Ultracentrifugal analysis of staphylococcal alpha toxin

Alpha-toxin of Staphylococcus aureus

Alpha-toxin, the major cytotoxic agent elaborated by Staphylococcus aureus, was the first bacterial exotoxin to be identified as a pore former. The protein is secreted as a single-chain, water-soluble molecule of Mr 33,000. At low concentrations (less than 100 nM), the toxin binds to as yet unidentified, high-affinity acceptor sites that have been detected on a variety of cells including rabbit erythrocytes, human platelets, monocytes and endothelial cells. At high concentrations, the toxin additionally binds via nonspecific absorption to lipid bilayers; it can thus damage both cells lacking significant numbers of the acceptor and protein-free artificial lipid bilayers. Membrane damage occurs in both cases after membrane-bound toxin molecules collide via lateral diffusion to form ring-structured hexamers. The latter insert spontaneously into the lipid bilayer to form discrete transmembrane pores of effective diameter 1 to 2 nm. A hypothetical model is advanced in which the pore is lined by amphiphilic beta-sheets, one surface of which interacts with lipids whereas the other repels apolar membrane constitutents to force open an aqueous passage. The detrimental effects of alpha-toxin are due not only to the death of susceptible targets, but also to the presence of secondary cellular reactions that can be triggered via Ca2+ influx through the pores. Well-studied phenomena include the stimulation of arachidonic acid metabolism, triggering of granule exocytosis, and contractile dysfunction. Such processes cause profound long-range disturbances such as development of pulmonary edema and promotion of blood coagulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Secondary structure and assembly mechanism of an oligomeric channel protein

The alpha-toxin of Staphylococcus aureus is secreted as a water-soluble, monomeric polypeptide (Mr 33 182) that can assemble into an oligomeric membrane channel. By chemical cross-linking, we have confirmed that the major form of the channel is a hexamer. The circular dichroism spectrum of this hexamer in detergent revealed that it contains a high proportion of beta-sheet that we deduce must lie within the lipid bilayer when the protein is associated with membranes. The circular dichroism spectrum of the monomeric toxin in the presence or absence of detergent was closely similar to the spectrum of the hexamer, suggesting that the secondary structure of the polypeptide is little changed on assembly. Results of experiments involving limited proteolysis of the monomer and hexamer are consistent with the idea that assembly involves the movement of two rigid domains about a hinge located near the midpoint of the polypeptide chain. The hydrophilic monomer is thereby converted to an amphipathic rod that becomes a subunit of the hexamer.

Purification of oligomeric staphylococcal alpha-toxin by affinity chromatography on digitonin-sepharose

An effective concentration of alpha-toxin from Staphylococcus aureus Wood 46, directly from the culture supernatant, could be achieved by adsorption on digitonin-sepharose and elution with 3 mol/l sodium thiocyanate (NaSCN). The toxin was further purified by gelchromatography. The purified product yielded 1 single protein band upon SDS-polyacrylamide electrophoresis. It was nonhemolytic, but reacted with anti-alpha-toxin under complement fixation. Dialysis against 0.14 mol/l NaCl with hydrophobic amino acids partially reactivated the alpha-hemolytic activity of the toxin. Ultracentrifugal analysis yielded sedimentation coefficients for the purified toxin of approximately 3,7 S when dissolved in 3 mol/l NaSCN and of about 12 S after dialysis against 0.14 mol/l NaCl (Table 1). The spontaneous oligomerization of the alpha-toxin during dialysis against 0.14 mol/l NaCl possibly resulted from a change in configuration induced by its adsorption to digitonin-sepharose.

Action of staphylococcal alpha-toxin on membranes: some recent advances

Recent developments in the area of Staphylococcal alpha-toxin studies are presented which modify the concepts previously held with respect to both biological and physical properties of alpha-toxin. New data concerning the nature of the binding site for alpha-toxin on rabbit erythrocyte membranes and a model to explain the various observed complexes of alpha-toxin and membrane receptor are discussed. Finally, evidence suggesting that Staphylococcal alpha-toxin is a potent demyelinating agent is presented.

Cholesteryl de-esterifying enzyme from Staphylococcus aureus: separation from alpha toxin, purification, and some properties

A cholesteryl de-esterifying enzyme found in partially purified preparations of alpha toxin produced by the Wood 46 strain on Staphylococcus aureus has been separated from other staphylococcal proteins and from alpha toxin by isoelectric focusing and gel filtration. Preparations of alpha toxin from Bi-Gel P-60 columns and of the cholesteryl esterase from Bio-Gel P-200 columns showed a high degree of purity, as determined by analytical ultracentrifugation, gel diffusion, immunoelectrophoresis, and polyacrylamide gel electrophoresis. The molecular weight of the cholesteryl esterase determined by polyacrylamide gel electrophoresis in sodium dodecyl sulfate was 25,500 and on Bio-Gel P-300 columns it was 175,000, indicating an associating system. The density of the enzyme was lower than expected for simple proteins (about 1.19 g/ml). Chloroform-methanol extracts showed the presence of a neutral lipid that did not contain cholesterol. This material, possibly a glycolipid, might play a role in the stabilization of the enzymatically active protomer. The isoelectric point of the esterase was 9.1. Cholesteryl esterase was labile and lost its activity easily. It could bind reversibly to agarose-containing gels. After elution, it was enzymatically inactive, with an isoelectric point of less than 6.2. The W46M mutant of the Wood 46 strain, which does not produce alpha toxin, also does not produce cholesteryl esterase.

The hemolysins of Staphylococcus aureus

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Cellular location of alpha-hemolysin in Staphylococcus aureus

Protoplast fractionation techniques were used to prepare subcellular fractions of three strains of Staphylococcus aureus for the analysis of alpha-hemolysin activity. More than 90% of the cellular hemolytic activity was localized in the periplasm, only a trace was detected in the cytoplasm, and none was found in either plasma membranes or mesosomes. This cellular portion constituted approximately 4 to 10% of the total hemolytic activity of the culture. The erythrocyte lysis spectrum and Sephadex G-100 filtration studies indicated that the cellular hemolysin closely resembles the extracellular form.

Comparison of purified alpha-toxins from various strains of Staphylococcus aureus

Alpha-toxin from five strains of Staphylococcus aureus, including Wood 46, was purified by isoelectric focusing. The alpha-toxins obtained from different strains were similar. The isoelectric point of the purified toxins was 8.65 +/- 0.15. Sharp concentration peaks were not always obtained. In the ultracentrifuge the alpha-toxins migrated usually as three peaks which could be dissociated with propionic acid to yield one peak. A single line of identity was obtained in immunoelectrophoresis when a heterologous antiserum was reacted with the five purified toxins. It was concluded that the widespread use of the Wood 46 strain for the production of alpha-toxin is justified.

Factors affecting interaction of staphylococcal alpha toxin with membranes

Staphylococcal alpha toxin interacts not only with membranes of erythrocytes but also with membranes of other kinds of mammalian cells (platelets, hepatocytes, and lysosomes from polymorphonuclear leukocytes) with the formation of characteristic ring-like structures that can be seen by electron microscopy. Such structures are not observed when alpha toxin is added to membranes derived from various bacteria. The rings seen on mammalian cell membranes tend to be either randomly disposed or in square array. The frequency with which square arrays are seen is influenced by the presence of staphylococcal delta toxin, by the negative staining agent, and by the kind of cell from which the membrane is derived. Synthetic membranes in the form of liposomes, prepared individually from phosphatidyl choline, phosphatidyl serine, phosphatidyl inositol, and cardiolipin, produced randomly disposed rings upon addition of alpha toxin. Liposomes made from phosphatidyl ethanolamine did not yield rings. Alpha toxin-treated liposomes prepared from chloroform-methanol extracts of brain white matter consistently showed rings that were rectangularly ordered. Ordered rings on membranes derived from toxin-treated platelets and those on toxin-treated brain extract liposomes were seen in freeze-etched as well as in negatively stained preparations.

Production of staphylococcal alpha toxin. I. Relationship between cell growth and toxin formation

Alpha toxin production and its relationship to cell growth were studied in the Wood 46 strain of Staphylococcus aureus. Toxin first appeared in the culture in the late logarithmic stage, but at least 80% was produced during the subsequent period of slower cell growth. The toxin concentration per unit of cell mass or viable count increased continually throughout the period of toxin production and and at its maximum represented 1.6 to 2.0% of the dry weight of the cells. The possibility that alpha toxin is released as a result of cell lysis was examined by using the appearance of cellular deoxyribonucleic acid in the medium as an indicator of lysis. The results showed that no appreciable amount of lysis occurred during toxin production; at a time when almost maximum amounts of toxin were present in the culture, less than% 4 of the cells had lysed. This finding, together with the observation that less than 0.25% of the total amount of toxin in the culture could be found intracellularly, indicates that alpha toxin is released from intact cells shortly after it is synthesized.