KINETICS OF STREPTOLYSIN-O SELF-ASSEMBLY

Streptolysin O is a member of a family of membrane-damaging toxins tha t bind to cell membranes containing cholesterol and then polymerize to form large pores. We have examined the kinetics of toxin action using I-125-labelled streptolysin O. Binding of toxin monomers to membranes displays first-order kinetics and is reversible, the rate of desorpti on from red cells shows a marked dependence on temperature. To study o ligomerization, toxin was bound to erythrocytes at 0 degrees C. Oligom er formation was then triggered by a sudden temperature shift and stop ped by solubilization of membranes with deoxycholate. While at moderat ely high streptolysin O concentrations oligomerization behaves as a re action of second order, the kinetic pattern changes with increasing to xin concentration. We show that this can be accounted for by the assum ption of a two-step reaction mechanism: two membrane-bound monomers fi rst associate into a start complex, which then is rapidly extended by the sequential addition of further monomers up to the final oligomer s ize.