A single native ganglioside GM1-binding site is sufficient for cholera toxin to bind to cells and complete the intoxication pathway.

Abstract

ABSTRACTCholera toxin (CT) from Vibrio cholerae is responsible for the majority of the symptoms of the diarrheal disease cholera. CT is a heterohexameric protein complex with a 240-residue A subunit and a pentameric B subunit of identical 103-residue B polypeptides. The A subunit is proteolytically cleaved within a disulfide-linked loop to generate the A1 and A2 fragments. The B subunit of wild-type (wt) CT binds 5 cell surface ganglioside GM1 (GM1) molecules, and the toxin-GM1 complex traffics from the plasma membrane (PM) retrograde through endosomes and the Golgi apparatus to the endoplasmic reticulum (ER). From the ER, the enzymatic A1 fragment retrotranslocates to the cytosol to cause disease. Clustering of GM1 by multivalent toxin binding can structurally remodel cell membranes in ways that may assist toxin uptake and retrograde trafficking. We have recently found, however, that CT may traffic from the PM to the ER by exploiting an endogenous glycosphingolipid pathway (A. A. Wolf et al., Infect. Immun. 76:1476–1484, 2008, and D. J. F. Chinnapen et al., Dev. Cell 23:573–586, 2012), suggesting that multivalent binding to GM1 is dispensable. Here we formally tested this idea by creating homogenous chimeric holotoxins with defined numbers of native GM1 binding sites from zero (nonbinding) to five (wild type). We found that a single GM1 binding site is sufficient for activity of the holotoxin. Therefore, remodeling of cell membranes by mechanisms that involve multivalent binding of toxin to GM1 receptors is not essential for toxicity of CT.