Binding of Bacillus thuringiensis Cry1Ac toxin to Manduca sexta aminopeptidase-N receptor is not directly related to toxicity

Bacillas thuringiensis Cry1Ac delta-endotoxin specifically binds a 115-kDa aminopeptidase-N purified from Manduca sexta midgut, Cry1Ac domain III mutations were constructed around a putative sugar-binding pocket and binding to purified aminopeptidase-N and brush border membrane vesicles (BBMV) was compared to toxicity. Q509A, R511A, Y513A, and 509-511 (QNR-AAA) eliminated aminopeptidase-N binding and reduced binding to BBMV, However, toxicity decreased no more than two-fold, indicating activity is not directly correlated with aminopeptidase-N binding. Analysis of toxin binding to aminopeptidase-N in M, sexta is therefore insufficient for predicting toxicity, Mutants retained binding, however, to another BBMV site, suggesting alternative receptors may compensate in vivo. (C) 1999 Federation of European Biochemical Societies.