A cytolysin, theta-toxin, preferentially binds to membrane cholesterol surrounded by phospholipids with 18-carbon hydrocarbon chains in cholesterol-rich region.

We have previously suggested the existence of two distinctive states of cholesterol in erythrocyte and lymphoma cell membranes as revealed by high- and low-affinity binding sites for theta-toxin of Clostridium perfringens Ohno-Iwashita, Y., Iwamoto, M., Mitsui, K., Ando, S., & Nagai, Y. (1988) Eur. J. Biochem. 176, 95-101; Ohno-Iwashita, Y., Iwamoto, M., Ando, S., Mitsui, K., & Iwashita, S. (1990) Biochim. Biophys. Acta 1023, 441-448]. To understand factor(s) which determine membrane cholesterol heterogeneity, we analyzed toxin binding to large unilamellar liposomes composed of cholesterol and phospholipids (phosphatidylcholine/phosphatidylglycerol = 82:18, mol/mol). Liposomes containing phospholipids with 18-carbon hydrocarbon chains at both positions 1 and 2 of the glycerol have both high- and low-affinity toxin-binding sites with Kd values similar to those of intact erythrocytes, whereas liposomes with hydrocarbon chains containing 16 or fewer carbons at either position 1 or 2 have only low-affinity toxin-binding sites. The cholesterol/phospholipid ratio, in addition to the length of phospholipid hydrocarbon chain, also determines the number of toxin-binding sites, indicating that at least these two factors determine the topology of membrane cholesterol by creating distinctively different affinity sites for the toxin. Since theta-toxin binding detects specific populations of membrane cholesterol that are not detectable by the measurements of susceptibility to cholesterol oxidase and cholesterol desorption from membranes, the toxin could provide a unique probe for studying the organization of cholesterol in membranes.