Guangxitoxin, also known as GxTX, is a peptide toxin found in the venom of the tarantula Plesiophrictus guangxiensis. It primarily inhibits outward voltage-gated Kv2.1 potassium channel Electric current, which are prominently expressed in Beta cell, thus increasing insulin secretion.
Sources
Guangxitoxin is found in the venom of the tarantula
Plesiophrictus guangxiensis, which lives mainly in
Guangxi province of southern China.
Chemistry
Subtypes
Guangxitoxin consists of multiple subtypes, including GxTX-1D, GxTX-1E and GxTX-2.
GxTX-2 shows
Peptide sequence similarities with
Hanatoxin (HaTX),
Stromatoxin-1 (ScTx1), and
Scodra griseipes toxin (SGTx) peptides.
GxTX-1 shows sequence similarities with
Jingzhaotoxin (JZTX-III),
Grammostola spatulata mechanotoxin-4 (GsMTx-4), and Voltage-sensor toxin-1 (VSTX1) peptides.
GxTX-1 consists of two variants, GxTX-1D and GxTX-1E, of which GxTX-1E is a more potent inhibitor of K
v2.1.
Sequence
GxTX-1D and GxTX-1E consist of 36 amino acids, differing only a single amino acid at the
N-terminus,
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
Structure
The three-dimensional NMR structure of the toxin reveals an
Amphiphile part and an inhibitor cystine knot (ICK) motif.
The amphipathic part is composed of a large cluster characterized by
Solvent exposure Hydrophobe 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.
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.
Target
GxTX-1E inhibits voltage-gated K
v2.1 channels by modifying its voltage-dependent gating,.
in the S3b-S4 paddle motif of the voltage-sensing
Protein 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
KCND channels.
K
v2.2 is located predominantly in
Delta cell of
primate islets.
K
v4.3 is mainly of importance in the
heart.
The Kv2.1 channel is predominantly expressed in pancreatic β-cells
GxTX-1E has no effect on voltage-gated Na+ or Ca2+ channels.
Mode of action
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.
Therapeutic use
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.
