Kinetic properties of the channels formed by the bacillus thuringiensis insecticidal crystal protein Cry1C in the plasma membrane of Sf9 cells.

Spectrofluorimetric measurements were conducted to quantify, in real-time, membrane permeability changes resulting from the treatment of Sf9 insect cells (Spodoptera frugiperda, Lepidoptera) with different Bacillus thuringiensis Cry insecticidal proteins. Coumarin-derived CD222 and Merocyanin-540 probes were respectively used to monitor extracellular K(+) and membrane potential variations upon Sf9 cells incubation with Cry toxins. Our results establish that Cry1C induces, after a delay, the depolarization of the cell membrane and the full depletion of intracellular K(+). These changes were not observed upon Sf9 cells treated with Cry1A family toxins. Both the rate of the K(+) efflux and the delay before its onset were dependent on toxin concentration. Both parameters were sensitive to temperature but only the delay was affected by pH. Cry1C-induced K(+) efflux was inhibited by lanthanum ions in a dose-dependent manner. This study provides the first kinetic and quantitative characterization of the ion fluxes through the channels formed by a Cry toxin in the plasma membrane of a susceptible insect cell line.