CELLULAR TOXICITIES AND MEMBRANE-BINDING CHARACTERISTICS OF INSECTICIDAL CRYSTAL PROTEINS FROM BACILLUS-THURINGIENSIS TOWARD CULTURED INSECT CELLS

The pattern of in vitro toxicity of activated toxins from several classes of entomocidal inclusion genes from Bacillus thuringiensis was measured using eight established cell lines from lepidopteran insects. Protoxins representing CryIA(b), CryIA(c), and a mixture of all three CryIA toxins (subtypes a, b, and c; from B. thuringiensis subsp. kurstaki HD-1) were compared with the protoxin representing CryIC in a bioassay which measured the viability of cultured insect cells upon exposure to entomocidal toxin proteins. The responses of the various cell lines were very specific toward the individual toxin proteins. CryIC activated protoxin was toxic for cells of Manduca sexta, Plodia interpunctella, and to a lesser extent Spodoptera frugiperda. CryIA(b) and CryIA(c) proteins were toxic toward M. sexta but relatively nontoxic for P. interpunctella or S. frugiperda. The toxicity of CryIA(b), CryIA(c), and the composite CryIA activated toxins toward cells of Choristoneura fumiferana varied substantially, with the CryIA mixture being slightly more toxic than CryIA(c) alone. CryIC toxin had no effect toward C. fumiferana cells. Probit regression analysis of dose-response relationships between insect species and crystal protein composition demonstrated specific patterns of toxicity which may be related to membrane-receptor site binding by specific toxins. Membrane binding analysis of I-125-labeled CryIA(b), CryIA(c), and CryIC toxins to insect cells from three of the cell lines yielded high specific binding only with bl. sexta cells toward CryIA(c) toxin. Lower levels of binding were observed with CryIA(b) and CryIC toward cells of C. fumiferana and P. interpunctella. Although relatively low binding levels for CryIC were observed with P. interpunctella cells, toxicity was high for these cells. The results suggest that at least for CryIC, binding may be unrelated to toxicity in cultured insect cells. (C) 1994 Academic Press, Inc.