Novel venom proteins produced by differential domain-expression strategies in beaded lizards and gila monsters (genus Heloderma)

The origin and evolution of venom proteins in helodermatid lizards was investigated by multidisciplinary techniques. Our analyses elucidated novel toxin types resultant from three unique domain-expression processes: i) the first full-length sequences of Lethal Toxin isoforms (helofensins) revealed this toxin type to be constructed by an ancestral mono-domain, mono-product gene (beta-defensin) which underwent three tandem domain duplications to encode a tetra-domain, mono-product with a possible novel protein fold; (ii) an ancestral mono-domain gene (encoding a natriuretic peptide) was medially extended to become a penta-domain, penta-product through the additional encoding of four tandemly repeated proline-rich peptides (helokinestatins), with the five discrete peptides liberated from each other by post-translational proteolysis; and iii) an ancestral multi-domain, multi-product gene belonging to the VIP/glucagon family being mutated to encode for a mono-domain, mono-product (exendins) followed by duplication and diversification into two variant classes (exendins 1 2 and exendins 3 4). Bioactivity characterization of exendin and helokinestatin elucidated variable cardioactivity between isofroms within each class. These results highlight the importance of utilising evolutionary-based search strategies for biodiscovery and the virtually unexplored potential of lizard venoms in drug design and discovery.