BACILLUS-THURINGIENSIS CRYLAA DELTA-ENDOTOXIN AFFECTS THE K-ACID SYMPORT IN BOMBYX-MORI LARVAL MIDGUT( AMINO)

We have examined the type of inhibition exerted by an activated prepar ation of the Bacillus thuringiensis delta-endotoxin CrylAa on K+-depen dent leucine transport into midgut brush border membrane vesicles or e pithelial cells of the isolated midgut from Bombyx mori to study its p ossible interaction with the amino acid symporter. K+ permeability and the cation-dependent amino acid translocation into brush border membr ane vesicles were evaluated by monitoring the fluorescence of the volt age-sensitive cyanine dye 3,3'-dipropylthiadicarbocyanine iodide. The symporter ability to accept Na+ instead of K+ was exploited and the di ssipation of an imposed inside-negative potential (K+ gradient in>out and valinomycin) was registered in the presence of a Na+ gradient (out >in) and of the amino acid. The fluorescence quenching dissipated more rapidly when the amino acid was present. Preincubation of brush borde r membrane vesicles with CrylAa caused a significant decrease of the a mino acid-dependent recovery of fluorescence, whereas K+ permeability was sparely affected. In the isolated midgut, CrylAa inhibits leucine uptake as well as the transepithelial electrical potential difference. The strong inhibition exerted by the delta-endotoxin was observed als o in the absence of potassium and the transepithelial electrical poten tial difference. The results obtained strongly suggest a direct intera ction of CrylAa delta-endotoxin with the K+/amino acid symporter.