Guangxitoxin

Guangxitoxin is found in the venom of the tarantula Plesiophrictus guangxiensis, which lives mainly in Guangxi province of southern China.

Guangxitoxin consists of multiple subtypes, including GxTX-1D, GxTX-1E and GxTX-2. GxTX-2 shows sequence similarities with Hanatoxin (HaTX), Stromatoxin-1 (ScTx1), and Scodra griseipes toxin (SGTx) peptides.{{cite journal | vauthors = Swartz KJ, MacKinnon R | title = An inhibitor of the Kv2.1 potassium channel isolated from the venom of a Chilean tarantula | journal = Neuron | volume = 15 | issue = 4 | pages = 941–9 | date = October 1995 | pmid = 7576642 | doi = 10.1016/0896-6273(95)90184-1 | doi-access = free }}{{cite journal | vauthors = Escoubas P, Diochot S, Célérier ML, Nakajima T, Lazdunski M | title = Novel tarantula toxins for subtypes of voltage-dependent potassium channels in the Kv2 and Kv4 subfamilies | journal = Molecular Pharmacology | volume = 62 | issue = 1 | pages = 48–57 | date = July 2002 | pmid = 12065754 | doi = 10.1124/mol.62.1.48 }}{{cite journal | vauthors = Lee CW, Kim S, Roh SH, Endoh H, Kodera Y, Maeda T, Kohno T, Wang JM, Swartz KJ, Kim JI | title = Solution structure and functional characterization of SGTx1, a modifier of Kv2.1 channel gating | journal = Biochemistry | volume = 43 | issue = 4 | pages = 890–7 | date = February 2004 | pmid = 14744131 | doi = 10.1021/bi0353373 }} GxTX-1 shows sequence similarities with Jingzhaotoxin-III (JZTX-III), Grammostola spatulata mechanotoxin-4 (GsMTx-4), and Voltage-sensor toxin-1 (VSTX1) peptides.{{cite journal | vauthors = Xiao Y, Tang J, Yang Y, Wang M, Hu W, Xie J, Zeng X, Liang S | title = Jingzhaotoxin-III, a novel spider toxin inhibiting activation of voltage-gated sodium channel in rat cardiac myocytes | journal = The Journal of Biological Chemistry | volume = 279 | issue = 25 | pages = 26220–6 | date = June 2004 | pmid = 15084603 | doi = 10.1074/jbc.M401387200 | doi-access = free }}{{cite journal | vauthors = Suchyna TM, Johnson JH, Hamer K, Leykam JF, Gage DA, Clemo HF, Baumgarten CM, Sachs F | title = Identification of a peptide toxin from Grammostola spatulata spider venom that blocks cation-selective stretch-activated channels | journal = The Journal of General Physiology | volume = 115 | issue = 5 | pages = 583–98 | date = May 2000 | pmid = 10779316 | pmc = 2217226 | doi = 10.1085/jgp.115.5.583 }}{{cite journal | vauthors = Ruta V, Jiang Y, Lee A, Chen J, MacKinnon R | title = Functional analysis of an archaebacterial voltage-dependent K+ channel | journal = Nature | volume = 422 | issue = 6928 | pages = 180–5 | date = March 2003 | pmid = 12629550 | doi = 10.1038/nature01473 | bibcode = 2003Natur.422..180R | s2cid = 4396796 }} GxTX-1 consists of two variants, GxTX-1D and GxTX-1E, of which GxTX-1E is a more potent inhibitor of K_v2.1.

GxTX-1D and GxTX-1E consist of 36 amino acids, differing only a single amino acid at the NH₂-terminal, aspartate or glutamate, respectively:

Asp/Glu-Gly-Glu-Cys-Gly-Gly-Phe-Trp-Trp-Lys-Cys-Gly-Ser-Gly-Lys-Pro-Ala-Cys-Cys-Pro-Lys-Tyr-Val-Cys-Ser-Pro-Lys-Trp-Gly-Leu-Cys-Asn-Phe-Pro-Met-Pro

GxTX-2 consists of 33 amino acids, which has only 9 identical amino acids in corresponding sequence compared to GxTX-1D and GxTX-1E:

Glu-Cys-Arg-Lys-Met-Phe-Gly-Gly-Cys-Ser-Val-Asp-Ser-Asp-Cys-Cys-Ala-His-Leu-Gly-Cys-Lys-Pro-Thr-Leu-Lys-Tyr-Cys-Ala-Trp-Asp-Gly-Thr

The three-dimensional NMR structure of the toxin reveals an amphipathic part and an inhibitor cystine knot (ICK) motif.{{cite journal | vauthors = Lee S, Milescu M, Jung HH, Lee JY, Bae CH, Lee CW, Kim HH, Swartz KJ, Kim JI | title = Solution structure of GxTX-1E, a high-affinity tarantula toxin interacting with voltage sensors in Kv2.1 potassium channels | journal = Biochemistry | volume = 49 | issue = 25 | pages = 5134–42 | date = June 2010 | pmid = 20509680 | pmc = 2918519 | doi = 10.1021/bi100246u }}

The amphipathic part is composed of a large cluster characterized by solvent-exposed hydrophobic residues which is enclosed by acidic and basic residues. The ICK motif contains three disulfide bonds stabilizing the toxin structure. The conserved amphipathic structure assists in binding the toxin and can be explained since similar toxins allocate into lipid membranes effectively with the help of this structure and interact with K_v channels from within the membrane.{{cite journal | vauthors = Lee SY, MacKinnon R | title = A membrane-access mechanism of ion channel inhibition by voltage sensor toxins from spider venom | journal = Nature | volume = 430 | issue = 6996 | pages = 232–5 | date = July 2004 | pmid = 15241419 | doi = 10.1038/nature02632 | bibcode = 2004Natur.430..232L | s2cid = 4329147 }}{{cite journal | vauthors = Phillips LR, Milescu M, Li-Smerin Y, Mindell JA, Kim JI, Swartz KJ | title = Voltage-sensor activation with a tarantula toxin as cargo | journal = Nature | volume = 436 | issue = 7052 | pages = 857–60 | date = August 2005 | pmid = 16094370 | doi = 10.1038/nature03873 | bibcode = 2005Natur.436..857R | s2cid = 4426640 }}{{cite journal | vauthors = Milescu M, Vobecky J, Roh SH, Kim SH, Jung HJ, Kim JI, Swartz KJ | title = Tarantula toxins interact with voltage sensors within lipid membranes | journal = The Journal of General Physiology | volume = 130 | issue = 5 | pages = 497–511 | date = November 2007 | pmid = 17938232 | pmc = 2151668 | doi = 10.1085/jgp.200709869 }}{{cite journal | vauthors = Milescu M, Bosmans F, Lee S, Alabi AA, Kim JI, Swartz KJ | title = Interactions between lipids and voltage sensor paddles detected with tarantula toxins | journal = Nature Structural & Molecular Biology | volume = 16 | issue = 10 | pages = 1080–5 | date = October 2009 | pmid = 19783984 | pmc = 2782670 | doi = 10.1038/nsmb.1679 }} Differences in distribution of acidic and basic residues compared to SGTx-1 may contribute to the difference in affinity of GxTX-1E for the K_v2.1 channel. Dissimilarities in orientation of loops and turns compared to JZTX-III may contribute to the discrepancy in selectivity of GxTX-1E to the K_v2.1 channel.

GxTX-1E inhibits voltage-gated K_v2.1 channels by modifying its voltage-dependent gating,.{{cite journal | vauthors = Schmalhofer WA, Ratliff KS, Weinglass A, Kaczorowski GJ, Garcia ML, Herrington J | title = A KV2.1 gating modifier binding assay suitable for high throughput screening | journal = Channels | volume = 3 | issue = 6 | pages = 437–47 | date = November 2009 | pmid = 21150283 | doi = 10.4161/chan.3.6.10201 | doi-access = free }} mutations in the S3b-S4 paddle motif of the voltage-sensing domain of K_v2.1 reduce affinity for tarantula toxins. Two other voltage-gated potassium channels inhibited by GxTX-1 are the K_v2.2 and K_v4.3 channels. K_v2.2 is located predominantly in δ-cells of primate islets.{{cite journal | vauthors = Yan L, Figueroa DJ, Austin CP, Liu Y, Bugianesi RM, Slaughter RS, Kaczorowski GJ, Kohler MG | title = Expression of voltage-gated potassium channels in human and rhesus pancreatic islets | journal = Diabetes | volume = 53 | issue = 3 | pages = 597–607 | date = March 2004 | pmid = 14988243 | doi = 10.2337/diabetes.53.3.597 | doi-access = free }} K_v4.3 is mainly of importance in the heart.{{cite journal | vauthors = Oudit GY, Kassiri Z, Sah R, Ramirez RJ, Zobel C, Backx PH | title = The molecular physiology of the cardiac transient outward potassium current (I(to)) in normal and diseased myocardium | journal = Journal of Molecular and Cellular Cardiology | volume = 33 | issue = 5 | pages = 851–72 | date = May 2001 | pmid = 11343410 | doi = 10.1006/jmcc.2001.1376 }}

The K_v2.1 channel is predominantly expressed in pancreatic β-cells{{cite journal | vauthors = MacDonald PE, Wheeler MB | title = Voltage-dependent K(+) channels in pancreatic beta cells: role, regulation and potential as therapeutic targets | journal = Diabetologia | volume = 46 | issue = 8 | pages = 1046–62 | date = August 2003 | pmid = 12830383 | doi = 10.1007/s00125-003-1159-8 | doi-access = free }} and in the central nervous system.{{cite journal | vauthors = Frech GC, VanDongen AM, Schuster G, Brown AM, Joho RH | title = A novel potassium channel with delayed rectifier properties isolated from rat brain by expression cloning | journal = Nature | volume = 340 | issue = 6235 | pages = 642–5 | date = August 1989 | pmid = 2770868 | doi = 10.1038/340642a0 | bibcode = 1989Natur.340..642F | s2cid = 4352853 }}{{cite journal | vauthors = Misonou H, Mohapatra DP, Trimmer JS | title = Kv2.1: a voltage-gated k+ channel critical to dynamic control of neuronal excitability | journal = Neurotoxicology | volume = 26 | issue = 5 | pages = 743–52 | date = October 2005 | pmid = 15950285 | doi = 10.1016/j.neuro.2005.02.003 }} In pancreatic β-cells, K_v2.1 comprises 60% of the currents mediated by K_v channels.{{cite journal | vauthors = MacDonald PE, Ha XF, Wang J, Smukler SR, Sun AM, Gaisano HY, Salapatek AM, Backx PH, Wheeler MB | title = Members of the Kv1 and Kv2 voltage-dependent K(+) channel families regulate insulin secretion | journal = Molecular Endocrinology | volume = 15 | issue = 8 | pages = 1423–35 | date = August 2001 | pmid = 11463864 | doi = 10.1210/mend.15.8.0685 | doi-access = free }} Furthermore, the K_v2.1 channel shows similar biophysical properties to the delayed rectifier K⁺ current (I_DR) of the β-cells.{{cite journal | vauthors = Roe MW, Worley JF, Mittal AA, Kuznetsov A, DasGupta S, Mertz RJ, Witherspoon SM, Blair N, Lancaster ME, McIntyre MS, Shehee WR, Dukes ID, Philipson LH | title = Expression and function of pancreatic beta-cell delayed rectifier K+ channels. Role in stimulus-secretion coupling | journal = The Journal of Biological Chemistry | volume = 271 | issue = 50 | pages = 32241–6 | date = December 1996 | pmid = 8943282 | doi = 10.1074/jbc.271.50.32241 | doi-access = free}} This makes GxTX appropriate to study the physiological role of the aforementioned current as it inhibits 90% of the β-cell I_DR. The I_DR is thought to play an important role in repolarization of action potentials.{{cite journal | vauthors = Smith PA, Bokvist K, Arkhammar P, Berggren PO, Rorsman P | title = Delayed rectifying and calcium-activated K+ channels and their significance for action potential repolarization in mouse pancreatic beta-cells | journal = The Journal of General Physiology | volume = 95 | issue = 6 | pages = 1041–59 | date = June 1990 | pmid = 2197368 | pmc = 2216351 | doi = 10.1085/jgp.95.6.1041 }} Both the K_v2.2 and K_v4.3 channels are believed not to contribute significantly to the β-cell I_DR.

GxTX-1E has no effect on voltage-gated Na⁺ or Ca²⁺ channels.

Inhibition of K_v2.1 by GxTX-1E causes a shift in voltage-dependency of activation toward more positive potentials of almost 100 mV. Moreover, GxTX-1E also exhibits properties of decreasing the velocity of hK_v2.1 channel opening and increasing the velocity of K_v2.1 channel closing approximately sixfold.

By inhibiting K_v2.1 potassium channels, GxTX-1E boosts action potentials of pancreatic β-cells causing mainly increased glucose-stimulated intracellular calcium oscillations which in turn intensifies glucose-stimulated insulin secretion.

How GxTX-1E can generate distinctive calcium oscillations in different cells remains unclear (broader oscillations, increased frequency or restoration of oscillations), however, the specificity of GxTX-1E points in the direction of I_DR inhibition causing these effects. Notably, GxTX-1E stimulated insulin secretion is specifically glucose-dependent, considering that I_DR is only active above -20mV membrane potentials which is only seen in raised glucose levels.

Unlike K_ATP channel blockers, GxTX-1 primarily blocks I_DR and demonstrates a potential target for future drugs in diabetes mellitus type 2 treatment, since a blockade of I_DR should not provoke hypoglycaemia.

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Category:Ion channel toxins

Category:Spider toxins