(3FTx) [1]
Toxin Tx7335.
Three Finger Toxin
Three Finger Toxins (3FTx) [1] are small proteins ranging in size from 57-82 amino acids and feature a core of four or five disulfide bonds with a series of three β-sheets extending from the core. The location of these disulfide bonds within the protein determine their biological activity [2]. Conventional 3FTx will have 8 cysteines and nonconventional 3FTx will have 10 cysteines (3). 3FTx may also exist as dimers, again effecting their biological activity. 3FTx will bind to nicotinic acetylcholine receptors via either competitive or allosteric binding. In doing so, the neurotoxin will cause the nervous system to shut down and cause the same effects in the body. 3FTx and Phospholipases are the two most prevalent proteins in coral snake venom due to gene duplication. This duplication creates a greater concentration of the toxins as well as increases the likelihood of gene mutation (1).
Media:Disulphide bond locations.mp4
[2]
Function and Disease
Table of Three Finger Toxins: 
Discovery
Toxin Tx7335 was discovered during testing of eastern mamba venom. The experiment was attempting to identify the toxins that interact with the KcsA pathway but received false positives upon isolation of all known proteins that interact with this pathway. The experimenters proceeded to remove the known toxins from the venom and run the remaining venom over a free Co2+ resin before running toxin pull-down experiments. These experiments showed the presence of a single toxin with a mass of 7333.5 daltons (4). The mass of this protein was then used to name the toxin.
Media: Toxin Tx7335 discovery.mp4
Structural highlights
Nonconventional 3FTx will have cysteines located at the 3rd, 6th, 11th, 17th, 24th, 39th, 43rd, 55th, 56th, and 61st positions. Conventional 3Ftx lack cysteines at the 6th and 11th position. The disulfide bonds will form between the 3rd and 24th positions, 6th and 11th positions, 17th and 39th positions, 43rd and 55th positions, and the 56th and 61st positions in nonconventional 3FTx, and conventional 3Ftx will lack the cysteine bond between the 6th and 11th positions (3). There is a single known 3FTx isolated from eastern green mamba venom called Tx7335 that doesn’t maintain the conventional cysteine pattern in the other 3FTx. It switches the location of a cysteine for a tyrosine at the 43rd position where all other known 3FTx proteins have a cysteine. The cysteine is replaced in the 25th position where most other 3FTx proteins have a tyrosine. The probable location for the new disulfide bond in this protein would be between the 26th and 54th cysteines (3).
Relevance
This change in configuration causes the activation of pH-gated potassium channels Media: pH-gated channel.jpg that no other 3FTx has been shown to cause. Activation of this potassium channel is caused by increasing the mean open time of the channel and the open probability of the channel. These changes are dose-dependent. The addition of 2 micromolar Tx7335 caused a roughly 8-fold increase in open-gate time for the wild-type KcsA and roughly 13-fold increase in the pmut3 KcsA. The open probability increased about 40-fold for the wild-type and about 90-fold for the pmut3 channels. The toxin added to the intracellular side of the cells did not cause any change in the channel (4).
The Potassium-ion channel in Streptomyces lividans (KcsA) are a good model for K+-selective ion channel pores found in humans. These pores are necessary in the cardiac and neuronal electronic signaling pathways. Failure in these pores has been linked to cardiac and neuronal diseases as well as various cancers (4). Ion flow through these channels are generally regulated by the activation due to a stimulus like voltage increase or ligand bonding and inactivation that is not dependent on a stimulus. Tx7335 is a ligand binding activator, and only activated the KcsA single channel system when it was introduced outside of the cell. This means that it does not directly affect the internal pH gate, but rather shifts the equilibrium towards the conductive state of the inactivation gate. Since this protein has a unique pathway to affect KcsA, it will be useful in studying the allosteric regulation of the KcsA, and other potassium channel inactivation by extension.
