Identification of a ganglioside recognition domain of tetanus toxin using a novel ganglioside photoaffinity ligand

Tetanus toxin entry into vertebrate motorneurons may involve binding of neuronal surface gangliosides containing the ''1b'' substructure (a NeuAc alpha 2,8NeuAc group on an internal galactose residue), The domains of tetanus toxin involved in ganglioside binding are known to reside within the carboxyl-terminal half of the toxin's heavy chain (''C fragment''). We developed a navel photoaffinity reagent based upon the structure of the 1b ganglioside G(D1b) (I-125-azido-G(D1b)) to define the ganglioside-binding domains of tetanus toxin. Using this ligand, we observed radiolabeling of tetanus toxin C fragment which could be specifically inhibited by a ganglioside of the 1b series (G(T1b)) but not by a non-1b series ganglioside (G(M3)). When tetanus toxin C fragment was proteolyzed with clostripain, whether before or after reaction with I-125-azido-G(D1b), a radiolabelled band was observed by SDS-polyacrylamide gel electrophoresis autoradiography, which was selectively inhibited by G(T1b) Protein sequencing of proteolyzed tetanus toxin C fragment comigrating with that band revealed the carboxyl-terminal 34 amino acid residues of tetanus toxin. Matrix-assisted laser desorption/ionization mass spectrometry of a photoaffinity labeled synthetic polypeptide representing the 34-amino acid domain revealed modification at a single residue (His(1293)). We propose that this domain of tetanus toxin is sufficient for ganglioside binding.