Proteases as insecticidal agents

Abstract

Proteases from a variety of sources (viruses, bacteria, fungi, plants, and insects) have toxicity towards insects. Some of these insecticidal proteases evolved as venom components, herbivore resistance factors, or microbial pathogenicity factors, while other proteases play roles in insect development or digestion, but exert an insecticidal effect when over-expressed from genetically engineered plants or microbial pathogens. Many of these proteases are cysteine proteases, although insect-toxic metalloproteases and serine proteases have also been examined. The sites of protease toxic activity range from the insect midgut to the hemocoel (body cavity) to the cuticle. This review discusses these insecticidal proteases along with their evaluation and use as potential pesticides.

Keywords: basement membrane; cuticle; insecticides; microbial defense; peritrophic matrix; plant defense.

Figure 1

Diagram of generic insect anatomy, showing targets sites of insecticidal proteases (). Besides degrading the cuticle, basement membranes, and peritrophic matrix, proteases in the hemocoel may convert prophenoloxidase (PPO) to phenoloxidase (PO) either directly or indirectly by activating the cascade of serine proteases that lead to the conversion step.

Figure 2

Cuticular melanization resulting from baculovirus expression of ScathL in larvae of Heliothis virescens. Control (uninfected) and wild type virus-infected larvae are shown for comparison.

Figure 3

Impact of ScathL on internal tissues of H. virescens. A, B, Scanning electron micrographs of the basement membrane overlying the fat body of larvae infected with recombinant baculoviruses expressing the catalytic site mutant ScathL-C146A (A) or wild-type ScathL (B) showing ScathL-mediated perforations. Bars, 2.5 µm. C, D, Light microscopy images of trachea of larvae infected with viruses expressing ScathL-C146A (A) or wild-type ScathL (B) showing melanized tracheal tips (t). Scale bar, 0.5 mm. (Reprinted with permission from Elsevier, [95].)