Characterization of baculovirus insecticides expressing tailored Bacillus thuringiensis CryIA(b) crystal proteins

Full-length, truncated, and mature forms of the CryIA(b) insecticidal crystal protein gene of Bacillus thuringiensis were engineered into the p10 locus of Autographa californica nuclear polyhdrosis virus (AcNPV). A signal sequence of Heliothis virescens juvenile hormone esterase was introduced at the N-terminus of these constructs to induce secretion. All recombinants, except those containing the mature toxin, produced high levels of CryIA(b) ICPs in insect cells. Thirty percent of the intracellular protoxin was N-glycosylated, suggesting that the protoxin was translocated across the ER membrane. Secretion into the medium, however, was limited. The production of the mature toxin was poor as a result of its cytotoxicity to insect cells. In a bioassay against second instar Spodoptera exigua larvae, using a recombinant expressing the Androctonus australis scorpion toxin gene in the same p10 locus as a positive control, the median survival time of AcNPV recombinants expressing the various B. thuringiensis CryIA(b) ICP constructs was not significant different from that of wild-type AcNPV. This suggests that production and/or secretion of B. thuringiensis (pro)toxins by AcNPV p10 recombinant viruses does not increase insecticidal activity since (i) the protoxins produced are inactive and not likely to be activated in vivo; (ii) secretion of the B. thuringiensis protoxins is poor; and (iii) production of the mature toxins results in cytotoxicity.