Comparative effects of cholesterol and thiocholesterol on streptolysin O

The cholesterol-dependent cytolysins

In view of the recent studies on the CDCs, a reasonable schematic of the stages leading to membrane insertion of the CDCs can be assembled. As shown in Fig. 3, we propose that the CDC first binds to the membrane as a monomer. These monomers then diffuse laterally on the membrane surface to encounter other monomers or incomplete oligomeric complexes. Presumably, once the requisite oligomer size is reached, the prepore complex is converted into the pore complex and a large membrane channel is formed. During the conversion of the prepore complex to the pore complex, we predict that the TMHs of the subunits in the prepore complex insert into the bilayer in a concerted fashion to form the large transmembrane beta-barrel, although this still remains to be confirmed experimentally. Many intriguing problems concerning the cytolytic mechanism of the CDCs remain unsolved. The nature of the initial interaction of the CDC monomer with the membrane is currently one of the most controversial questions concerning the CDC mechanism. Is cholesterol involved in this interaction, as previously assumed, or do specific receptors exist for these toxins that remain to be discovered? Also, the trigger for membrane insertion and the regions of these toxins that facilitate the [figure: see text] interaction of the monomers during prepore complex formation are unknown. In addition, the temporal sequence of the multiple structural changes that accompany the conversion of the soluble CDC monomer into a membrane-inserted oligomer have yet to be defined or characterized kinetically.

Cholesterol-binding cytolytic protein toxins

Cholesterol-binding cytolysins (CBCs) are a large family of 50- to 60-kDa single-chain proteins produced by 23 taxonomically different species of Gram-positive bacteria from the genera Streptococcus, Bacillus, Clostridium, Listeria and Arcanobacterium. Apart pneumolysin, which is an intracytoplasmic toxin, all the other toxins are secreted in the extracellular medium. Among the species producing CBCs, only L. monocytogenes and L. ivanovii are intracellular pathogens which grow and release their toxins in the phagocytic cells of the host. CBCs are lethal to animals and highly lytic toward eukaryotic cells, including erythrocytes. Their lytic and lethal properties are suppressed by sulfhydryl-group-blocking agents and reversibly restored by thiols or other reducing agents. These properties are irreversibly abrogated by very low concentrations of cholesterol and other 3beta-hydroxysterols. Membrane cholesterol is thought to be the toxin-binding site at the surface of eukaryotic cells. Toxins molecules bind as monomers to the membrane surface with subsequent oligomerization into arc-and ring-shaped structures surrounding large pores generated by this process. Thirteen structural genes of the toxins (all chromosomal) have been cloned and sequenced to date. The deduced primary structure of the proteins shows obvious sequence homology particularly in the C-terminal part and a characteristic common consensus sequence containing a unique Cys residue (ECTGLAWEWWR) near the C-terminus of the molecules (except pyolysin and intermedilysin). However, another Cys residue outside this undecapeptide and closer to the C-terminus occurs in ivanolysin. Genetic replacement of the Cys residue in the consensus undecapeptide by certain amino acids demonstrated that this residue was not essential for toxin function. Other residues in the undecapeptide have been mutagenized, particularly the Trp residues. One of these Trp appeared critical for lytic activity. The recent elucidation of the 3-D structure of perfringolysin O provided interesting information on the structure-activity relationship. The molecule was divided into four domains. Three domains are arranged in a row, giving an elongated shape. Domain 3 is covalently connected to the N-terminal domain 1 and packed laterally against domain 2. Membrane interaction of the monomer appears to be mediated by domain 4, while, oligomerization involves several sites scattered throughout the sequence. The Trp-rich region around the conserved Cys residue within domain 4 is assumed to conformationally adapt to cholesterol, and domain 3 is envisaged to move across the "hinge" by which it is connected to domain 1.

Maintenance of motility in mouse sperm permeabilized with streptolysin O

One approach to studying the mechanisms governing sperm motility is to permeabilize sperm and examine the regulation of motility by manipulating the intracellular milieu of the cell. The most common method of sperm permeabilization, detergent treatment, has the disadvantage that the membranes and many proteins are extracted from the cell. To avoid this problem, we have developed a method that uses streptolysin O to create stable pores within the plasma membrane while leaving internal membranes intact. Sperm were permeabilized, preincubated, and then treated with 0.6 U/ml of streptolysin O. Permeabilization was assessed by fluorescent dye technologies and endogenous protein phosphorylation using exogenously added [gamma-32P]ATP. Streptolysin O-induced permeabilization rendered the sperm immotile, and the effect was Ca2+-dependent. When the cells were treated simultaneously with a medium containing ATP, streptolysin O-treated sperm maintained flagellar movement. These results demonstrate that the streptolysin O permeabilization model system is a useful experimental method for studying the mechanisms that regulate sperm motility since it allows the flagellar apparatus to be exposed to various exogenously added molecules.

The role of sterols in the functional reconstitution of water-soluble mitochondrial porins from plants

Water-soluble porins were prepared from native mitochondrial porins isolated from different plants (pea and corn). In the water-soluble form the porins have lost their channel-forming properties. The water-soluble porins were investigated for the influence of different sterols on their membrane activity and their channel-forming properties in lipid bilayer membranes. Our experiments demonstrated that the water-soluble porins regained channel forming activity when the protein was preincubated with different sterols in the presence of a detergent. The channels formed in lipid bilayer membranes after this procedure regain in many but not all cases the original properties of the native mitochondrial porins. Preincubation with other sterols led to a change in the single-channel conductance or to a complete loss of the voltage dependence. The sterols had also a strong influence on the channel-forming activity of the porins. Preincubation of water-soluble pea porin with the plant sterol beta-sitosterol resulted in a considerable higher channel-forming activity than with all the other sterols used for preincubation. The role of the sterols in the channel-forming complex is discussed.

Role of sterols in the functional reconstitution of water-soluble mitochondrial porins from different organisms

Experiments were performed on lipid bilayer membranes with water-soluble mitochondrial porins from different eukaryotic organisms, such as Dictyostelium discoideum, Paramecium, and rat liver, to study the requirements of functional reconstitution of the porins. The water-soluble porins lost their associated lipids and sterols and are unable to form channels in lipid bilayer membranes. We demonstrate that the water-soluble porins regain their channel-forming ability after preincubation of the polypeptides with sterols in the presence of detergents. Mitochondrial porin from Dictyostelium discoideum maintained after this procedure its original properties, in particular the voltage dependence. Water-soluble mitochondrial porins from Paramecium tetraurelia and from rat liver were also activated upon preincubation with different sterols in detergent but showed voltage-dependences that were different from those of detergent-purified porins. Furthermore, the voltage dependence depended on the sterol used for preincubation. Interestingly, the preincubation with sterols can likewise be used to activate detergent-purified mitochondrial porins that may have lost associated sterol during isolation and purification procedures.

Effect of lipidic factors on membrane cholesterol topology--mode of binding of theta-toxin to cholesterol in liposomes

We have previously suggested the existence of two distinct states for cholesterol in cell membranes as revealed by high- and low-affinity binding sites for theta-toxin of Clostridium perfringens. In liposomes, phospholipid and cholesterol compositions, but not membrane protein composition, have been shown to be major determinants for the topology of membrane cholesterol. The effects of lipidic factors on cholesterol topology were investigated in detail by analyzing toxin binding to large unilamellar liposomes composed of cholesterol and phospholipids (neutral phospholipids/phosphatidylglycerol = 82:18, mol/mol). The numbers of high- and low-affinity toxin-binding sites depend strictly on the cholesterol mole percentage in liposomes. High-affinity toxin-binding sites appear only in liposomes with high cholesterol contents. Liposomes whose cholesterol/phospholipid ratio is 0.4 or less have no high-affinity sites regardless of their phospholipid compositions, while low-affinity sites appear in liposomes with lower cholesterol contents. The threshold values for the cholesterol mole percentage above which high-affinity toxin-binding sites appear were examined. The values decrease in accordance with the increase in the mole fraction of 18-carbon hydrocarbon chains among the total 14-18 carbon-hydrocarbon chains of the liposomal phospholipids. Furthermore, both the partial replacement of phosphatidylcholine with phosphatidylethanolamine and the digestion of phospholipids with phospholipase C also affect the threshold values. Thus the cholesterol mole percentage, in combination with phospholipid chain length and other factors, determines the topology of membrane cholesterol providing distinctively different affinity sites for theta-toxin.

A rapid turbidimetric immunoassay for serum antistreptolysin-O

We designed a rapid, quantitative turbidimetric immunoassay for serum antistreptolysin-O (ASO), based on the immunoprecipitating velocity with purified streptolysin-O (SLO), which can be measured turbidimetrically at 340 nm. The assay system involves polyoxyethylene mono-p-nonylphenyl ether to accelerate and enhance the immunoprecipitation reaction. No antibody excess phenomenon is observed, up to 1,700 U/ml. Grossly icteric sera can be assayed. Patient sera gave acceptable results. Free cholesterol offers no interference. The standard curve is linear for values up to 800 U/ml. Results measured by the proposed method and those by the conventional microtitration method correlated well. Analytical recovery averaged 90.1% (80.3%-102.5%). Within-run and day-to-day CVs ranged from 2.1% to 3.5% and from 2.5% to 6.9%, respectively. Working SLO is stable for at least 15 days at 4 degrees C. The proposed method has proven to be superior to other available methods for measuring serum ASO.

Surface properties of bacterial sulfhydryl-activated cytolytic toxins. Interaction with monomolecular films of phosphatidylcholine and various sterols

Sulfhydryl-activated cytolysins are a group of bacterial protein toxins which, in the reduced state, lyse eukaryotic cells by disruption of the cytoplasmic membrane. Cell surface cholesterol is thought to be the target of the toxins. In the present work, the monolayer technique was used to investigate the interaction of four SH-activated toxins (streptolysin 0, alveolysin , perfringolysin 0, pneumolysin ) with various lipid films as a model for studying toxin-induced membrane disruption. A surface pressure increase up to very high values was elicited by reduced toxins (approximately equal to 10 nM) on films of cholesterol, other toxin-binding 3 beta-hydroxy-sterols, thiocholesterol and cholesterol-phosphatidylcholine mixtures suggesting deformation or penetration of the films. The surface-active potency of the toxins was of the same order as that of melittin and snake cardiotoxins at similar concentrations. No pressure increase was observed on films made of pure phosphatidylcholine, lanosterol and other sterols lacking the 3 beta-OH group. Optimal efficiency was at cholesterol/phosphatidylcholine molar ratio of 1 to 1. The critical pressures for toxin interaction with phosphatidylcholine and cholesterol monolayers were 25 mN X m-1 and 45 mN X m-1 respectively. Toxin interaction with phosphatidylcholine [14C]-cholesterol films did not modify monolayer radioactivity, indicating no cholesterol desorption. No pressure increase was elicited by toxins inactivated by SH-group reagents, heating or neutralization with antibody. Toxin effect was dependent temperature and pH. The overall potency of the four toxins tested was streptolysin 0 greater than alveolysin approximately equal to perfringolysin 0 greater than pneumolysin . The monolayer system mimicked in several respects toxin interaction with eukaryotic cells.