Defensins are proteins common to many plants and animals that fight off viral, bacterial and fungal pests.
Researchers investigated the relationship between these proteins and the neurotoxins present in scorpion venom.
Their results showed how just a single genetic mutation could convert such a protein into a deadly toxin. The findings, published in the journal Molecular Biology and Evolution, are the first evidence of an evolutionary relationship between these defensins and toxins, according to scientists.
A scorpion’s venom is a potent mix of genetically-encoded toxic proteins used to kill or paralyse prey and defend against predators or competitiors. Previous evidence suggested a common ancestor between a family of neurotoxins found in this venom and defensins, insect proteins which defend against tiny pests known as microbes.
But Prof Shunyi Zhu from the Chinese Academy of Sciences, who undertook the study, explained that the similarity of the two in terms of their genetic structure was relatively low which “left a puzzle for more than 20 years” for researchers. In order to confirm the functional link, the team of researchers from China and Belgium analysed the scorpion neurotoxin to find its “signature” – the region of the protein responsible for its structure and function.
They then searched for this key sequence in some of the insect defensive proteins. It was found in green shield bugs, spined soldier bugs and three species of backswimmer.
“It is surprising that only insect defensins from venomous insects contain scorpion toxin signatures,” said Prof Zhu. “These defensins clearly represent an evolutionary intermediate and could have the potential to develop into a toxin with similar action to scorpion toxins.”
To test this theory, the researchers went on to engineer the insect defensive protein to give it scorpion neurotoxin function. They were able to do so by deleting just one single loop in the protein’s genetic structure. “This is a typical example of divergent evolution,” said Prof Zhu describing how the shift from microbe immunity to predator defence is a key element in the evolutionary origins of scorpions and their stings.