A mutant cholera toxin B subunit that binds GM1-ganglioside but lacks immunomodulatory or toxic activity

CM1-ganglioside receptor binding by the B subunit of cholera toxin (CtxB) i s widely accepted to initiate toxin action by triggering uptake and deliver y of the toxin A subunit into cells. More recently, GM1 binding by isolated CtxB, or the related B subunit of Escherichia coli heat-labile enterotoxin (EtxB), has been found to modulate leukocyte function, resulting in the do wn-regulation of proinflammatory immune responses that cause autoimmune dis orders such as rheumatoid arthritis and diabetes. Here, we demonstrate that GM1 binding, contrary to expectation, is not sufficient to initiate toxin action. We report the engineering and crystallographic structure of a mutan t cholera toxin, with a His to Ala substitution in the B subunit at positio n 57. Whereas the mutant retained pentameric stability and high affinity bi nding to GM1-ganglioside, it had lost its immunomodulatory activity and, wh en part of the holotoxin complex, exhibited ablated toxicity. The implicati ons of these findings on the mode of action of cholera toxin are discussed.