Fluorescence characterization of the low pH-induced change in diphtheria toxin conformation: effect of salt.

We have examined the effect of pH on diphtheria toxin conformation using intrinsic protein fluorescence and a new fluorescence quenching method. In aqueous solutions, fluorescence indicates toxin conformation undergoes a drastic change at low pH. This conformational change is closely associated with a switch from a hydrophilic conformation to a hydrophobic one, as judged by quenching-detected detergent binding. In the absence of NaC1 these changes occur around pH 4-4.5. However, in 150 mM NaCl the conformational change occurs in the pH 5-5.5 range, close to the pH the toxin is expected to encounter in endosomes and lysosomes. Therefore, the conformational change observed at low pH is likely to be physiologically significant.