MEMBRANE DISORGANIZATION INDUCED BY PERFRINGOLYSIN-O (THETA-TOXIN) OFCLOSTRIDIUM-PERFRINGENS - EFFECT OF TOXIN BINDING AND SELF-ASSEMBLY ON LIPOSOMES

theta-Toxin (perfringolysin O) of Clostridium perfringens binds to mem brane cholesterol with high (K-d approximate to 10(-9) M) and low (K-d approximate to 10(-7) M) affinities and causes membrane lysis of inta ct cells and liposomes. In order to understand the lytic process at th e molecular level, the lysis of liposomes was investigated in comparis on with that of intact cells. The toxin dose required to cause 50% lys is (RD(50)) of phosphatidylcholine phosphatidylglycerol (82:18, mol/mo l) liposomes containing 36-40 mol% cholesterol was 300-1400-times high er than the RD(50) value for sheep or human erythrocytes when samples with the same cholesterol concentration were compared. However, the av erage number of toxin molecules bound per liposome vesicle at 50% lysi s was estimated as 10-18 from the RD(50) values, close to the number o n erythrocytes at 50% lysis, suggesting that the number of toxin molec ules adsorbed per vesicle is important for lysis. As to the toxin dose required for membrane lysis, no significant difference was observed b etween liposomes containing both high- and low-affinity toxin-binding sites and those containing only low-affinity sites, suggesting that th eta-toxin molecules bound to low-affinity sites can assemble and cause membrane lsis as well as those bound to high-affinity sites. theta-To xin assembles on liposomal membranes, as on erythrocytes, in a high-mo lecular-weight polymeric form as judged from sedimentation patterns in sucrose density-gradient centrifugation. The high-molecular-weight po lymers were detected only under conditions where cell or liposome lysi s occurred. At low toxin doses, slower sedimenting toxin oligomers and monomers were predominant on liposomal membranes. These results indic ate that toxin assembly on membranes is essential for liposome lysis a s it is for cell lysis and that assembly occurs on membranes without m embrane proteins.