Tryptophan residues of cholera toxin and its A and B protomers. Intrinsic fluorescence and solute quenching upon interacting with the ganglioside GM1, oligo-GM1, or dansylated oligo-GM1.

Solute quenching of the tryptophan fluorescence of cholera toxin and its A and B protomers has been investigated in the prsence and absence of the monosialoganglioside, galactosyl-N-acetylgalactosaminyl-[N-acetylneuraminyl]galactosylglucosylceramide (GM~), or the oligosaccharide moiety of GMI.Quenching constants obtained with a negatively charged quencher, iodide ion, in conjunction with a positively charged quencher, N-methyl-4-picolinium ion, indicate that the tryptophan residues of the B protomer are exposed to solvent and quencher.Fluorescence quenching by iodide ion of cholera toxin and its B protomer, but not the A protomer, is almost completely prevented or reversed upon the addition of G M ~ or OligO-GMl.The magnitude of the calculated quenching constants is in agreement with a collisional quenching process.Complex formation with GM, or OligO-GM1 does not, however, completely reduce the quenching by N-methyl-l-picolinium ion.The different effects of complex formation on the quenching efficiency of positively and negatively charged quenchers are interpreted to indicate that the single tryptophan residue at position 88 in the B protomer (i) is located in a positively charged microenvironment and (ii) loses its solvent exposure upon the interaction between B protomer and G M ~ receptor.GMl can cause a 12-nm blue shift in the tryptophan emission spectrum of 5-dimethylamino-1-naphthalene sulfonate (dansyl) derivatives of cholera toxin in the absence of any shift in the emission spectrum of the dansyl fluorophore.The B protomer of cholera toxin, upon interacting with a 5-dimethylamino-1-naphthalene sulfonate derivative of the oligosaccharide moiety Of G M ~, exhibits a fluorescence emission spectrum when excited at 282 nm compatible with an energy transfer between the single tryptophan residue of each constituent B protomer polypeptide chain and the dansyl group on the oligosaccharide.These data suggest that the tryptophan residue is very near the oligosaccharide moiety of G M ~ and that the two may even directly interact. ~~ ~Cholera toxin (M, -84,000) is composed of A and B protomers (M, -29,000 and -55,000, respectively) (1-5).The B protomer contains five identical polypeptide chains ( M , -11,600) arranged in a noncovalently associated, ring-like, pentameric configuration (1-5).The A protomer is composed