Mechanism of membrane damage by streptolysin-O

Abstract

Streptolysin-O (SLO) is a thiol-activated, membrane-damaging protein toxin of Mr 69,000 that is produced by most strains of I-hemolytic group A streptococci. Native, primarily water-soluble toxin molecules bind to cholesterol-containing target membranes to assemble into supramolecular curved rod structures (25 to 100 nm long by ca. 7.5 nm wide), forming rings and arcs that penetrate into the apolar domain of the bilayer. Electron microscopic analyses of toxin polymers in their native and reconstituted membrane-bound form indicate that the convex surface of the rod structures is a hydrophobic, lipid-binding domain, whereas the concave surfaces appear to be hydrophilic. The embedment of the rings and arcs generates large transmembrane slits or pores of up to 30-nm diameter that can be directly visualized by negative staining and freeze-fracture electron microscopy. SLO oligomers were isolated in extensively delipidated form in detergent solution, and cholesterol was found not to detectably contribute to the observed rod structures. The rods are stable structures that resist prolonged exposure to trypsin and chymotrypsin. They can be reincorporated into cholesterol-free phosphatidylcholine liposomes to generate lesions identical to those observed on erythrocytes lysed by native SLO. Thus, although cholesterol plays a key role in the initial binding of SLO to the membrane, it does not directly participate in the formation of the membrane-penetrating toxin channels. Membrane damage by SLO is basically analogous to that mediated by previously studied channel formers, namely, the C5b-9 complement complex and staphylococcal ot-toxin. Lancefield group A P-hemolytic streptococci are the ma-* Corresponding author.

Assembly mechanism of the oligomeric streptolysin O pore: the early membrane lesion is lined by a free edge of the lipid membrane and is extended gradually during oligomerization

“…On electron micrographs of toxin-treated erythrocyte membranes, the typical wild-type SLO oligomers appear as rings that are accompanied by some arcs (Bhakdi et al, 1985). A very similar picture was obtained with the functional mutant C530A ( Figure 5A).…”

“…These difficulties are avoided in a different model of SLO pore assembly which we are advancing ( Figure 9B). The model was based originally on electron microscopic observations (Bhakdi et al, 1985) in conjunction with kinetic data on the oligomerization process (Palmer et al, 1995), and has been corroborated further here.…”

“…Deprived of favorable hydrophobic interaction with this protein surface, the adjacent membrane lipids will become subject to surface tension and hence retract to form the edge facing the arc that is visualized by electron microscopy. With arcs reconstituted into liposomes, this edge appears as a straight line (Bhakdi et al, 1985), which would yield the maximum relaxation of surface tension. The shape of the lipid edge seems somewhat more varied in the case of arc lesions on erythrocytes.…”

“…Instead, their stability derives from relieving the membrane of two free edges. Accordingly, twin arcs are no longer observed once the membrane is dissolved with detergent (Bhakdi et al, 1985).…”

Ehrlichiasecretes Etf-1 to induce autophagy and capture nutrients for its growth through RAB5 and class III phosphatidylinositol 3-kinase

“…86 SLO is a thiol-activated protein toxin that binds cholesterol to form membrane-penetrating channels in a temperature-dependent manner. 87 Cytoplasmic Etf-1 was labeled with anti-Etf-1 after SLO treatment (Fig. 6B).…”