Ostreolysin is a 15 kDa thermolabile protein isolated from the edible oyster mushroom Pleurotus ostreatus. Previous studies show that nanomolar concentrations of ostreolysin exerts cytolytic effects on erythrocytes and tumor cells. Moreover, it is also toxic to rodents when intravenously injected (LD₅₀ = 1170 mg/kg), leading to death by cardiorespiratory arrest. These effects are assumed to result from ostreolysin-induced pores in plasma membrane. Therefore, we have investigated the ability of ostreolysin to form pores when incorporated into both planar lipid bilayers and plasma membranes of CHO cells, using electrophysiological techniques and have characterized their conductance and ionic selectivity.

Ostreolysin was able to form pores in cholesterol-enriched planar lipid phosphatidylcholine bilayers. The amplitude of the resulting transmembrane current varied linearly with the applied membrane potential with a mean conductance of 700 pS at 20 mM KCl. According to this result, the pore-forming activity was investigated in cells. Addition of ostreolysin to the CHO cells resulted in formation of pores with wide size distribution. The toxin induced a current that reversed at about 0 mV. The current reversal potential was not markedly modified when Na⁺ and Cl^- concentrations were changed in the extracellular medium, indicating that the corresponding pores are not selective for one of these ion species.

These results suggest that the toxicity of ostreolysin on mammalian cells may be explained by its ability to induce non-selective ion-conducting pores.