Tetanus toxin fragment forms channels in lipid vesicles at low pH.

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Abstract
Single-walled asolectin vesicles [soybean] loaded with K+ at pH 7.00 released their K+ content upon incubation with tetanus toxin fragment B but only when the incubation was at pH below 5.00. Whole tetanus toxin exhibited only a weak releasing activity. Toxin light .alpha. chain and the carboxyl-terminal 48,000-dalton moiety of the heavy chain (fragment IIC) were unable to provoke K+ release from vesicles at any pH. K+ release from lipid vesicles could also be detected with tetanus toxin heavy .beta. chain at low pH. Using a detergent binding assay ([3H]Triton X-100), hydrophobic domain, localized in the 50,000-dalton terminal polypeptide of tetanus toxin heavy chain, was shown to be detectable at pH 3.60 but not at pH 5.00. Evidently, the ability of tetanus toxin fragment B to release K+ from asolectin vesicles at low pH is due to the 50,000-dalton amino-terminal polypeptide of the heavy chain present in toxin fragment B. This phenomenon may be caused by channel formation across the vesicle membrane as has been observed for the 23,000-dalton amino-terminal moiety of diphtheria toxin fragment B.