Low pH induces a hydrophobic domain in the tetanus toxin molecule.

Binding of the non-ionic detergent [3H]Triton X-100 by tetanus toxin, by its fragment C and by its alpha chain has been studied. At pH 4.00 or above, tetanus toxin does not bind Triton X-100. At pH lower than 4.00, binding of detergent to the toxin occurs. At pH 3.00, a maximum of 100 mol bound/mol of protein is reached only when the detergent concentration exceeds its critical micelle concentration. No measurable amount of Triton X-100 is bound by the toxin C fragment at pH 3.00. Most of the tetanus toxin alpha chain precipitates out in Triton X-100 at pH 3.00. Leakage of K+ from single-walled asolectin vesicles loaded with potassium was observed with tetanus toxin at pH lower than 4.00. When ganglioside GDlb was present on the asolectin vesicles, release of K+ was obtained with tetanus toxin between pH 4.00 to 5.00. We suggest that, as for diphtheria toxin, entry of tetanus toxin into an acidic compartment of target cells might be required for the expression of its biological activity.