The C terminus of component C2II of Clostridium botulinum C2 toxin is essential for receptor binding

The binary Clostridium botulinum C2 toxin consists of two separate proteins, the binding component C2II (80.5 kDa) and the actin-ADP-ribosylating enzyme component C2I (49.4 kDa). For its cytotoxic action, C2II binds to a cell membrane receptor and induces cell entry of C2I via receptor-mediated endocytosis. Here we studied the structure-function relationship of C2II by constructing truncated C2II proteins and producing polyclonal antisera against selective regions of C2II. An antibody raised against the C terminus (amino acids 592 to 721) of C2II inhibited binding of C2II to cells. The antibody prevented pore formation by C2II oligomers in artificial membranes but did not influence the properties of existing channels. To further define the region responsible for receptor binding, we constructed proteins with deletions in C2II; specifically, they lacked amino acid residues 592 to 721 and the 7 C-terminal amino acid residues. The truncated proteins still formed sodium dodecyl sulfate-stable oligomers but were unable to bind to cells. Our data indicate that the C terminus of C2II mediates binding of the protein to cells and that the 7 C-terminal amino acids are structurally important for receptor binding.