Synthetic peptides with amino acid sequences corresponding to predicted transmembrane segments of tetanus toxin were used as probes to identify a channel-forming motif. A peptide denoted TeTx II, with sequence GVVLLLEYIPEITLPVIAALSIA, forms cation-selective channels when reconstituted in planar lipid bilayers. The single channel conductance in 0.5 M NaCl or KCl is 28 ± 3 and 24 ± 2 pS, respectively. In contrast, a peptide with sequence NFIGALETTGVVLLLEYIPEIT, denoted as TeTx I. or a peptide with the same amino acid composition as TeTx II but with a randomized sequence, do not form channels. Conformational energy calculations show that a bundle of four amphipathic α-helices is a plausible structural motif underlying observable pore properties. The identified functional module may account for the channel-forming activity of both tetanus toxin and the homologous botulinum toxin A.
