Reduced sensitivity of Niemann-Pick C1-deficient cells to θ-toxin (perfringolysin O): Sequestration of toxin to raft-enriched membrane vesicles

θ-Toxin (perfringolysin O) binds to cell surface cholesterol and forms oligomeric pores that cause membrane damage. Both in cytotoxicity and cell survival assays, a mutant Chinese hamster ovary cell line NPC1(-) that lacked Niemann-Pick C1 showed reduced sensitivity to θ-toxin, compared with wild-type (wt) cells. BCθ is a derivative of θ-toxin that retains cholesterol-binding activity but lacks cytotoxicity. Confocal and electron microscopy revealed the presence of multiple vesicles which bound BCθ, both on the cell surface and in the extracellular space of these cells. BCθ binding to raft microdomains was verified by its resistance to 1% Triton X-100 at 4°C and recovery of bound BCθ in floating low-density fractions on sucrose density gradient fractionation. BCθ-labeled vesicles were abolished when NPC1(-) cells were depleted of lipoproteins and also when treated with a Rho-associated kinase inhibitor Y-27632. In addition, similar vesicles were observed in wt cells treated with progesterone. In parallel with these results, θ-toxin sensitivity of NPC1(-) cells was increased when cells were depleted of lipoproteins or treated with Y-27632, whereas that of wt cells was decreased by progesterone. Our findings suggest that sequestration of toxin to raft-enriched cell surface vesicles may underlie reduced sensitivity of NPC1-deficient cells to θ-toxin.