Mode of action of parasporin-4, a cytocidal protein from Bacillus thuringiensis

Parasporin-4 (PS4) is a cytotoxic protein produced by Bacillus thuringiensis strain A1470. It exhibits specific cytotoxicity against human cancer cell lines, CACO-2, Sawano, and MOLT-4 cells, in particular. When cells were administrated with PS4, cell swelling and nuclear shrinkage were induced, and, the ballooned cells burst within 24 h. PSI-BLAST search showed that the protein shared homology not only with B. thuringiensis Cry toxins but also with aerolysin-type β-pore-forming toxins. Circular dichroism measurements suggested that PS4 was a β-sheet-rich protein. PS4 aggregated into oligomers on the plasma membrane of PS4-susceptible CACO-2 cells, but not on that of PS4-resistant HeLa cells. Leakage of lactate dehydrogenase and influx of extracellular FITC-dextrans were observed only in susceptible cells. The activation of effectors caspase 3 and/or 7 was not observed in PS4-treated CACO-2 cells. It was shown that cytotoxicity of the PS4 against CACO-2 cells was exhibited when treated by cyclodextrin which induces cholesterol depletion. These results suggest that PS4 is a unique β-pore-forming toxin with a cholesterol-independent activity.