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Poneratoxin is a paralyzing neurotoxic peptide made by the bullet ant Paraponera clavata. It prevents inactivation of voltage gated sodium channels and therefore blocks the synaptic transmission in the central nervous system. Specifically, poneratoxin acts on voltage gated sodium channels in skeletal muscle fibers, causing paralysis, and nociceptive fibers, causing pain. It is rated as a 4 plus on the Schmidt sting pain index, the highest in the world, and its effects can cause waves of pain up to twelve hours after a single sting. . Schmidt describes it as "pure, intense, brilliant pain...like walking over flaming charcoal with a three-inch nail embedded in your heel." It is additionally being studied for its uses in biological insecticides.

Structure

The poneratoxin peptide is stored in an inactive 25-residue peptide in the venom reservoir of Paraponera clavata. The secondary structure is characterized by a helix-turn-helix motif: two alpha helices connected by a beta turn.

Poneratoxin inserting into the plasma membrane. Blue regions are hydrophillic, and red regions are hydrophobic

The two alpha helices are formed by residues 3-9 at the N-terminus, and residues 17-24 at the C-terminus, and they are connected by the beta turn at residues 11-16. From a three dimensional perspective, this structure forms a preferential V-shape with the two helices undergoing loose non-covalent interactions with each other. This is notable because of it's structural similarity to other peptides that interact with the membrane, and indicates that poneratoxin will also interact with the membrane and thereby affect imbedded voltage gated sodium channels. Furthermore, the structure of the peptide shifts from a random coil to the structured helix-turn-helix when introduced to a lipid bilayer environment, which indicates that this motif is important for interacting with the membrane.

The two alpha helices, however, have markedly different characteristics. The N-terminal alpha helix is apolar, containing a central hydrophobic core with hydrophilic residues at either end, and is uncharged. It is similar in structure to a transmembrane signal peptide and implies that it will anchor at the membrane by burying the hydrophobic core within the bilayer. In particular, the bulky and very hydrophobic phenylalanine residue is important for interacting with uncharged lipid bilayers, such as those composed of phosphatidylcholine. The C-terminal alpha helix is amphipathic with one side displaying polar and charged residues, and the other displaying non-polar residues, which drives insertion into the plasma membrane. Specifically, the positively charged arginine and the non-polar alanine residues were both shown to be essential for poneratoxin potency. See figure, where the hydrophobic (red) and hydrophillic (blue) regions of poneratoxin and the lipid bilayer align, demonstrating that the structure is designed to be inserted into the membrane, which will promote interaction with the voltage gated sodium channels.

Evolution of Painful Stings

The Schmidt Sting Pain Index arose from the pursuit of a larger hypothesis: that the evolution of sociality in Hymenoptera was dependent on the evolution of venom that was both painful and toxic. Pain is an advertisement of damage in the body, but molecules that produce pain and those that are toxic, and actively cause damage, are not the same. Although the painful signal acts as a deterrent, intelligent predators will learn the dishonesty of this signal with repeated exposure – that there is no real damage being done. For the early Hymenoptera that were primarily solitary, the pain alone would allow them the chance to escape. Furthermore, solitary insects do not provide a high energy reward for predators, and therefore predators do not expend significant effort to hunt them. However, with the evolution of sociality where many Hymenoptera cluster together in colonies, nests become a nutritionally rich and therefore worthwhile target. If there were no defenses, predators would devour the defenseless society, leaving few surviving individuals and eliminating social reproduction. Sociality would therefore not be beneficial. In order for sociality to evolve, Hymenoptera needed a defense beyond a painful sting to protect their whole colony. Their sting was an advertisement of damage, and toxicity evolved as its truth. With a toxic sting, and thus the ability to protect against predators, Hymenoptera were able to progress towards sociality and its associated evolutionary benefits of the shared raising of youth, individual task specialization, inter-colony communication, and food storage.

To approach studying this evolutionary connection between toxicity and sociality, Schmidt recognized there needed to be a quantitative measure with which to score the painfulness of stings. Assays for toxicity are already well characterized and can be quantified, but without the Schmidt Sting Pain Index, there would be no way to relate the amount of sociality to the level of pain, and therefore this hypothesis could not have been studied.