Lysenin is a sphingomyelin (SM)-binding pore-forming toxin. To reveal the interaction of lysenin with lipid membranes, we investigated lysenin-induced membrane permeation of a fluorescent probe, calcein, through dioleoylphosphatidylcholine (DOPC)/SM, DOPC/SM/cholesterol (chol), and SM/chol membranes, using the single-giant unilamellar vesicle (GUV) method. The results clearly show that lysenin formed pores in all the membranes, through which membrane permeation of calcein occurred without disruption of GUVs. The membrane permeation began stochastically, and the membrane permeability coefficient increased over time to reach a maximum, steady value, P-s, which persisted for a long time (100-500 s), indicating that the pore concentration increases over time and finally reaches its steady value, N-P(s). The P-s values increased as the SM/lysenin ratio decreased, and at low concentrations of lysenin, the P-s values of SM/DOPC/chol (42/30/28) GUVs were much larger than those of SM/DOPC (58/42) GUVs. The dependence of P-s on the SM/lysenin ratio for these membranes was almost the same as that of the fraction of sodium dodecyl sulfate (SDS)-resistant lysenin oligomers, indicating that N-P(s) increases as the SDS-resistant oligomer fraction increases. On the other hand, lysenin formed pores in GUVs of SM/chol (60/40) membrane, which is in a homogeneous liquid-ordered phase, indicating that the phase boundary is not necessary for pore formation. The P-s values of SM/chol (60/40) GUVs were smaller than those of SM/DOPC/chol (42/30/28) GUVs even though the SDS-resistant oligomer fractions were similar for both membranes, suggesting that not all of the oligomers can convert into a pore. On the basis of these results, we discuss the elementary processes of lysenin-induced pore formation.
