Expression efficiency of a scorpion neurotoxin, AaHIT, using baculovirus in insect cells

Insect-selective neurotoxins are useful agents for developing recombinant baculoviral insecticides. AaHIT, one such toxin, was expressed in AcMNPV and had a dramatic effect on pest insects. However, the toxin production was relatively low. In order to identify reasons for the low production of recombinant AaHIT (rAaHIT) in insect cells, we analyzed the expression efficiency using Northern blotting, enzyme-linked immunosorbent assay, Western blotting, and a bioassay using Sarcophaga falculata larvae. The levels of rAaHIT mRNA and protein in Sf21 cells were comparable with those of another secreted protein, recombinant juvenile hormone esterase (rJHE), which was expressed in the same baculovirus system. However, the secretion efficacy of rAaHIT was significantly lower than that of rJHE. In addition, the toxic activity in the medium was approximately 1% of the total secreted rAaHIT. Western blot analyses suggested that rAaHIT aggregated and accumulated in cells. We attempted to increase the amount of soluble rAaHIT in cells using a co-expressed chaperonin, BiP. Although co-expressed BiP apparently increased the amount of soluble rAaHIT in cells, no improvement of active toxin production in the medium was detected. These results suggest that secretion and folding of rAaHIT are insufficient in insect cells.