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

Parasporin-4 (P54) 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 beta-pore-forming toxins. Circular dichroism measurements suggested that PS4 was a beta-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 P54 against CACO-2 cells was exhibited when treated by cyclodextrin which induces cholesterol depletion. These results suggest that PS4 is a unique beta-pore-forming toxin with a cholesterol-independent activity. (C) 2010 Elsevier B.V. All rights reserved.