The heavy chain of tetanus toxin can mediate the entry of cytotoxic gelonin into intact cells.

An artificial conjugate of the heavy chain of tetanus toxin linked by a disulphide bond to the impermeant ribosome-inactivating protein gelonin is cytotoxic to intact HT29 cells by inhibiting intracellular protein synthesis. Neither toxin nor gelonin alone has any significant effect. This shows that the heavy chain has the ability to mediate internalization of a protein to which it is bound by a disulphide bond. Thus the normal role of the tetanus toxin heavy chain may be to allow entry of the light chain into a cell.