Photolabeling of staphylococcal alpha-toxin from within rabbit erythrocyte membranes

Biological relevance of natural alpha-toxin fragments from Staphylococcus aureus

Serine proteases represent an essential part of cellular homeostasis by generating biologically active peptides. In bacteria, proteolysis serves two different roles: a major housekeeping function and the destruction of foreign or target cell proteins, thereby promoting bacterial invasion. In the process, other virulence factors such as exotoxins become affected. In Staphylococcus aureus culture supernatant, the pore-forming alpha-toxin is cleaved by the coexpressed V8 protease and aureolysin. The oligomerizing and pore-forming abilities of five such spontaneously occurring N- and C-terminal alpha-toxin fragments were studied. (3)H-marked alpha-toxin fragments bound to rabbit erythrocyte membranes but only fragments with intact C termini, missing 8, 12 and 71 amino acids from their N-terminal, formed stable oligomers. All isolated fragments induced intoxication of mouse adrenocortical Y1 cells in vitro, though the nature of membrane damage for a fragment, degraded at its C terminus, remained obscure. Only one fragment, missing the first eight N-terminal amino acids, induced irreversible intoxication of Y1 cells in the same manner as the intact toxin. Four of the isolated fragments caused swelling, indicating altered channel formation. Fragments missing 12 and 71 amino acids from the N terminus occupied the same binding sites on Y1 cell membranes, though they inhibited membrane damage caused by intact toxin. In conclusion, N-terminal deletions up to 71 amino acids are tolerated, though the kinetics of channel formation and the channel's properties are altered. In contrast, digestion at the C terminus results in nonfunctional species.

Staphylococcus aureus alpha-toxin: characterization of protein/lipid interactions, 2D crystallization on lipid monolayers, and 3D structure

Staphylococcus aureus alpha-toxin was characterized with respect to surface activity and its interaction with lipid monolayers. The protein alone had a detergent-like behavior at the air/water interface. Its affinity was higher for negatively charged than for neutral phospholipids. The interaction was pH dependent, showing a maximum increase at pH 7.0. Only a small part of the protein oligomer appeared to be inserted into the monolayers. Crystalline sheets of alpha-toxin were formed using negatively charged phospholipids. Electron microscopy of such areas, at different tilt angles, allowed reconstruction of a three-dimensional model following image processing. The sheets analyzed consisted of two protein layers arranged on a tetragonal lattice. Under the conditions used to grow the crystals the toxin formed 90-A-wide cylinders with a height of 70 A. One of the imposed fourfold axes running perpendicular to the plane of the crystalline layer is positioned at a protein-deficient region which forms a 25-A-wide pore through the oligomer.

Photolabeling of a pore-forming toxin with the hydrophobic probe 2-[3H]diazofluorene. Identification of membrane-inserted segments of Staphylococcus aureus alpha-toxin

The identification of membrane-inserted segments of pore-forming soluble proteins is crucial to understanding the action of these proteins at the molecular level. A distinct member of this class of proteins is alpha-toxin, a 293-amino acid-long 33-kDa hemolytic toxin secreted by Staphylococcus aureus that can form pores in both artificial and natural membranes. We have studied the interaction of alpha-toxin with single bilayer vesicles prepared from asolectin using a hydrophobic photoactivable reagent, 2-[3H]diazofluorene ([3H]DAF) (Pradhan, D., and Lala, A. K. (1987) J. Biol. Chem. 262, 8242-8251). This reagent readily partitions into the membrane hydrophobic core and on photolysis labels the lipid and protein segments that penetrate the membrane. Current models on the mode of action of alpha-toxin indicate that, on interaction with membranes, alpha-toxin forms an oligomer, which represents the active pore. In keeping with these models, we observe that [3H]DAF photolabels the membrane-bound alpha-toxin oligomer. Cyanogen bromide fragmentation of [3H]DAF-labeled alpha-toxin gave several fragments, which were subjected to Edman degradation. We could thus sequence residues 1-19, 35-60, 114-139, 198-231, and 235-258. Radioactive analysis and phenylthiohydantoin-derivative analysis during sequencing permitted analysis of DAF insertion sites. The results obtained indicated that the N and C termini (residues 235-258) have been extensively labeled. The putative pore-forming glycine-rich central hinge region was poorly labeled, indicating that the apposing side of the lumen of the pore does not form the lipid-protein interface. The DAF labeling pattern indicated that the major structural motif in membrane-bound alpha-toxin was largely beta-sheet.

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)

Heat shock proteins induce pores in membranes

Human heat shock protein (hsp) 70 and bacterial protein groEL promote leakage of calcein from liposomes induced by human serum albumin signal peptide, by S. aureus alpha toxin or by diphtheria toxin. Hsp 70 and groEL, as well as two mycobacterial homologues hsp 71 and hsp 65, induce ion conducting pores across planar lipid bilayers at low or neutral pH. It is concluded that hsp induce pores in membranes and that this may contribute to their action within cells.

Oligomerization of 3H-labelled staphylococcal alpha-toxin and fragments on adrenocortical Y1 tumour cells

Staphylococcus aureus alpha-toxin has previously been shown to bind to erythrocyte membranes and the isolated membranes contain the toxin in both monomeric and hexameric form. The hexamers are believed to form the ring-shaped structures observed by electron microscopy on toxin-treated erythrocytes. It has not previously been shown that hexamers are formed also on nucleated mammalian cells although it has been assumed that hexamers in both systems create transmembrane channels, responsible for the toxin-induced membrane damage. Here we demonstrate by autoradiography that 3H-alpha-toxin bound to and formed high molecular weight complexes-presumably hexamers-on cultured adrenocortical Y1 tumour cells. The binding kinetics suggested a non-specific association of alpha-toxin with the membrane, rather than specific receptor-binding. The pH during toxin binding did not influence the subsequently induced membrane damage. Non-membrane damaging alpha-toxin fragment preparations also bound firmly to the cell membranes. Upon contact with Y1 cells the fragments formed complexes of the same apparent molecular size as those generated from intact alpha-toxin. Two interpretations are possible: either the fragment oligomers are somehow defective i.e. not able to form transmembrane structures or the functional relevance of toxin oligomerization for alpha-toxin-induced membrane damage must be questioned.

Staphylococcal alpha toxin--recent advances

The elucidation of the amino acid sequence of alpha toxin in 1984 has greatly promoted our understanding of the basic biochemistry and interaction of this toxin with membranes. These aspects are discussed and the concept of alpha toxin as a channel forming protein is critically evaluated. The lethal action of alpha toxin has not yet been clarified, but the previously postulated action as a neurotoxin is not supported by recent observations.

Distribution of 3H-labeled staphylococcal alpha-toxin and a toxin fragment in mice

Staphylococcal alpha-toxin and a toxin fragment were labeled with N-succinimidyl[2,3-3H]propionate. The labeled compounds retained greater than 95% biological activity. The distribution of labeled staphylococcal alpha-toxin and alpha-toxin fragment after intravenous administration to BALB/c mice was studied with whole-body and microautoradiography. The animals were divided into three groups that received (i) labeled alpha-toxin only, labeled alpha-toxin after prior injection of unlabeled fragment, or labeled fragment only. After 5 min, the distribution patterns were similar in groups 1 and 2, with the highest amounts of radioactivity found in the blood vessels, liver, spleen, lungs, and kidneys, whereas the labeled fragment alone showed no initial accumulation in the lungs. The kidneys continued to show a high concentration of radioactivity, whereas the levels at 60 min had decreased in the other organs. The toxin showed continued stable binding to the proximal tubuli, whereas the toxin fragment seemed to dissociate and was found only in small amounts in the glomeruli. No radioactivity was found in the central nervous system.

Characterization of domain borders and of a naturally occurring major fragment of staphylococcal alpha-toxin

A naturally occurring staphylococcal alpha-toxin fragment with an apparent membrane-binding capacity but without toxic activities is shown to be derived from the C-terminal half of the intact polypeptide chain by cleavage between position 134 and 135 in the parent molecule. The resulting N-terminus is slightly ragged with a fragment start not only at position 135 but also at the adjacent position 136. Another naturally occurring fragment starts at position 9, derived from an original cleavage between position 8 and 9 in the parent molecule. Analysis of non-purified fragment mixtures confirmed these positions and established that only one further region, at positions 71-72, is partly sensitive to proteolysis under natural conditions. Trypsin treatment has limited effects on the native toxin molecule, giving essentially only two initial cleavages with resultant large fragments. One of these cleavages is at the peptide bond between position 131 and 132, thus only three residues away from the position of the major naturally occurring cleavage. The other bond sensitive to trypsin is between position 8 and 9, thus identically positioned to the cleavage occurring naturally. Together, all the cleavages define a region in a central segment of the polypeptide chain that has all the properties of an inter-domain segment. The C-terminal half appears to constitute a membrane-binding domain, and the N-terminal half a structure needed for full biological activity, functionally subdividing the parent polypeptide chain.

Interactions between membranes and cytolytic peptides

The physico-chemical and biological properties of cytolytic peptides derived from diverse living entities have been discussed. The principal sources of these agents are bacteria, higher fungi, cnidarians (coelenterates) and the venoms of snakes, insects and other arthropods. Attention has been directed to instances in which cytolytic peptides obtained from phylogenetically remote as well as from related sources show similarities in nature and/or mode of action (congeneric lysins). The manner in which cytolytic peptides interact with plasma membranes of eukaryotic cells, particularly the membranes of erythrocytes, has been discussed with emphasis on melittin, thiolactivated lysins and staphylococcal alpha-toxin. These and other lytic peptides are characterized in Table III. They can be broadly categorized into: (a) those which alter permeability to allow passage of ions, this process eventuating in colloid osmotic lysis, signs of which are a pre-lytic induction or latent period, pre-lytic leakage of potassium ions, cell swelling and inhibition of lysis by sucrose. Examples of lysins in which this mechanism is involved are staphylococcal alpha-toxin, streptolysin S and aerolysin; (b) phospholipases causing enzymic degradation of bilayer phospholipids as exemplified by phospholipases C of Cl. perfringens and certain other bacteria; (c) channel-forming agents such as helianthin, gramicidin and (probably) staphylococcal delta-toxin in which toxin molecules are thought to embed themselves in the membrane to form oligomeric transmembrane channels.

Staphylococcal alpha-toxin-induced PGI2 production in endothelial cells: role of calcium

Studies in erythrocytes indicate that staphylococcal alpha-toxin generates discrete transmembrane channels with an effective diameter of 2-3 nm. In cultured, confluent, pig pulmonary arterial endothelial cells we studied the triggering of the arachidonic acid cascade and its dependence on calcium influx, possibly through toxin-created pores. In endothelial cells alpha-toxin time dependently (5-30 min) and dose dependently (0.1-8 micrograms/ml) stimulated the release of radiolabeled arachidonic acid and prostacyclin (PGI2) production in similar amounts as the calcium ionophore A23187 (10 microM). Preincubation of alpha-toxin with neutralizing antibodies abolished the effect. The toxin response was strictly dose dependent on extracellular calcium but not on magnesium. The toxin effect was accompanied by an up to 10-fold increased passive permeability of pulmonary arterial endothelial cells for 45Ca. Interference with calcium-calmodulin function (trifluoperazine, W7) dose dependently reduced production of PGI2, but blockers of physiological calcium channels (verapamil, nimodipine, nisoldipine, and diltiazem) did not. In contrast to the effect of the ionophore A23187, the toxin effect was accompanied by a release of potassium, but in neither system was there a release of lactate dehydrogenase. In addition, alpha-toxin but not ionophore-exposed endothelial cells showed an increased passive influx of small radiolabeled markers (45Ca and [3H]sucrose) but not of large markers [( 3H]inulin and [3H]dextran). These data are consistent with the concept that alpha-toxin triggers the arachidonic acid cascade in pulmonary arterial endothelial cells by calcium influx and suggest that this calcium influx may proceed through toxin-created transmembrane channels.

Primary sequence of the alpha-toxin gene from Staphylococcus aureus wood 46

The complete DNA sequence of a cloned alpha-toxin gene from Staphylococcus aureus was determined. The amino acid sequence of the alpha-toxin protein, predicted from the DNA sequence, was described and compared with published data. The primary product of the cloned alpha-toxin gene contained a 26-amino-acid leader sequence which possessed characteristic features of a signal sequence involved in secretion. The mature alpha-toxin protein had a molecular size of 33,000 and contained only three short regions of high hydrophobicity in addition to a number of short, weakly hydrophobic regions.