APEKTx1
Identifiers
SymbolN/A
RefSeqP86862.1
UniProtP86862
Search for
StructuresSwiss-model
DomainsInterPro

Kunitz-type serine protease inhibitor APEKTx1 is a peptide toxin derived from the sea anemone Anthopleura elegantissima. This toxin has a dual function, acting both as a serine protease inhibitor and as a selective and potent pore blocker of Kv1.1, a shaker related voltage-gated potassium channel.

Sources

APEKTx1 is a potent toxin purified from the sea anemone A. elegantissima.[1] Besides APEKTx1, other toxins such as APETx1,[2] APE1-1, APE1-2, APE2-2, ApC,[3] and APETx2[4] have been identified in A. elegantissima.

Chemistry

APEKTx1 monomer structure prediction (Alpha-fold model). The average per residue confidence (pLDDT) is 91.46%. The levels of confidence of the model are color coded: dark blue very high (pLDDT > 90), light blue high (90 > pLDDT > 70), orange low (70 > pLDDT > 50), and yellow very low (pLDDT < 50).

This peptide has 65 amino acids crosslinked by 3 disulphide bridges, and has a molecular mass of 7475 Da. It acts as a monomer. The toxin belongs to the type 2 sea anemone peptides targeting voltage-gated K channels. Other type 2 toxins are the kalicludines from Anemonia sulcata, which selectively block Kv1.2 channels,[5] and SHTX II from Stichodactyla haddoni.[6] Structural homology is also shared with the basic pancreatic trypsin inhibitor (BPTI), a very potent Kunitz-type protease inhibitor, and dendrotoxins (DTX I and α-DTX), which are potent inhibitors of voltage-gated potassium channels.[1]

APEKTx1
Amino acid sequence
INSICLLPKKQGFCRARFPRFYYNSSTRRCEMFY

YGGCGGNANNFNTLEECEKVCLGYGEAWKAP

Target

APEKTx1 is a highly selective blocker of the voltage-gated potassium channel Kv1.1 with no effect on other tested potassium channels (Kv1.2, Kv1.3, Kv1.4, Kv1.5, Kv1.6, Shaker IR, Kv2.1, Kv3.1, Kv4.2 and Kv4.3).[1] APEKTx1 selectively blocks Kv1.1 channels with an IC50 value of 0.9 nM, which makes it between a 700 to 3000 times more potent inhibitor than the two known sea anemone peptides targeted against Kv channels (kalicludines and SHTX II).[1] APEKTx1 is thought to interact with Kv1.1 through the aliphatic residue alanine (A352), an acidic residue glutamate (E353), and an aromatic residue tyrosine (Y379), as a mutation in these sites causes a loss in affinity of the toxin for Kv1.1.[1][7] These residues are located in the H5-loop between the S5 and S6 domains and are part of the channel’s pore.[7][8]

In addition, APEKTx1 acts as a potent trypsin inhibitor (Kd= 124 nM), probably a competitive one. However, trypsin inhibition is more potent (as it has a higher affinity) in BPTI, which can be explained by the presence of Phe13 and Pro19 in APEKTx1, causing an unfavorable interaction.[1]

Mode of action

APEKTx1 works by blocking the Kv1.1 channel in its open conformation, showing no effect on the voltage-dependence of channel gating.[1] In contrast to certain homologous toxins like DTX-K, APEKTx1 can completely block the K+ current. The binding site of the toxin is presumed to be located at the extracellular side, as evidenced by the rapid inhibition of current through Kv1.1 channels and the reversible binding upon washout. Unlike the irreversible binding of DTX K dendrotoxins (which are homologous to APEKTx1), APEKTx1's action is reversible. This is possibly due to the substitution of a lysine (either Lys3 or Lys26) with a neutral arginine in APEKTx1.[1] APEKTx1 likely contains a dyad: a pair of amino acid residues composed of a basic residue and a hydrophobic residue separated from each other by 6.6 ± 1.0 Å that interacts with the aromatic residues on the P-loop participating in the channel-toxin interaction affinity.[1][9] APEKTx1’s dyad is formed by Arg15 and Phe13, separated by 6.2 Å, contributing to the formation of a physical barrier that opposes the K+ efflux.[10] Presumably, the side chain of Arg15 enters the Kv1.1 pore and interacts with the Asp377 residues, while Phe13 interacts with hydrophobic residues, including Tyr375, which is critically involved in the interaction affinity with the sea anemones toxins.[1][11]

The interaction of APEKTx1 with Kv1.1 channels follows the kinetic behavior of a bimolecular reaction, meaning that the rate of binding depends on the concentration of both reactants (toxin and channel).[1]

References

  1. 1 2 3 4 5 6 7 8 9 10 11 Peigneur S, Billen B, Derua R, Waelkens E, Debaveye S, Béress L, Tytgat J (July 2011). "A bifunctional sea anemone peptide with Kunitz type protease and potassium channel inhibiting properties". Biochemical Pharmacology. 82 (1): 81–90. doi:10.1016/j.bcp.2011.03.023. PMID 21477583. S2CID 20886727.
  2. ↑ Diochot S, Loret E, Bruhn T, Béress L, Lazdunski M (July 2003). "APETx1, a new toxin from the sea anemone Anthopleura elegantissima, blocks voltage-gated human ether-a-go-go-related gene potassium channels". Molecular Pharmacology. 64 (1): 59–69. doi:10.1124/mol.64.1.59. PMID 12815161.
  3. ↑ Bruhn T, Schaller C, Schulze C, Sanchez-Rodriguez J, Dannmeier C, Ravens U, et al. (May 2001). "Isolation and characterisation of five neurotoxic and cardiotoxic polypeptides from the sea anemone Anthopleura elegantissima". Toxicon. 39 (5): 693–702. doi:10.1016/s0041-0101(00)00199-9. PMID 11072049.
  4. ↑ Diochot S, Baron A, Rash LD, Deval E, Escoubas P, Scarzello S, et al. (April 2004). "A new sea anemone peptide, APETx2, inhibits ASIC3, a major acid-sensitive channel in sensory neurons". The EMBO Journal. 23 (7): 1516–1525. doi:10.1038/sj.emboj.7600177. PMC 391081. PMID 15044953.
  5. ↑ Schweitz H, Bruhn T, Guillemare E, Moinier D, Lancelin JM, Béress L, Lazdunski M (October 1995). "Kalicludines and kaliseptine. Two different classes of sea anemone toxins for voltage sensitive K+ channels". The Journal of Biological Chemistry. 270 (42): 25121–25126. doi:10.1074/jbc.270.42.25121. PMID 7559645.
  6. ↑ Honma T, Kawahata S, Ishida M, Nagai H, Nagashima Y, Shiomi K (April 2008). "Novel peptide toxins from the sea anemone Stichodactyla haddoni". Peptides. 29 (4): 536–544. doi:10.1016/j.peptides.2007.12.010. PMID 18243416. S2CID 207356560.
  7. 1 2 Hurst RS, Busch AE, Kavanaugh MP, Osborne PB, North RA, Adelman JP (October 1991). "Identification of amino acid residues involved in dendrotoxin block of rat voltage-dependent potassium channels". Molecular Pharmacology. 40 (4): 572–576. PMID 1921987.
  8. ↑ Tytgat J, Debont T, Carmeliet E, Daenens P (October 1995). "The alpha-dendrotoxin footprint on a mammalian potassium channel". The Journal of Biological Chemistry. 270 (42): 24776–24781. doi:10.1074/jbc.270.42.24776. PMID 7559595.
  9. ↑ Dauplais M, Lecoq A, Song J, Cotton J, Jamin N, Gilquin B, et al. (February 1997). "On the convergent evolution of animal toxins. Conservation of a diad of functional residues in potassium channel-blocking toxins with unrelated structures". The Journal of Biological Chemistry. 272 (7): 4302–4309. doi:10.1074/jbc.272.7.4302. PMID 9020148.
  10. ↑ Mouhat S, De Waard M, Sabatier JM (February 2005). "Contribution of the functional dyad of animal toxins acting on voltage-gated Kv1-type channels". Journal of Peptide Science. 11 (2): 65–68. doi:10.1002/psc.630. PMID 15635666. S2CID 31444823.
  11. ↑ Gilquin B, Braud S, Eriksson MA, Roux B, Bailey TD, Priest BT, et al. (July 2005). "A variable residue in the pore of Kv1 channels is critical for the high affinity of blockers from sea anemones and scorpions". The Journal of Biological Chemistry. 280 (29): 27093–27102. doi:10.1074/jbc.M413626200. PMID 15890656.
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