Studies on the Role of a Nucleoside‐Phosphate‐Binding Site of Diphtheria Toxin in the Binding of Toxin to Vero Cells or Liposomes

Using Fab antibody fragments directed against protein crm 45 (non‐toxic cross‐reacting material serologi‐cally‐related to diphtheria toxin mutant protein, the product of a non‐sens mutation in the diphtheria toxin gene) or the B 45 domain of this molecule (the remaining portion of the toxin B fragment present in protein (crm 45), we have shown that the diphtheria toxin polyphosphate‐binding site (P site) [Lory and Collier (1980) Proc. Nut1 Acad. Sci. USA , 1, 267–271], evidenced by the ability of the toxin to bind ATP, is affected upon incubation of this protein with the Fab fragment of anti‐protein (crm 45) IgG but not with the Fab fragment of anti‐polypeptide (B 45) IgG. After incubation with an excess of the Fab fragment of anti‐polypeptide (B 45) IgG, 125 I‐labeled toxin does not bind to toxin‐sensitive Vero cells. We therefore suggest that the P site is not by itself the binding domain of diphtheria toxin to the cell membrane receptor and that the region of the toxin molecule devoted to this particular activity overlapped probably on the B 45 polypeptide. In contrast with the results obtained by Alving et al. ( Proc. Nat1 Acad. Sci. USA , 1980, 4, 1986–1990) with or without a functional P site, diphtheria toxin is able to bind liposomes. However interaction with phospholipid vesicles with toxin is blocked upon incubation of the protein with the Fab fragment of anti‐polypeptide (B 45) IgG. Our data indicate that the P site of toxin does not participate in the ability of this molecule to bind liposomes.