Interaction of diphtheria toxin with phosphorylated molecules

Specific cleavage of diphtheria toxin by human urokinase

Diphtheria toxin must undergo a specific cleavage reaction and subsequent reduction to express the enzymatic ADP-ribosyltransferase activity that is responsible for its toxicity. In an effort to identify potential cellular enzymes that might be involved in this process we have found that a human urinary plasminogen activator, urokinase, is capable of specifically cleaving diphtheria toxin to yield an enzymatically active A fragment (more homogeneous than that produced by trypsin cleavage) and a B fragment (with an identical amino-terminal sequence to that produced by trypsin cleavage). The results raise the possibility that urokinase or urokinase-like enzymes play a role in diphtheria toxin-mediated intoxication.

Identification of diphtheria toxin receptor and a nonproteinous diphtheria toxin-binding molecule in Vero cell membrane

Two substances possessing the ability to bind to diphtheria toxin (DT) were found to be present in a membrane fraction from DT-sensitive Vero cells. One of these substances was found on the basis of its ability to bind DT and inhibit its cytotoxic effect. This inhibitory substance competitively inhibited the binding of DT to Vero cells. However this inhibitor could not bind to CRM197, the product of a missense mutation in the DT gene, and did not inhibit the binding of CRM197 to Vero cells. Moreover, similar levels of the inhibitory activity were observed in membrane fractions from DT-insensitive mouse cells, suggesting the inhibitor is not the DT receptor which is specifically present in DT-sensitive cells. The second DT-binding substance was found in the same Vero cell membrane preparation by assaying the binding of 125I-labeled CRM197. Such DT-binding activity could not be observed in membrane preparation from mouse L cells. From competition studies using labeled DT and CRM proteins, we conclude that this binding activity is due to the surface receptor for DT. Treatment of these substances with several enzymes revealed that the inhibitor was sensitive to certain RNases but resistant to proteases, whereas the DT receptor was resistant to RNase but sensitive to proteases. The receptor was solubilized and partially purified by chromatography on CM-Sepharose column. Immunoprecipitation and Western blotting analysis of the partially purified receptor revealed that a 14.5-kD protein is the DT receptor, or at least a component of it.

Evidence that membrane phospholipids and protein are required for binding of diphtheria toxin in Vero cells

Treatment with phospholipase C strongly protected monkey kidney (Vero) cells against diphtheria toxin and reduced the ability of the cells to bind 125I-labelled toxin. Treatment with phospholipase D and with trypsin also protected the cells, although to a lesser extent. Phospholipase A2 had no protective effect. Phospholipase C also protected fetal hamster kidney cells against the toxin. After removal of the enzymes, as well as after treatment of the cells with 4-acetamide 4'-isothiocyanostilbene 2,2'-disulfonic acid, diphtheria toxin binding capability was restored slowly, apparently by a process requiring protein synthesis, since cycloheximide blocked the restoration. The data indicate that both phospholipids and protein are involved in the binding sites for diphtheria toxin.

Role of glycosylation in expression of functional diphtheria toxin receptors

We have previously demonstrated, by using a detergent-solubilized system, the existence of specific diphtheria toxin-binding glycoproteins on the surface of toxin-sensitive cells. We have now tested the effect of tunicamycin treatment on the sensitivity of cells in culture to diphtheria toxin and have investigated the toxin sensitivity of mutant cells with known defects in glycosylation of asparagine-linked glycoproteins. Treatment of CHO-K1 cells with tunicamycin, which blocks the synthesis of both high-mannose-type and complex-type oligosaccharide chains of asparagine-linked glycoproteins, resulted in a 50- to 100-fold decrease in sensitivity to diphtheria toxin. In contrast, CHO-K1 mutants, defective in the synthesis of either high-mannose-type or complex-type oligosaccharides, showed no difference in toxin sensitivity compared with that of their parental cell lines. When we used an acid shock system, which is believed to result in receptor-dependent direct toxin penetration at the cell surface, the toxin sensitivity of tunicamycin-treated cells was not restored to that of untreated cells, suggesting that tunicamycin treatment results in a decrease in functional toxin receptors. Direct binding studies with 125I-labeled toxin demonstrated that this decrease in functional receptors is due to a decrease in the affinity of the receptors rather than to a change in the number of receptors. Taken together, these data are consistent with the interpretation that the diphtheria toxin receptor is a glycoprotein and suggest that the toxin binds neither to carbohydrate residues unique to the high-mannose-type oligosaccharides nor to those unique to the complex-type oligosaccharides. Furthermore, these data are consistent with the hypothesis that diphtheria toxin binds to the peptide backbone of the glycoprotein receptor.

Polyphosphate-mediated protection from cellular intoxication with Clostridium difficile toxin B

The influence of polyphosphorylated compounds on intoxication of human lung fibroblasts with Clostridium difficile toxin B was studied. ATP, as well as other nucleoside di-, tri-, and tetraphosphates, inorganic polyphosphates and polyphosphorylated sugars, caused a dose-dependent (1-5 mM range) delay in the appearance of the cytopathogenic effect. With a longer phosphate chain, the delay was more pronounced, although the cytopathogenic effect always developed finally, reaching the level of the control within 20 h. Toxin preparations contained one fraction of molecules able to bind ATP, besides one non-binding fraction. The protective effect of ATP did not depend on its energy producing ability. Neither was the protective effect due to an inactivation of the toxin per se, or to an interference with binding of the toxin to the cells. ATP was protective even upon addition 10 min after the toxin binding step. In the presence of ATP, the toxin remained accessible to neutralization with antitoxin. In analogy with the P-site on diphtheria toxin, we postulate that C. difficile toxin B contains a polyphosphate-binding site. This site is separate from the receptor-binding site, but involved in the interaction of toxin B with the cell surface shortly after the binding step.

Effect of polymers of L-lysine on the cytotoxic action of diphtheria toxin

L-Lysine and poly(L-lysine) polymers of defined sizes were tested for their effect on diphtheria toxin-mediated cytotoxicity of Chinese hamster ovary cells. Poly(L-lysine)8-12 and poly(L-lysine)15 were protective, whereas lysine and lysine polymers of three and four residues were not. The protection by poly(L-lysine)8-12 occurred by affecting the initial toxin receptor interaction. Furthermore, poly(L-lysine)8-12 acted as a competitive inhibitor with an apparent dissociation constant of 6 X 10(-7) M. These observations are consistent with a previously proposed model (Proia et al., J. Biol. Chem. 256:4991-4997, 1981) in which (i) the cationic polyphosphate-binding site (P site) on the B fragment of diphtheria toxin interacts with a putative polyanionic binding site on the toxin receptor, and (ii) the protection of cells from diphtheria toxin by poly(L-lysine) and other polycationic molecules would be the result of the competition between these polycationic molecules and the cationic domain of the diphtheria toxin molecule for the polyanionic toxin-binding site on the cell surface receptor.

Binding of diphtheria toxin to phospholipids in liposomes

Diphtheria toxin bound to the phosphate portion of some, but not all, phospholipids in liposomes. Liposomes consisting of dimyristoyl phosphatidylcholine and cholesterol did not bind toxin. Addition of 20 mol% (compared to dimyristoyl phosphatidylcholine) of dipalmitoyl phosphatidic acid, dicetyl phosphate, phosphatidylinositol phosphate, cardiolipin, or phosphatidylserine in the liposomes resulted in substantial binding of toxin. Inclusion of phosphatidylinositol in dimyristol phosphatidylcholine/cholesterol liposomes did not result in toxin binding. The calcium salt of dipalmitoyl phosphatidic acid was more effective than the sodium salt, and the highest level of binding occurred with liposomes consisting only of dipalmitoyl phosphatidic acid (calcium salt) and cholesterol. Binding of toxin to liposomes was dependent on pH, and the pattern of pH dependence varied with liposomes having different compositions. Incubation of diphtheria toxin with liposomes containing dicetyl phosphate resulted in maximal binding at pH 3.6, whereas binding to liposomes containing phosphatidylinositol phosphate was maximal above pH 7. Toxin did not bind to liposomes containing 20 mol% of a free fatty acid (palmitic acid) or a sulfated lipid (3-sulfogalactosylceramide). Toxin binding to dicetyl phosphate or phosphatidylinositol phosphate was inhibited by UTP, ATP, phosphocholine, or p-nitrophenyl phosphate, but not by uracil. We conclude that (a) diphtheria toxin binds specifically to the phosphate portion of certain phospholipids, (b) binding to phospholipids in liposomes is dependent on pH, but is not due only to electrostatic interaction, and (c) binding may be strongly influenced by the composition of adjacent phospholipids that do not bind toxin. We propose that a minor membrane phospholipid (such as phosphatidylinositol phosphate or phosphatidic acid), or that some other phosphorylated membrane molecule (such as a phosphoprotein) may be important in the initial binding of diphtheria toxin to cells.