Assembly of Staphy lococcus aureus alpha-toxin on target membranes

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Staphylococcus aureus alpha-toxin is a 33-kDa polypeptide endowed with hemolytic, dermonecrotic and lethal activities. It is thought to bind to membranes, assemble into pore-forming hexamers, eventually causing colloid osmotic lysis of cells. Our previous studies showed that staphylococcal alpha-toxin binds specifically to the choline-type phospholipids, and that the increase in membrane fluidity promotes hexamerization of the toxin on target membranes. In this project, we prepared fragments of alpha-toxin and assayd their binding and pore-forming abilities. The results indicated (1) that several fragments, such as Alal-Glu71, Gly72-Asn293, Thr9-Lys131 and Ile132-Asn293 retained ability to bind to phosphatidylcholine-cholesterol liposomes and erythrocyte membranes, (2) that these fragments did not form membrane pores, although they assembled partially into dimers on the membranes, and (3) that a larger-sized fragment (Thr9-Asn293) showed little pore-forming activity towards liposomes and erythrocytes, although it retained the binding and oligomerizing abilities. These data suggest that both of the N-terminal and the C-terminal portions of alpha-toxin are indispensable for its pore-forming ability. Thus, our data are not consistent with the previous proposal that different domains of alpha-toxin are responsible for defferent functions (e.g., C-Terminal 18-kDa fragment of the toxin was reported to be hemolytic). Our results may suggest that monomer molecules of alpha-toxin are linked in a head-to-head manner when they assemble into hexamers.