Over hundreds of millions of years of evolutionary fine-tuning, animal venoms have evolved into potent chemical cocktails utilized for predation and defence. Venoms typically contain hundreds of pharmacologically diverse bioactive peptides, thus venomous animals are natural repositories containing vast arrays of toxins with potential pharmaceutical and agrochemical applications. Since many venomous animals are insectivorous and rely on their venom for prey capture and subdual, their venoms often contain numerous toxins that rapidly incapacitate or kill insects. With pest species fast becoming resistant to current insecticides, there is an urgent need for novel insecticidal compounds. Animal venoms are therefore an emergent source of prospective lead molecules for insecticidal development, and the functional and structural diversity of known venom peptides that are selectively active on insects are reviewed in Chapter 2.
Although scorpion venoms have been studied for over fifty years, the majority of research has focussed primarily on members of the medically important Buthidae species. Additionally, venoms of the estimated 200 species of scorpion native to Australia have received very little attention. Chapter 3 presents the first venom mass profiles of six non-buthid and one buthid scorpion species, four of which are endemic to Australia. While masses under 5 kDa dominated the venoms of all species, the buthid venom contained considerably more masses between 7 and 8 kDa than those of the non-buthids, corroborating the emergent trend that buthids are richer in longchain neurotoxins than non-buthids. The Australian scorpion venom fractions were also analysed with the relatively new MALDI-ToF matrix 1,5-DAN. Over forty partial sequences were obtained, the majority of which are homologous to scorpion antimicrobials.
Chapter 4 describes the isolation, characterization, and structure of a novel toxin (U1-LITX-Lw1a) from the venom of the Australian scorpion Liocheles waigiensis. U1-LITX-Lw1a is the first example of a native peptide that adopts a structural motif called the disulfide-directed β-hairpin (DDH) fold. Over a decade ago it was proposed that the DDH is an ancestral two-disulfide precursor fold of the three-disulfide inhibitor cystine knot (ICK) motif that is common to venom peptides from spiders, scorpions and aquatic cone snails. U1-LITX-Lw1a not only represents the discovery of a missing link in venom protein evolution, it is the first member of a fourth structural fold to be adopted by scorpion venom peptides. Additionally, U1-LITX-Lw1a was shown to have potent insecticidal activity across a broad range of insect pest species, thereby providing a novel structural scaffold for bioinsecticide development.
Chapter 5 details the activity of U1-LITX-Lw1a at mammalian intracellular calcium release channels called ryanodine receptors (RyRs). Since all scorpion venom ICK toxins isolated to date target RyRs, the ability of U1-LITXLw1a to activate RyRs provides the first functional link between DDH and ICK scorpion toxins. Additionally, support for the two-binding site hypothesis of scorpion toxins on RyRs is provided by the activity of a C-terminal U1-LITXLw1a mutant, which did not enhance full receptor openings seen with the wildtype toxin but retained the ability to induce receptor subconductance states. U1-LITX-Lw1a and analogues are therefore potential pharmacological tools and may have applications as drug leads for disease states involving aberrant RyRs.