Differential effects of pH on the pore-forming properties of Bacillus thuringiensis insecticidal crystal toxins

The effect of pH on the pore-forming ability of two Bacillus thuringiensis toxins, Cry1Ac and Cry1C, was examined with midgut brush border membrane ve sicles isolated from the tobacco hornworm, Manduca sexta, and a light-scatt ering assay. In the presence of Cry1Ac, membrane permeability remained high over the entire pH range tested (6.5 to 10.5) for KCI and tetramethylammon ium chloride, but was much lower at pll 6.5 than at higher pHs for potassiu m gluconate, sucrose, and raffinose. On the other hand, the Cry1C-induced p ermeability to all substrates tested was much higher at pH 6.5, 7.5, and 8. 5 than at pll 9.5 and 10.5. These results indicate that the pores formed by Cry1Ac are significantly smaller at pH 6.5 than under alkaline conditions, whereas the pore-forming ability of Cry1C decreases sharply above pH 8.5. The reduced activity of Cry1C at high pH correlates well with the fact that its toxicity for AT. sexta is considerably weaker than that of Cry1Aa, Cry 1Ab, and Cry1Ac. However, Cry1E, despite having a toxicity comparable to th at of Cry1C, formed channels as efficiently as the Cry1A toxins at pH 10.5. These results strongly suggest that although pH can influence toxin activi ty, additional factors also modulate toxin potency in the insect midgut.