Effect of ammonium chloride on receptor-mediated uptake of diphtheria toxin by Vero cells

Membrane assembly of the cholesterol-dependent cytolysin pore complex

The cholesterol-dependent cytolysins (CDCs) are a large family of pore-forming toxins that are produced, secreted and contribute to the pathogenesis of many species of Gram-positive bacteria. The assembly of the CDC pore-forming complex has been under intense study for the past 20 years. These studies have revealed a molecular mechanism of pore formation that exhibits many novel features. The CDCs form large β-barrel pore complexes that are assembled from 35 to 40 soluble CDC monomers. Pore formation is dependent on the presence of membrane cholesterol, which functions as the receptor for most CDCs. Cholesterol binding initiates significant secondary and tertiary structural changes in the monomers, which lead to the assembly of a large membrane embedded β-barrel pore complex. This review will focus on the molecular mechanism of assembly of the CDC membrane pore complex and how these studies have led to insights into the mechanism of pore formation for other pore-forming proteins. This article is part of a Special Issue entitled: Protein Folding in Membranes.

Ion-conducting channels produced by botulinum toxin in planar lipid membranes

The interaction of botulinum neurotoxin (Botx) with planar lipid membranes was studied by measuring the ability of the toxin to form ion-conducting channels. Channel formation was pH dependent. At physiological pH, Botx formed no channels, whereas at pH 6.6, the toxin formed channels with a unit conductance of 12 pS in 0.1 M NaCl. The rate of channel formation increased with decreasing pH, reaching a maximum at pH 6.1, and then decreased at lower values of pH. The channels, once formed, were permanent entities in the membrane throughout the course of an experiment and fluctuated between an open and a closed state. The rate of channel formation depended upon the square of the toxin concentration, suggesting an aggregation step is involved in channel formation. The data were consistent with the hypothesis that Botx enters cells through endocytosis, followed by its release into the cytoplasm at low pH.

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.

Binding and uptake of diphtheria toxin by toxin-resistant Chinese hamster ovary and mouse cells

We investigated two phenotypically distinct types of diphtheria toxin-resistant mutants of Chinese hamster cells and compared their resistance with that of naturally resistant mouse cells. All are resistant due to a defect in the process of internalization and delivery of toxin to its target in the cytosol, elongation factor 2. By cell hybridization studies, analysis of cross-resistance, and determination of specific binding sites for 125I-labeled diphtheria toxin, we showed that these cell strains fall into two distinct complementation groups. The Dipr group encompasses Chinese hamster strains that are resistant only to diphtheria toxin, as well as mouse LM cells. These strains possess a normal complement of high-affinity binding sites for diphtheria toxin, but these receptors are unable to deliver active toxin fragment A to the cytosol. Cells of the DPVr group have a broader spectrum of resistance, including Pseudomonas exotoxin A and several enveloped viruses as well as diphtheria toxin. In these studies, which investigate the resistance of these cells to diphtheria toxin, we demonstrate that they possess a reduced number of specific binding sites for this toxin and behave, phenotypically, like cells treated with the proton ionophore monensin. Their resistance is related to a defect in a mechanism required for release of active toxin from the endocytic vesicle.

Strains of CHO-K1 cells resistant to Pseudomonas exotoxin A and cross-resistant to diphtheria toxin and viruses

We have investigated two phenotypically distinct types of mutants of CHO-K1 cells that are resistant to Pseudomonas exotoxin A due to a defect in the delivery of active toxin to the target site in the cell, elongation factor 2. Both types contain normal levels of toxin-sensitive elongation factor-2. Hybridization studies have shown that these cells fall into two distinct complementation groups. One group, designated DPVr, is resistant to Pseudomonas toxin, diphtheria toxin, and four enveloped RNA viruses. This group is also hypersensitive to ricin. The resistance of this group is apparently related to a defect in a mechanism for the acidification of endocytic vesicles. The other group, designated PVr, is resistant to Pseudomonas toxin and to three enveloped RNA viruses. The resistance of this group appears to be related to a defect in a cellular mechanism required for the maturation of Sindbis virus that is likewise required for the entry of active Pseudomonas toxin.

Binding and uptake of diphtheria toxin by toxin-resistant Chinese hamster ovary and mouse cells

We investigated two phenotypically distinct types of diphtheria toxin-resistant mutants of Chinese hamster cells and compared their resistance with that of naturally resistant mouse cells. All are resistant due to a defect in the process of internalization and delivery of toxin to its target in the cytosol, elongation factor 2. By cell hybridization studies, analysis of cross-resistance, and determination of specific binding sites for 125I-labeled diphtheria toxin, we showed that these cell strains fall into two distinct complementation groups. The Dipr group encompasses Chinese hamster strains that are resistant only to diphtheria toxin, as well as mouse LM cells. These strains possess a normal complement of high-affinity binding sites for diphtheria toxin, but these receptors are unable to deliver active toxin fragment A to the cytosol. Cells of the DPVr group have a broader spectrum of resistance, including Pseudomonas exotoxin A and several enveloped viruses as well as diphtheria toxin. In these studies, which investigate the resistance of these cells to diphtheria toxin, we demonstrate that they possess a reduced number of specific binding sites for this toxin and behave, phenotypically, like cells treated with the proton ionophore monensin. Their resistance is related to a defect in a mechanism required for release of active toxin from the endocytic vesicle.