Characteristics of channel pores formed by Bacillus thuringiensis mosquito-larvicidal Cry4Aa toxin

Cry4Aa toxin produced by Bacillus thuringiensis subsp. israelensis exhibits specific toxicity to larvae of medically important mosquito genera. In the present study, we analyzed the characteristics of channel pores formed by recombinant Cry4Aa in a solvent-free planar lipid bilayer. Stable channel currents were observed in electrophysiologic measurements, and the single-channel conductance was 187 +/- 10 pS in symmetrical buffer containing 150 mM KCl. The channel pores formed by Cry4Aa were cation-selective, with an estimated P-K/P-Cl permeability ratio of 4.9. In addition, Cry4Aa channel pores exhibited apparent cation preference in the order Na+ > K+, Na+ > Ca2+, and K+ > Ca2+. Although the effect was limited, the cation preference of Cry4Aa channel pores seemed to be correlated with toxicity. Culex pipiens mosquito larvae reared in NaCl solution exhibited greater sensitivity to Cry4Aa, particularly early period after exposure. The presence of cations that preferentially translocate through Cry4Aa channel pores may facilitate excessive influx of water into the midgut cells, leading to colloid-osmotic lysis. Whereas CaCl2 had some effect on the mosquito-larvicidal activity of Cry4Aa, KCl had no effect. The effect of some cations may be mitigated by the variety of ion channels present on the midgut cell membrane.