STREPTOLYSIN O - A PROPOSED MODEL OF ALLOSTERIC INTERACTION BETWEEN APORE-FORMING PROTEIN AND ITS TARGET LIPID BILAYER

Streptolysin O, a polypeptide of 571 amino acids, belongs to the famil y of thiol-activated toxins that permeabilize animal cell membranes. T he protein binds as a monomer to membrane cholesterol, Binding involve s a conserved region close to the C-terminus and triggers subsequent p olymerization Ito large are-and ring-shaped structures surrounding por es of up to 30 nm. Besides the C-terminus, a distantly located region spanning residues 213-305 is involved in oligomerization and in membra ne insertion. Here, we searched for conformational effects of monomer binding to the latter functionally important region. To this end, sing le cysteine substitution mutants were produced and derivatized with th e polarity-sensitive fluorophore acrylodan. Fluorimetric measurements revealed that binding of the monomer to membranes is accompanied by di stinct environmental changes at amino acid residues 218, 248, 266, and 277. Conspicuously, the environment of residues 218 and 266 became mo re hydrophilic, suggesting movement of these residues out of hydrophob ic protein pockets. Upon oligomerization. further alterations in all s ide-chain environments were observed. The membrane-bound monomer thus differs in conformation from both the monomer in solution and the subu nit of the oligomer. The putative binding site of the molecule is Link ed to remote domains involved in oligomerization and membrane insertio n in an apparently allosteric fashion. It is proposed that allostery i s responsible for restricting oligomerization to the membrane-bound st ate of the toxin.