| Poneratoxin | |
|---|---|
 | |
| Identifiers | |
| Symbol | Poneratoxin |
| SCOP2 | 1G92 /SCOPe /SUPFAM |
| OPM superfamily | 151 |
| OPM protein | 1g92 |
Poneratoxin is a paralyzingneurotoxicpeptide made by the bullet antParaponera clavata. It prevents inactivation ofvoltage gated sodium channels and therefore blockssynaptic transmission in thecentral nervous system.[1] Specifically, poneratoxin acts on voltage gated sodium channels inskeletal muscle fibers, causing paralysis, andnociceptive fibers, causing pain.[2] It is rated as a 4 plus on theSchmidt sting pain index, the highest possible rating with that system, and its effects can cause waves of pain up to twelve hours after a single sting.[3] It is additionally being studied for its uses in biological insecticides.[4]
Overall, poneratoxin disrupts normal function ofvoltage-gated sodium channels in bothvertebrates andinvertebrates. It causes repetitive firing and prolongation ofaction potentials, particularly in thecentral nervous system of insects.[4] The increase in signaling is what causes the intense pain frombullet ant stings.[5]
Poneratoxin is in an inactive state when stored in the ant venom reservoir due to the reservoir's acidic conditions, but it becomes toxic when activated via a multistep process. The combination of poneratoxin binding to acell membrane (in order to act upon a voltage-gated sodium channel) and the movement from acidic conditions in the ant venom reservoir to basic conditions at the target site leads to poneratoxin undergoing aconformational change that activates it.[1]
Catterallet al. hypothesized that somepolypeptideneurotoxins modifyvoltage-gated channels function via a "voltage-sensor trapping" mechanism. The hypothesis states that neurotoxins similar to poneratoxin, such asalpha-scorpion toxins, act uponsodium channels via binding to the channels'receptor site 3, which normally affects the channels' ability to inactivate. Therefore, receptor site 3 neurotoxins often affect sodium channels by slowing or blocking inactivation.[2][6] Normally, the region of the channel where neurotoxin receptor site 3 is undergoes a conformational change of an outward movement to lead to inactivation. Receptor site 3 neurotoxins are proposed to prevent this conformational change via interaction withacidic andhydrophobicamino acid residues at that site.[6]
When frogskeletal muscle fibers were exposed to poneratoxin, it was found that poneratoxin primarily affected voltage-dependent sodium channels by decreasing the peak sodium current and also inducing a slow sodium current. This combination resulted in the sodium channels activating at very negative potentials and deactivating very slowly, a phenomenon commonly seen in excitable tissues.[7] Poneratoxin is considered as a slow-actingagonist forsmooth muscles.[8]
The poneratoxin peptide is stored in an inactive 25-residue peptide (amino acid sequence FLPLLILGSLLMTPPVIQAIHDAQR) in the venom reservoir ofParaponera clavata. Thesecondary structure is characterized by ahelix-turn-helix motif: twoalpha helices connected by abeta-turn.
The twoalpha helices are formed by residues 3–9 at the N-terminus, and residues 17–24 at the C-terminus, and they are connected by thebeta-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.[1] This is notable because of its structural similarity to other peptides that interact with the membrane, and indicates that poneratoxin will also interact with the membrane and thereby affect embeddedvoltage gated sodium channels.[9] Furthermore, the structure of the peptide shifts from arandom coil to the structuredhelix-turn-helix when introduced to alipid bilayer environment, which indicates that this motif is important for interacting with the membrane.[1]
The two alpha helices, however, have markedly different characteristics. The N-terminal alpha helix is apolar, containing a centralhydrophobic core withhydrophilic residues at either end, and is uncharged. It is similar in structure to a transmembranesignal peptide, which implies that it will anchor at the membrane by burying the hydrophobic core within the bilayer.[1] In particular, the bulky and very hydrophobicphenylalanine residue is important for interacting with uncharged lipid bilayers, such as those composed ofphosphatidylcholine. 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.[10] Specifically, the positively chargedarginine and the non-polaralanine residues were both shown to be essential for poneratoxin potency.[2] See figure, where the hydrophobic (red) and hydrophilic (blue) regions of poneratoxin and the lipid bilayer align, demonstrating that the structure is evolved to insert into the membrane, which will promote interaction with thevoltage gated sodium channels.
Many people consider a sting from abullet ant to resemble the sensation of getting shot.Justin Schmidt, anentomologist who developed theSchmidt sting pain index, described it as "pure, intense, brilliant pain...like walking over flaming charcoal with a three-inch nail embedded in your heel," and considers the sting from a bullet ant to be the most painful insect sting he has experienced.[11] The pain from bullet ant stings can last for many hours, even up to 24 hours. Both the immense pain and the duration of the sting are due to the effects of poneratoxin.[5] In addition to the notorious pain, symptoms of stings from bullet ants (as well as stings from other ants of the genusParaponera as well as the genusDinoponera) include fever, cold sweats, nausea, vomiting,lymphadenopathy andcardiac arrhythmias.[citation needed]
Toxicity assays have found that theLT50 of poneratoxin, delivered via injections of genetically engineered viruses, toS. frugiperda larvae, was at 131 hours post-injection. A dose of 105pfu of poneratoxin was sufficient to kill theS. frugiperda larvae, and a dose of 10 ng could paralyze them.[1] Based on these experiments, scientists believe poneratoxin can make a good candidate as a bio-insecticide because of itsneurotoxicity to other insects, making it capable of immobilizing or even killing insects infected with it. The making of arecombinant virus by engineering abaculovirus that expresses poneratoxin has been proposed.[1]