The Clostridium neurotoxin family is composed of tetanus neurotoxin and seven serotypes of botulinum neurotoxin. The structure of the botulinum neurotoxin reveals a four domain protein. The N-terminal catalytic domain (Pfam:PF01742), the central translocation domains and two receptor binding domains [1]. Subsequent to cell surface binding and receptor mediated endocytosis of the neurotoxin, an acid induced conformational change in the neurotoxin translocation domain is believed to allow the domain to penetrate the endosome and from a pore, thereby facilitating the passage of the catalytic domain across the membrane into the cytosol [1]. The structure of the translocation reveals a pair of helices that are 105 Angstroms long and is structurally distinct from other pore forming toxins [1].

1: Crystal structure of botulinum neurotoxin type A and implications for toxicity. Lacy DB, Tepp W, Cohen AC, DasGupta BR, Stevens RC; Nat Struct Biol 1998;5:898-902. PMID:9783750