The subunits of cholera toxin: structure, stoichiometry, and function.

The toxin of Vibrio cholerae dissociates into subunit A and an aggregate of subunit B (choleragenoid); the dissociation is rapid under denaturing conditions and slow at neutral pH. Subunit A has a molecular weight of 27,000 daltons (measured by sedimentation equilibrium or gel chromatography) and has two polypeptide chains (mol wt, approximately 22,000 and 5,000 daltons) joined by disulfide bonds. The molecular weight of subunit B in 6 M guanidine hydrochloride is 14,000 daltons when determined by sedimentation equilibrium or gel chromatography, although dodecylsulfate gel electrophoresis suggests a lower value. These results suggest a structure of AB4 for the toxin; studies of cross-linking with methyl-4-mercaptobutyrimidate confirm this structure. The properties of antibodies both to cholera toxin and to choleragenoid are compatible with this structure, but subunit A has very low immunogenicity. Subunit A by itself is active, and this activity is abolished by a large excess of antitoxin but not by choleragenoid, anticholeragenoid, or ganglioside GM1 (galactosyl-N-acetylgalactosaminyl [sialosyl] lactosyl ceramide; GGnSLC). It is suggested that the function of subunit B is not to interact directly with the adenylate cyclase system, but to bind to the cell membrane and facilitate the interaction of subunit A.