Ionic permeabilities induced by Bacillus thuringiensis in Sf9 cells

Abstract

The effect of Bacillus thuringiensis insecticidal toxins on the monovalent cation content and intracellular pH (pH _i ) of individual Sf9 cells of the lepidopteran species Spodoptera frugiperda (fall armyworm) was monitored with the fluorescent indicators potassium-binding benzofuran isophthalate (PBFI) and 2′,7′-bis(carboxyethyl)-5,6-carboxyfluorescein (BCECF). The sequential removal of K⁺ and Na⁺ from the medium, in the presence of CryIC, a toxin which is highly active against Sf9 cells, caused sharp shifts in the fluorescence ratio of PBFI, demonstrating a rapid efflux of these ions. In Sf9 cells, pH _i depends strongly on the activity of a K⁺/H⁺ exchanger. In the absence of toxin, removal of K⁺ from the external medium resulted in a reversible acidification of the cells. In the presence of CryIC, pH _i equilibrated rapidly with that of the bathing solution. This effect was both time- and concentration-dependent. In contrast with CryIC, CryIIIA, a coleopteran-specific toxin, and CryIA(a), CryIA(b) and CryIA(c), toxins which are either inactive or poorly active against Sf9 cells, had no detectable effect on pH _i . B. thuringiensis endotoxins thus appear to act specifically by increasing the permeability of the cytoplasmic membrane of susceptible cells to at least H⁺, K⁺ and Na⁺.