From venoms to insecticides: Exploring the structure, function, and evolution of peptide toxins found in the venom of Australian funnel-web spiders

Date of Completion

January 2006

Keywords

Biology, Molecular|Biology, Genetics|Chemistry, Biochemistry

Degree

Ph.D.

Abstract

Arthropod pests adversely affect humans by destroying a significant amount of the world's food supply and by transmitting numerous deadly diseases. Generally, these pests have been controlled by spraying non-specific chemical insecticides. However, this is becoming increasingly ineffective due to the evolution of insecticide resistance in most medically and agriculturally important arthropods. In addition, there is a growing concern about the human health risks associated with certain agrochemicals. Thus, there is an urgent need to develop new insect control methods. Since the primary role of spider venoms is to kill or immobilize arthropod prey, one thought is that spider venoms should be a rich source of insecticidal toxins. By screening the venom of the deadly Australian funnel-web spider, our lab discovered several families of insect-specific peptide neurotoxins that appear to be suitable for novel bioinsecticide development. ^ Analysis of cDNA libraries from the venom glands of Australian funnel-web spiders suggests that these neurotoxins are generated via a remarkable combinatorial peptide library strategy. The mature toxins are derived from an mRNA translation product consisting of an N-terminal signal sequence, a central propeptide, and a C-terminal mature toxin sequence. However, rather than making the toxins as "one-offs", the spider appears to generate a library (or "family") of peptide toxins which sometimes vary by as little as one amino acid residue. Intriguingly, within each toxin family, there is a marked difference in the level of sequence conservation within the signal peptide and mature toxin sequences. The signal peptide, which is critical for targeting the toxin to a specific secretory pathway, is highly conserved within each family. In contrast, the mature toxin sequence is poorly conserved---it appears to have been hypermutated during the course of venom evolution, with only the cystine framework remaining conserved. By grouping functionally disparate toxins into large "superfamilies" (which we define as toxins that contain the same signal sequence and cystine framework) we have gained significant insight into the ongoing evolutionary process by which these spiders combat prey resistance and generate peptides with novel functions. ^ Superfamily analysis resulted in an intriguing lead toxin, the hybrid toxin. Analysis demonstrated that the hybrid toxin is the first dual-target self-synergistic toxin. This effect means that less material would be needed to kill targeted insects because of the increased potency of the dual-target insecticide, and activity of an insecticide on two distinctly different ion channels minimizes the possibility of insects evolving target site resistance as resistance mutations would have to evolve in two separate channels simultaneously. Thus, we have discovered an excellent lead for the development of a novel dual target, self-synergizing insecticide. ^

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