A RING-SHAPED STRUCTURE WITH A CROWN FORMED BY STREPTOLYSIN-O ON THE ERYTHROCYTE-MEMBRANE

Streptolysin O (SLO) is a membrane-damaging toxin produced by most str ains of group A beta-hemolytic streptococci. We performed ultrastructu ral analysis of SLO-derived lesions on erythrocyte membranes by examin ing electron micrographs of negatively stained preparations. SLO forme d numerous arc- and ring-shaped structures with or without holes on me mbranes. Rings formed on intact cell membranes had an inner diameter o f ca. 24 nm and had distinct borders of ca. 4.9 nm in width, but the d iameter of rings varied from 24 to 30 nm on membranes of erythrocyte g hosts. Image analysis of electron micrographs demonstrated that each r ing was composed of an inner and an outer layer. Each layer contained an array of 22 to 24 SLO molecules. On the top of the ring, we found a characteristic crown that projected from the cell membrane. The crown was separated by an electron-dense layer from the basal part of the r ing that was embedded in the lipid bilayer of the erythrocyte membrane . Heights of the three parts, namely, the crown (head), the space (nec k), and the basal portion (base), were ca. 3.2, 1.6, and 5.0 nm, respe ctively, and we postulated that these parts are the constituents of a single SLO molecule. The volumes of SLO molecules in the inner and out er layers were calculated to be 77 and 88 nm3. On the basis of a model of the structure of SLO, we propose some new details of the mechanism s of hemolysis by SLO toxin.