presence remains essential, since a truncated analogue missing the last three amino acids is inactive. Finally, a retrospective analysis of the enterotoxicity data associated with analogues containing D- or L-amino acid replacements supports the view that backbone as well as side chain perturbations in the central-turn region will strongly influence the biological activity of STIb.