Discrepin

Discrepin (α-KTx15.6) is a peptide from the venom of the Venezuelan scorpion Tityus discrepans.[1] It acts as a neurotoxin by irreversibly blocking A-type voltage-dependent K+-channels.

Etymology and source

Discrepin is named after its source: a Venezuelan scorpion called Tityus discrepans.[1] Its systematic number is α-KTx15.6.[2]

Chemistry

The subfamily α-KTx15 consists of 6 toxins. The first five toxins of this subfamily are very much alike, but discrepin only shares 50% amino acid homology with other members of this subfamily.[1] Discrepin contains 38 amino acid residues. It has a polyglutamic acid at its N-terminal region.[3] Discrepin has the α and β folds that are characteristic of scorpion toxins.[2] It consists of one α-helix and three β-sheet helix strands. The α-helix is formed from amino acid Ser11 until Arg21. The three antiparallel β-sheets are formed from amino acid Ile2 until Lys7, Ala27 until Cys29 and Arg33 until Cys36.

Target

Discrepin blocks voltage-gated Shal-type (Kv4.x) K+ channels in cerebellar granular cells.[1][2] These A-type K+ channels regulate firing frequency, spike initiation and the waveform of action potentials.[2] Discrepin has yet only been tested in cerebellar cells, however, Kv4.x family channels are in general highly expressed in the brain, heart and smooth muscles.[4] Competition experiments showed that discrepin inhibits the binding of scorpion toxin BmTx3 to its receptor site, where other K+ channel blockers (Kv1-, Kv3.4-, Kv4.2/4.3 family blockers) were unable to compete with this toxin. These results support the hypothesis that discrepin can bind to a very specific and unique type of Kv4.x receptor channels.[2] The residues of discrepin that are important for blocking these channels have not yet been clarified. However, it is clear that the N-terminal plays a role in the binding affinity.[3] The stoichiometry of toxin binding to the potassium channel is 1:1.[3]

Mode of action

Discrepin specifically blocks the IA currents (fast transient, low-voltage-activated currents) of voltage-dependent K+ channels. Inhibition of these K+ currents occurs in an irreversible manner, i.e. washing out of the toxin gives no recovery of the currents.[1][2] The kinetics of the channel are not affected by discrepin and the blockage is independent of the holding potential.[2]

Toxicity

The half-effective dose (IC50) is 190 ± 30 nM.[1]

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References
  1. D'Suze G, Batista CV, Frau A, et al. (October 2004). "Discrepin, a new peptide of the sub-family alpha-ktx15, isolated from the scorpion Tityus discrepans irreversibly blocks K+ -channels (IA currents) of cerebellum granular cells". Archives of Biochemistry and Biophysics. 430 (2): 256–63. doi:10.1016/j.abb.2004.07.010. PMID 15369825.
  2. Prochnicka-Chalufour A, Corzo G, Satake H, et al. (February 2006). "Solution structure of discrepin, a new K+-channel blocking peptide from the alpha-KTx15 subfamily". Biochemistry. 45 (6): 1795–804. doi:10.1021/bi0519248. PMID 16460026.
  3. Romeo S, Corzo G, Vasile A, Satake H, Prestipino G, Possani LD (April 2008). "A positive charge at the N-terminal segment of Discrepin increases the blocking effect of K+ channels responsible for the IA currents in cerebellum granular cells". Biochimica et Biophysica Acta (BBA) - General Subjects. 1780 (4): 750–5. doi:10.1016/j.bbagen.2008.01.012. PMID 18280256.
  4. Birnbaum SG, Varga AW, Yuan LL, Anderson AE, Sweatt JD, Schrader LA (July 2004). "Structure and function of Kv4-family transient potassium channels". Physiological Reviews. 84 (3): 803–33. doi:10.1152/physrev.00039.2003. PMID 15269337.
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