# Pandinus imperator toxin (Pi4)

Pi4 (α-KTx 6.4) is a short toxin from the scorpion Pandinus imperator that blocks specific potassium channels.

## Etymology

The name Pi4 is the fourth toxin isolated from the scorpion Pandinus imperator, from which Pi1, Pi2, Pi3, and Pi7 have also been isolated.

## Chemistry

Pi4 is a peptide, which consists of 38 amino-acids with the following sequence:
IEAIRCGGSRDCYRPCQKRTGCPNAKCINKTCKCYGCS
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. The cysteines of Pi4 are paired in the following order: $^6$C–$^{27}$C; $^{12}$C–$^{32}$C; $^{16}$C–$^{34}$C; $^{22}$C–$^{37}$C.
Most disulfide bridges show a left-handed conformation, although in the disulfide bridge between $^{22}$C–$^{37}$C some variation is found. Only the disulfide bridge between $^6$C–$^{27}$C shows a right-handed conformation.

## Target & Mode of action

Pi4 blocks different potassium channels, for instance, the Shaker B, K$_v$1.2, and SK potassium channels.

### Shaker B channel

Pi4 binds to Shaker B potassium channels, the Drosophila homologue of the voltage-gated potassium channel K$_v$1.1. Pi4 reversibly blocks this channel with an IC$_{50}$ of 3.0 ± 2.2 nM.
A Pi4 peptide, synthesized with a different C-terminus than the natural Pi4 (COO- instead of COH$_2$N), shows the same binding characteristics as natural Pi4. This suggests that the C-terminus of the peptide Pi4 is not involved in the binding of Pi4 to the Shaker B channel.
The residues $^{26}$K, $^{28}$I, $^{29}$N, $^{33}$K, and $^{35}$Y might be important for Pi4's interaction with Shaker B channels, especially $^{26}$K and $^{35}$Y, which form a conserved dyad of a lysine and aromatic cluster in other potassium channel toxins. It has been suggested that the positive charge of the lysine mimicks a potassium ion and enters the pore of the potassium channel, hence occluding the pore opening and inhibiting the ion flux.

### Kv1.2 channel

Pi4 blocks the voltage-gated potassium channel K$_v$1.2. at low concentrations (IC$_{50}$ 8.0 ± 5 pM). No significant effects have been observed on Kv1.1 and Kv1.3 channels at concentrations up to 10 μm.
In the binding of the peptide Pi4 to the K$_v$1.2 channel, the β-sheet structure is thought to play an important role. First, the residue $^{35}$Y, located in the β-sheet structure, tightly interacts via electrostatic forces with the aromatic cluster of Kv1.2 channels ($^{344}$W, $^{345}$W and $^{355}$Y). Second, the residue $^{26}$K becomes stabilized by the four carbonyl oxygen atoms located in the channel of pore formed by four Kv1.2 α-subunits ($^{357}$D). Finally, the four $^{332}$Q residues of the four α-subunits of Kv1.2 channels interact via salt bridges with four subunits of the toxin ring (composed of $^{10}$R, $^{19}$R, $^{30}$K, $^{33}$K).

### SK-channel

Furthermore, the Pi4 binds to SK channels, small conductance Ca$^{2+}$-activated potassium channels. Pi4 competes with apamin, another SK-channel toxin. IC$_{50}$ is 0.5 ± 0.2 μM.

## Toxicity

Scorpion venoms can be toxic for mammals, insects, and crustaceans. Pi4 is lethal in mice upon injection in the ventricular system of the brain at and LD$_{50}$ value of 0.2 μg/mouse.