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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 PK/PCl permeability ratio of 4.9. In addition, Cry4Aa channel-pores exhibited apparent cation preference in the order Na + > K + , Na + > Ca 2+ , and K + > Ca 2+ . 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 2 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.

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