Pandinus imperator toxin (Pi4)
Pi4 (α-KTx 6.4) is a short toxin from the scorpion Pandinus imperator that blocks specific potassium channels.[1][2][3]
Etymology
[edit]The name Pi4 is the fourth toxin isolated from the scorpion Pandinus imperator, from which Pi1, Pi2, Pi3, and Pi7 have also been isolated.[3]
Chemistry
[edit]Pi4 is a peptide, which consists of 38 amino-acids with the following sequence:
IEAIRCGGSRDCYRPCQKRTGCPNAKCINKTCKCYGCS[1][4]
It contains an α-helix and a β-sheet; it is stabilized by four cysteine-pairings that are crosslinked by four short disulfide bridges. The four disulfide bridges are characteristic for the α-KTX6 subfamily, while most other scorpion toxins contain only three disulfide bridges.[1] The cysteines of Pi4 are paired in the following order: 6C–27C; 12C–32C; 16C–34C; 22C–37C.[1][4]
Most disulfide bridges show a left-handed conformation, although in the disulfide bridge between 22C–37C some variation is found. Only the disulfide bridge between 6C–27C shows a right-handed conformation.[1]
Target & Mode of action
[edit]Pi4 blocks different potassium channels, for instance, the Shaker B, Kv1.2, and SK potassium channels.
Shaker B channel
[edit]Pi4 binds to Shaker B potassium channels, the Drosophila homologue of the voltage-gated potassium channel Kv1.1.[1] Pi4 reversibly blocks this channel with an IC50 of 3.0 ± 2.2 nM.[1][2]
A Pi4 peptide, synthesized with a different C-terminus than the natural Pi4 (COO- instead of COH2N), shows the same binding characteristics as natural Pi4.[1] This suggests that the C-terminus of the peptide Pi4 is not involved in the binding of Pi4 to the Shaker B channel.[1]
The residues 26K, 28I, 29N, 33K, and 35Y might be important for Pi4's interaction with Shaker B channels,[1] especially 26K and 35Y, which form a conserved dyad of a lysine and aromatic cluster in other potassium channel toxins.[5] It has been suggested that the positive charge of the lysine mimicks a potassium ion and enters the pore of the potassium channel,[6][7][8] hence occluding the pore opening and inhibiting the ion flux.
Kv1.2 channel
[edit]Pi4 blocks the voltage-gated potassium channel Kv1.2.[2] at low concentrations (IC50 8.0 ± 5 pM).[2] No significant effects have been observed on Kv1.1 and Kv1.3 channels at concentrations up to 10 μm.[2]
In the binding of the peptide Pi4 to the Kv1.2 channel, the β-sheet structure is thought to play an important role. First, the residue 35Y, located in the β-sheet structure, tightly interacts via electrostatic forces with the aromatic cluster of Kv1.2 channels (344W, 345W and 355Y).[2] Second, the residue 26K becomes stabilized by the four carbonyl oxygen atoms located in the channel of pore formed by four Kv1.2 α-subunits (357D).[2] Finally, the four 332Q residues of the four α-subunits of Kv1.2 channels interact via salt bridges with four subunits of the toxin ring (composed of 10R, 19R, 30K, 33K).[2]
SK-channel
[edit]Furthermore, the Pi4 binds to SK channels, small conductance Ca2+-activated potassium channels.[2] Pi4 competes with apamin, another SK-channel toxin. IC50 is 0.5 ± 0.2 μM.[2]
Toxicity
[edit]Scorpion venoms can be toxic for mammals, insects, and crustaceans.[1] Pi4 is lethal in mice upon injection in the ventricular system of the brain at and LD50 value of 0.2 μg/mouse.[2]
References
[edit]- ^ a b c d e f g h i j k Guijarro, J. Iñaki; M'Barek, Sarrah; Gómez-Lagunas, Froylan; Garnier, Damien; Rochat, Hervé; Sabatier, Jean-Marc; Possani, Lourrival D.; Delepierre, Muriel (2003). "Solution structure of Pi4, a short four-disulfide- bridged scorpion toxin specific of potassium channels". Protein Science. 12 (9): 1844–1855. doi:10.1110/ps.03186703. PMC 2323982. PMID 12930984.
- ^ a b c d e f g h i j k M'Barek, Sarrah; Mosbah, Amor; Sandoz, Guillaume; Fajloun, Ziad; Olamendi-Portugal, Timoteo; Rochat, Hervé; Sampieri, François; Guijarro, J. Iñaki; Mansuelle, Pascal (2003). "Synthesis and characterization of Pi4, a scorpion toxin from Pandinus imperator that acts on K + channels". Eur. J. Biochem. 270 (17). Muriel Delepierre, Michel De Waard, and Jean-Marc Sabatier: 3583–3592. doi:10.1046/j.1432-1033.2003.03743.x. PMID 12919322.
- ^ a b Olamendi-Portugal, Timoteo; Gómez-Lagunas, Froylan; Gurrola, Georgina B.; Possani, Lourival D. (1998). "Two similar peptides from the venom of the scorpion pandinus imperator, one highly effective blocker and the other inactive on K+ channels". Toxicon. 36 (5): 759–770. doi:10.1016/s0041-0101(97)00163-3. PMID 9655636.
- ^ a b "Kalium: Scorpion Toxins Active on Potassium Channels". kaliumdb.org. Retrieved 2016-10-09.
- ^ Dauplais, Marc; Lecoq, Alain; Song, Jianxing; Cotton, Joël; Jamin, Nadège; Gilquin, Bernard; Roumestand, Christian; Vita, Claudio; Medeiros, Cleane L. C. de (1997-02-14). "On the Convergent Evolution of Animal Toxins CONSERVATION OF A DIAD OF FUNCTIONAL RESIDUES IN POTASSIUM CHANNEL-BLOCKING TOXINS WITH UNRELATED STRUCTURES". Journal of Biological Chemistry. 272 (7): 4302–4309. doi:10.1074/jbc.272.7.4302. ISSN 0021-9258. PMID 9020148.
- ^ Goldstein, S.A.; Miller, C. (1993). "Mechanism of charybdotoxin block of a voltage-gated K+ channel". Biophysical Journal. 65 (4): 1613–1619. Bibcode:1993BpJ....65.1613G. doi:10.1016/s0006-3495(93)81200-1. PMC 1225887. PMID 7506068.
- ^ Giangiacomo, Kathleen M.; Garcia, Maria L.; McManus, Owen B. (1992-07-01). "Mechanism of iberiotoxin block of the large-conductance calcium-activated potassium channel from bovine aortic smooth muscle". Biochemistry. 31 (29): 6719–6727. doi:10.1021/bi00144a011. ISSN 0006-2960. PMID 1379069.
- ^ Park, Chul-Seung; Miller, Christopher (1992). "Interaction of charybdotoxin with permeant ions inside the pore of a K+ channel". Neuron. 9 (2): 307–313. doi:10.1016/0896-6273(92)90169-e. PMID 1379820. S2CID 43596648.