Interaction between tetanolysin and Mycoplasma cell membrane

Inactivation and Activity of Cholesterol-Dependent Cytolysins: What Structural Studies Tell Us

The homologous bacterially expressed cholesterol-dependent cytolysins (CDCs) form pores via oligomerization; this must occur preferentially once the target membrane has been engaged. Conformational changes in CDCs then drive partition from an aqueous environment to a lipidic one. This review addresses how premature oligomerization is prevented, how conformational changes are triggered, and how cooperativity between subunits brings about new functionality absent from isolated protomers. Variations are found in the answers provided by the CDCs to these issues. Some toxins use pH as a trigger of activity, but recent results have shown that dimerization in solution is an alternative way of preventing premature oligomerization, in particular for the CDC from Clostridium perfringens, perfringolysin. More controversially, there is still no resolution to the debate as to whether incomplete (arciform) oligomers form pores: recent results again suggest that they do.

Self-interaction of pneumolysin, the pore-forming protein toxin of Streptococcus pneumoniae

The pathogenically important cholesterol-binding pore-forming bacterial "thiol-activated" toxins (TATs) are commonly believed to be monomeric in solution and to undergo a transition on membrane binding mediated by cholesterol to an oligomeric pore. We present evidence, gained through the application of a number of biochemical and biophysical techniques with associated modelling, that the TAT from Streptococcus pneumoniae, pneumolysin, is in fact able to self-associate in solution to form the same oligomeric structures. The weak interaction leading to solution oligomerization is manifested at low concentrations in a dimeric toxin form. The inhibition of toxin self-interaction by derivatization of the single cysteine residue in pneumolysin with the thiol-active agent dithio (bis)nitrobenzoic acid indicates that self-interaction is mediated by the fourth domain of the protein, which has a fold similar to other proteins known to self-associate. This interaction is thought to have implications for the understanding of mechanisms of pore formation and complement activation by pneumolysin.

Transmembrane diffusion channels in Mycoplasma gallisepticum induced by tetanolysin

The permeability properties of Mycoplasma gallisepticum cells treated with a purified preparation of tetanolysin were investigated by determining the initial swelling rates of cells suspended in an isoosmotic solution of electrolytes or nonelectrolytes. The swelling, initiated by the tetanolysin, depended on the tetanolysin concentration and was markedly affected by the molecular size of the various osmotic stabilizers utilized. Thus, the initial swelling rates in an isoosmotic solution of monosaccharides were much higher than those in isoosmotic solutions of di-, tri-, or tetrasaccharides. Cell swelling induced by tetanolysin was much lower with energy-depleted M. gallisepticum cells, with arsenate-treated cells, or when the membrane potential (delta psi) was collapsed by valinomycin (10 microM) plus KCl (100 mM). Swelling was not affected by the proton-conducting ionophore carbonyl cyanide-m-chlorophenylhydrazone (1 to 10 microM) or by nigericin (5 microM). These results support the concept that the damage induced by tetanolysin is due to the formation of water-filled pores within the membranes of energized M. gallisepticum cells. Such pores allow the diffusion of hydrophilic molecules into the cells and may vary in size, depending on the tetanolysin concentration utilized.

Mechanism of tetanolysin-induced membrane damage: studies with black lipid membranes

Tetanolysin produced similar rates of leakage of K+ and hemoglobin from erythrocytes. When studied by using cholesterol-containing black lipid membranes, this hemolysin induced conductance steps with a broad frequency distribution. These findings are inconsistent with the formation of structural channels and suggest that tetanolysin acts by causing lipid perturbations.

Structural characteristics of tetanolysin and its binding to lipid vesicles

Tetanolysin binding to lipid vesicles was found to depend on the molar ratio of cholesterol to phospholipid, being low in vesicles containing up to 20 mol% cholesterol and high in vesicles containing more than 33 mol%. High concentrations of purified tetanolysin preparations formed arc- and ring-shaped structures. The structures were not readily detectable in diluted preparations unless incubated with lipid vesicles containing high molar ratios of cholesterol to phospholipid. It is suggested that the toxin is concentrated on the vesicles to local concentrations high enough to form the arcs and rings.

Cholesterol-dependent tetanolysin damage to liposomes

Tetanolysin caused membrane damage, resulting in release of trapped glucose from liposomes containing cholesterol. Maximum glucose release occurred from liposomes that contained 50 mol% cholesterol. At higher or lower levels of cholesterol, glucose release was reduced and glucose release did not occur at all below 40 mol% cholesterol. The apparent activity of tetanolysin was not influenced by temperature (24 degrees C compared to 32 degrees C) or by liposomal phospholipid fatty acyl chain length. We conclude that tetanolysin caused cholesterol-dependent lysin-mediated damage to liposomes, possibly by means of a pore consisting of a complex of toxin and cholesterol.

Increased resistance to streptolysin O in mammalian cells treated with oxygenated derivatives of cholesterol

L-cell fibroblast cultures were treated with certain oxygenated derivatives of cholesterol which are known to inhibit cholesterol biosynthesis in mammalian cells. After incubation in the presence of 20-alpha-hydroxycholesterol or 25-hydroxycholesterol for 18 h, the cells became increasingly resistant to streptolysin O. Maximum resistance to toxin was obtained by incubation for 48 h in 0.5 microgram of 20-alpha-hydroxycholesterol or 0.25 microgram of 25-hydroxycholesterol per ml; under these conditions, the cells were 10 to 50 times more resistant than were untreated controls. The ability of the hydroxycholesterol compounds to render the cells resistant was related to the age of the cultures. Maximum protection was found when more sparsely populated cultures were treated with 25-hydroxycholesterol. Older, heavily populated cultures could not be protected even with the high concentrations of 25-hydroxycholesterol. In contrast to control cultures, most of the toxin activity remained in the medium after being incubated with hydroxycholesterol-treated cultures. The results indicate that less toxin binds to the resistant cells and suggest that a reduction in membrane cholesterol content may account for resistance to streptolysin O.

Effect on sphingomyelin-containing liposomes of phospholipase D from Corynebacterium ovis and the cytolysin from Stoichactis helianthus

The toxic, sphingomyelin-specific phospholipase D (phosphatidylcholine phosphatidohydrolase EC 3.1.4.4) from Corynebacterium ovis was purified to near homogeneity. It has a molecular weight of 31 000 and a pI of approx. 9.8. Although not cytolytic itself, it protected red cells from hemolysis by staphylococcal sphingomyelinase (beta-hemolysin) and helianthus toxin. The apparently non-enzymatic cytolysin (helianthus toxin) from the sea anemone Stoichactis helianthus also interacts with membrane sphingomyelin. C. ovis and helianthus toxins were compared with regard to their effects on liposome model membranes, and they were found both to produce changes analogous to those in erythrocytes. Only helianthus toxin caused release of trapped glucose marker, but liposomes could be protected from release by pretreatment with C. ovis toxin. Both toxins demonstrated binding to sphingomyelin-containing liposomes, but only the bacterial sphingomyelinase catalyzed the release of choline from these vesicles.

Alteration by cereolysin of the structure of cholesterol-containing membranes

When erythrocyte membranes were treated with cereolysin, negatively stained and examined by electron microscopy, ring and arc-shaped structures were observed in the membrane. The outside diameter of the rings varied from 33 to 50 nm with a border thickness of 6.7 to 8.3 nm. The arcs varied in length from 33 to 170 nm with a border thickness of also 6.7 to 8.3 min. When right-side-out erythrocyte ghosts which had been treated with cereolysin were examined by electron microscopy after freeze-fracture, structures with a diameter of 31 to 63 nm were seen in the fracture face of the exoplasmic half of the membrane, but no alterations were visible in the fracture face of the protoplasmic half of the membrane bilayer. Thus the ring structures did not appear to form holes through the membrane. At cereolysin concentrations above 6 microgram/ml rings and arcs were seen when purified toxin alone was examined. At or below 6 microgram/ml toxin rings and arcs were seen only if toxin was incubated with free or membrane-bound cholesterol. Our interpretation is that cereolysin tends to aggregate into ring and arc-shaped structures, and that the tendency to aggregate is increased by cholesterol. Rings and arcs were not seen when erythrocyte ghosts were treated with low, but lytic amounts of cereolysin that significantly altered the premeability of the ghosts.

Digitonin induced alterations of the erythrocyte membrane as visible by freeze-fracturing

Treatment of erythrocytes by low concentrations of digitonin results in the formation of elongated, bulged membrane areas free from particles and intramembraneous tubular structures emerged from these domains. At higher concentrations the tubular structures are present also outside of the membrane. A second, not bulged type of particle-free areas is more or less rounded, but mostly of rectangular shape. The rate of this type of domains increases with the concentration of digitonin. Breaks in these areas lead to a formation of sheets which results in a breakdown of the membrane structure finally into a brittle mass of many small sheets lying irregularly one upon the other. Already during the first steps of this membrane breakdown hemolysis takes place. Cross-linking of the proteins with glutaraldehyde does not entirely block up the membrane alterations. In fixed erythrocytes the formation fo elongated domains is restricted, no tubular structures are present and no dislocations of particles can be observed. Nevertheless smooth sheets localized also outside of the membrane are formed. However the investigated ghosts are not stabilized by glutaraldehyde against the effects of digitonin. Particle dislocations as well as all the other membrane alterations are present. The implications of the obtained results are: 1. The elongated domains and the tubular structures are probably not digitonin-cholesterol-complexes. 2. The formation of crystalline regions by digitonin-cholesterol-complexes destroys the membrane structure.