Secretory and GM1 receptor binding role of N-terminal region of LTB in Vibrio cholerae.

Heat labile enterotoxin from enterotoxigenic Escherichia coli is similar to cholera toxin (CT) and is a leading cause of diarrhea in developing countries. It consists of an enzymatically active A subunit (LTA) and a carrier pentameric B subunit (LTB). In the current study, we evaluated the importance of the N-terminal region of LTB by mutation analysis. Deletion of the glutamine (DeltaQ3) residue and a substitution mutation E7G in the alpha1 helix region led to defects in LTB protein secretion. Deletion of the proline residue (DeltaP2) caused a decrease in alpha helicity. The DeltaP2 mutant affected GM(1) ganglioside receptor binding activity without affecting LTB pentamer formation. Upon refolding/reassembly, the DeltaP2 mutant showed defective biological activity. The single substitution mutation (E7D) strengthened the helix, imparting structural stability and thereby improved the GM(1) ganglioside receptor binding activity. Our results demonstrate the important role of N-terminal alpha1 helix in maintaining the structural stability and the integrity of GM(1) ganglioside receptor binding activity.