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Applied and Environmental Microbiology, October 2001, p. 4488-4494, Vol. 67, No. 10
0099-2240/01/$04.00+0   DOI: 10.1128/AEM.67.10.4488-4494.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

Differential Effects of pH on the Pore-Forming Properties of Bacillus thuringiensis Insecticidal Crystal Toxins

Le Binh Tran,1 Vincent Vachon,1 Jean-Louis Schwartz,1,2 and Raynald Laprade1,*

Groupe de recherche en transport membranaire, Université de Montréal, Montreal, Quebec H3C 3J7,1 and Biotechnology Research Institute, National Research Council, Montreal, Quebec H4P 2R2,2 Canada

Received 13 February 2001/Accepted 17 July 2001

The effect of pH on the pore-forming ability of two Bacillus thuringiensis toxins, Cry1Ac and Cry1C, was examined with midgut brush border membrane vesicles isolated from the tobacco hornworm, Manduca sexta, and a light-scattering assay. In the presence of Cry1Ac, membrane permeability remained high over the entire pH range tested (6.5 to 10.5) for KCl and tetramethylammonium chloride, but was much lower at pH 6.5 than at higher pHs for potassium gluconate, sucrose, and raffinose. On the other hand, the Cry1C-induced permeability to all substrates tested was much higher at pH 6.5, 7.5, and 8.5 than at pH 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 M. sexta is considerably weaker than that of Cry1Aa, Cry1Ab, and Cry1Ac. However, Cry1E, despite having a toxicity comparable to that of Cry1C, formed channels as efficiently as the Cry1A toxins at pH 10.5. These results strongly suggest that although pH can influence toxin activity, additional factors also modulate toxin potency in the insect midgut.

* Corresponding author. Mailing address: Groupe de recherche en transport membranaire, Université de Montréal, P.O. Box 6128, Centre Ville Station, Montreal, Quebec H3C 3J7, Canada. Phone: (514) 343-7960. Fax: (514) 343-7146. E-mail: raynald.laprade{at}umontreal.ca.

Applied and Environmental Microbiology, October 2001, p. 4488-4494, Vol. 67, No. 10
0099-2240/01/$04.00+0   DOI: 10.1128/AEM.67.10.4488-4494.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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